Week 3 Notes

Python,
Week 3

Loops, ranges and formatted output: the tools that let a few lines of code do real repetitive work.

8 chapters55 worked examples~47 min read

whilefor and rangef-stringsbreak / continue

Chapters

01The while LoopPart 1
02The for Loop and range()Parts 2-3
03for-Loop PracticePart 3
04Nested Loops and PracticeParts 4-5
05Formatted Printing, end and sepParts 6-7
06break, continue and passPart 8
07Practice ZonePractice zone
08Week 3 Summary SheetAll parts

Loops

The while Loop

Repeating while a condition holds, with while...else, and four classic digit programs.

1.1The while loop

The while loop executes a group of statements as long as the given expression is True.

Syntax

while expression: followed by an indented block of statement(s).

Python

i = 0
while i < 5:
    print(i)
    i += 1

Output

Figure 1. Test, run the body, update the counter, test again

The loop keeps going until i (the counter) is no longer less than 5. Incrementing the counter is important: if it is not changed, the condition never becomes false and the loop runs forever.

Round	i at the test	i < 5 ?	prints	i after
1	0	True	0	1
2	1	True	1	2
3	2	True	2	3
4	3	True	3	4
5	4	True	4	5
6	5	False	—	loop ends

Python

i = 0
while i < 5:
    print(i)
    # i += 1 is missing: i stays 0 and this prints 0 forever

Do not run this one. It is shown only to explain the infinite loop. (If it ever happens, stop the program with the Stop button or Ctrl + C.)

1.2while ... else

The else block runs once, when the condition becomes false, that is when the loop ends normally.

Python

i = 1
while i <= 2:
    print("Hello")
    i += 1
else:
    print("Loop end")

Output

Hello
Hello
Loop end

Extra. If the loop is left early with break (Part 6), the else block is skipped.

Python

i = 1
while i <= 5:
    if i == 3:
        break
    print(i)
    i += 1
else:
    print("Loop end")

Output

1
2

1.3Practice: factorial

The factorial of $n$ is $1 \times 2 \times \cdots \times n$ . Start with fact = 1 and multiply by i for i = 1, 2, ..., n. Two versions of the same program:

Python

print("Enter a no.: ")
n = int(input())
i, fact = 1, 1
while i <= n:
    fact = fact * i
    i += 1
print("Factorial is:", fact)

Output

Enter a no.: 
5
Factorial is: 120

Python

n = int(input("Enter: "))
i, fact = 1, 1
while i <= n:
    fact = fact * i
    i += 1
print("Factorial:", fact)

Output

Enter: 5
Factorial: 120

i	fact before	fact after
1	1	1
2	1	2
3	2	6
4	6	24
5	24	120

For n = 0 the loop never runs, so fact stays 1, which is the correct value of 0!.

1.4Practice: count the digits

Python

num = abs(int(input("Enter a no.: ")))
digits = 1
while num > 9:
    num = num // 10
    digits += 1
print(digits)

Output

Enter a no.: 12345
5

Each pass removes the last digit with // 10 and adds 1 to the count; the loop stops when only one digit is left (num > 9 is false). abs() makes negative numbers work.

Input	Output
12345	`5`
9	`1`
10	`2`
0	`1`
-987	`3`

Simpler alternative. len(str(x)) gives the digit count directly: len(str(12345)) is 5.

1.5Practice: reverse the digits

Python

num = int(input("Enter a number: "))
absNum = abs(num)
rev = 0
while absNum > 0:
    r = absNum % 10
    absNum = absNum // 10
    rev = rev * 10 + r
print(rev if num > 0 else -rev)

Output

Enter a number: 123
321

Each pass takes the last digit r = absNum % 10, drops it from absNum, and pushes it onto the right end of rev with rev * 10 + r. Trace for 123:

absNum (start)	r = absNum % 10	absNum after	rev
123	3	12	3
12	2	1	32
1	1	0	321

Input	Output	Note
123	`321`
-45	`-54`	sign restored at the end
100	`1`	leading zeros disappear in a number
0	`0`	the loop never runs

1.6Practice: palindrome check

Python

num = int(input("Enter a number: "))
absNum = abs(num)
rev = 0
while absNum > 0:
    r = absNum % 10
    absNum = absNum // 10
    rev = rev * 10 + r
if num < 0:
    rev = -rev
print("Palindrome" if rev == num else "Not a palindrome")

Output

Enter a number: 121
Palindrome

A number is a palindrome if it reads the same reversed. The program reverses it and compares with the original.

Input	Output
121	`Palindrome`
123	`Not a palindrome`
10	`Not a palindrome`
7	`Palindrome`
-121	`Palindrome`

Extra. The program calls -121 a palindrome because it restores the sign before comparing. By the usual definition negative numbers are not palindromes, so treat that as a quirk of this program.

1.7Going deeper

Choose the update. Three things make a good while: a starting value, a test, and an update that moves toward making the test false.
Unknown number of rounds. while is the right tool when you do not know in advance how many times to repeat (digits of a number, input until a sentinel).
Off-by-one. i < n versus i <= n changes how many times the loop runs (extra).

1.8Loop patterns you will use again and again

Almost every loop question is one of a handful of patterns. Spot the pattern first, then fill in the numbers.

A. Accumulator, counter, flag, running maximum

Accumulator: start a variable at the identity (0 for a sum, 1 for a product) and update it every round.
Counter: start at 0 and add 1 only when a condition holds.
Flag: start with found = False, set it to True when you meet the item, and test it after the loop.
Running maximum: start with the first value (not 0, which fails for all-negative data) and replace it whenever a larger value comes.
Average: accumulator divided by counter, after the loop.

Python

marks = [7, 3, 6, 2]
total = 0
count = 0
for m in marks:
    total = total + m
    count = count + 1
print("total", total, "count", count, "average", total / count)

temps = [-9, -4, -2, -7]
largest = temps[0]               # start from the first value, not from 0
for t in temps:
    if t > largest:
        largest = t
print("largest", largest)

found = False
for x in [4, 8, 15]:
    if x == 8:
        found = True
print("found", found)

Output

total 18 count 4 average 4.5
largest -2
found True

B. Euclid's gcd and the Fibonacci numbers with a while loop

Python

a, b = 252, 105
while b != 0:
    a, b = b, a % b              # both names change at once
print(a)

x, y = 0, 1
n = 8
while n > 0:
    print(x, end=" " if n > 1 else "\n")
    x, y = y, x + y
    n = n - 1

Output

21
0 1 1 2 3 5 8 13

The line a, b = b, a % b evaluates the whole right side first, then assigns, so it needs no temporary variable. The loop is Euclid's algorithm: $\gcd(a, b) = \gcd(b, a \bmod b)$ .

C. A loop that stops on a signal (sentinel) and the loop's else

Sentinel: while True: keeps going until a special value tells you to stop, then break leaves the loop. This is the pattern for "read numbers until the user types 0".
Loop else: the else block of a for or while runs when the loop ends normally, and is skipped if a break ended it. A prime test fits perfectly: if no divisor made us break, the number is prime.

Python

data = [5, 4, 3, 0, 9, 9]     # imagine these come from input()
total = 0
i = 0
while True:
    v = data[i]
    if v == 0:
        break                  # the sentinel
    total = total + v
    i = i + 1
print("sum before the 0 is", total)

for n in [7, 9]:
    for d in range(2, n):
        if n % d == 0:
            print(n, "is not prime")
            break
    else:
        print(n, "is prime")

Output

sum before the 0 is 12
7 is prime
9 is not prime

Off-by-one checklist. range(a, b) stops before b; to include b write range(a, b + 1). A while loop that never changes its condition runs forever; check that the variable in the condition really changes inside the loop.

1.9Exam tips and common mistakes

Exam tips

Trace with a table: columns for each variable, one row per round, and a final row for the failed test.
Count rounds by asking "when does the test first become false?".

Common mistakes

Forgetting to update the counter (infinite loop).
Updating the counter before printing and so shifting the output by one.
Starting fact at 0, which makes every product 0.

Practice questions (graded-assignment style)

Q1How many lines?

Multiple Choice Question (MCQ): choose the one correct option.

How many numbers does the program print?

Python

i = 0
while i < 5:
    print(i)
    i += 1

It never stops

Solution

Idea

Find the first i for which i < 5 is false.

Python

i = 0
while i < 5:
    print(i)
    i += 1

Output

i takes the values 0, 1, 2, 3, 4 inside the loop: five prints. At i = 5 the test fails.
Answer: option 2.

Q2Factorial of 4

Numerical Answer Type (NAT).

What does the program print for the input 4?

Python

n = int(input("Enter: "))
i, fact = 1, 1
while i <= n:
    fact = fact * i
    i += 1
print("Factorial:", fact)

Solution

Idea

Multiply 1, 2, 3, 4 in turn.

Python

n = int(input("Enter: "))
i, fact = 1, 1
while i <= n:
    fact = fact * i
    i += 1
print("Factorial:", fact)

Output

Enter: 4
Factorial: 24

fact goes 1 → 1 → 2 → 6 → 24.
Answer: 24

Q3Digit count

Numerical Answer Type (NAT).

What does the digit-count program print for the input 5000?

Python

num = abs(int(input("Enter a no.: ")))
digits = 1
while num > 9:
    num = num // 10
    digits += 1
print(digits)

Solution

Idea

Each pass removes one digit until one is left.

Python

num = abs(int(input("Enter a no.: ")))
digits = 1
while num > 9:
    num = num // 10
    digits += 1
print(digits)

Output

Enter a no.: 5000
4

5000 → 500 → 50 → 5, three removals, so the count goes 1 → 4.
Answer: 4

Q4Reverse with a zero

Multiple Choice Question (MCQ): choose the one correct option.

What does the reverse program print for the input 120?

Python

num = int(input("Enter a number: "))
absNum = abs(num)
rev = 0
while absNum > 0:
    r = absNum % 10
    absNum = absNum // 10
    rev = rev * 10 + r
print(rev if num > 0 else -rev)

021

21

12

120

Solution

Idea

rev is a number, not a string.

Python

num = int(input("Enter a number: "))
absNum = abs(num)
rev = 0
while absNum > 0:
    r = absNum % 10
    absNum = absNum // 10
    rev = rev * 10 + r
print(rev if num > 0 else -rev)

Output

Enter a number: 120
21

The digits come off as 0, 2, 1, so rev becomes 0, 2, 21. A number cannot keep a leading zero, so the result is 21.
Answer: option 2.

Q5while-else

Multiple Choice Question (MCQ): choose the one correct option.

What does the program print?

Python

i = 1
while i <= 2:
    print(i)
    i += 1
else:
    print("Loop end")

1 and 2 only

1, 2 and then Loop end

Loop end only

Nothing

Solution

Idea

The loop ends normally, so else runs.

Python

i = 1
while i <= 2:
    print(i)
    i += 1
else:
    print("Loop end")

Output

1
2
Loop end

The condition i <= 2 becomes false after two rounds, so the loop ends normally and else runs.
Answer: option 2.

Q6Palindrome

Multiple Choice Question (MCQ): choose the one correct option.

What does the palindrome program print for the input 10?

Python

num = int(input("Enter a number: "))
absNum = abs(num)
rev = 0
while absNum > 0:
    r = absNum % 10
    absNum = absNum // 10
    rev = rev * 10 + r
if num < 0:
    rev = -rev
print("Palindrome" if rev == num else "Not a palindrome")

Palindrome

Not a palindrome

10

An error

Solution

Idea

Reverse 10 as a number.

Python

num = int(input("Enter a number: "))
absNum = abs(num)
rev = 0
while absNum > 0:
    r = absNum % 10
    absNum = absNum // 10
    rev = rev * 10 + r
if num < 0:
    rev = -rev
print("Palindrome" if rev == num else "Not a palindrome")

Output

Enter a number: 10
Not a palindrome

10 reversed is the number 1 (the leading zero vanishes), and 1 != 10.
Answer: option 2.

Q1A running maximum

Numerical Answer Type (NAT).

What does the program print?

Python

data = [-5, -2, -8]
best = 0
for v in data:
    if v > best:
        best = v
print(best)

Solution

Idea

Starting from 0 assumes the data contains something at least as large as 0.

Python

data = [-5, -2, -8]
best = 0
for v in data:
    if v > best:
        best = v
print(best)

Output

No value in the list is greater than 0, so best never changes and the program prints 0, which is not even in the list. The correct start is best = data[0], which gives -2.
Answer: 0

Q2Euclid with a while loop

Numerical Answer Type (NAT).

What is printed?

Python

a, b = 84, 36
while b != 0:
    a, b = b, a % b
print(a)

Solution

Idea

Trace the pair $(a, b)$ round by round.

Python

a, b = 84, 36
while b != 0:
    a, b = b, a % b
print(a)

Output

$(84, 36) \to (36, 12) \to (12, 0)$ , since $84 \bmod 36 = 12$ and $36 \bmod 12 = 0$ . The loop stops with $a = 12$ .
Answer: 12

Q3The Fibonacci loop

Numerical Answer Type (NAT).

What is the last number printed?

Python

x, y = 0, 1
for _ in range(7):
    print(x, end=" ")
    x, y = y, x + y

Solution

Idea

The loop prints seven terms: each round prints x, then moves the pair along.

Python

x, y = 0, 1
for _ in range(7):
    print(x, end=" ")
    x, y = y, x + y

Output

0 1 1 2 3 5 8

The terms are $0, 1, 1, 2, 3, 5, 8$ . The seventh is 8.
Answer: 8

Q4The loop's else

Multiple Choice Question (MCQ): choose the one correct option.

What does the program print?

Python

for n in [4, 5]:
    for d in range(2, n):
        if n % d == 0:
            print(n, "composite")
            break
    else:
        print(n, "prime")

4 composite then 5 prime

4 prime then 5 prime

4 composite then 5 composite

4 composite, 4 prime, 5 prime

Solution

Idea

The else belongs to the inner for, and runs only if that loop finished without a break.

Python

for n in [4, 5]:
    for d in range(2, n):
        if n % d == 0:
            print(n, "composite")
            break
    else:
        print(n, "prime")

Output

4 composite
5 prime

n = 4: d = 2 divides 4, so it prints 4 composite and breaks; the else is skipped.
n = 5: no d in 2, 3, 4 divides 5, no break, so else prints 5 prime.
Answer: option 1.

Q5Stopping on a sentinel

Numerical Answer Type (NAT).

What does the program print?

Python

data = [3, 6, 0, 10, 10]
total = 0
for v in data:
    if v == 0:
        break
    total += v
print(total)

Solution

Idea

break leaves the loop at the first 0; the numbers after it are never read.

Python

data = [3, 6, 0, 10, 10]
total = 0
for v in data:
    if v == 0:
        break
    total += v
print(total)

Output

$3 + 6 = 9$ . If break were continue, the 0 would just be skipped and the answer would be $3 + 6 + 10 + 10 = 29$ .
Answer: 9

Revision overview

Syntaxwhile expression:
indented block

CounterStart, test, update
No update = infinite loop

elseRuns when the test turns
False (not after break)

The while loopWeek 3 · Part 1

Factorialfact = fact * i

Digits// 10 drops a digit
% 10 reads a digit

Reverserev = rev * 10 + r

Revision: repeating while a condition holds

Next · LoopsThe for Loop and range()

Loops

The for Loop and range()

Iterating over strings and lists, and counting with range(start, stop, step).

2.1The for loop

The for loop goes through (iterates over) a sequence or iterable object, such as a string or a list. The loop variable takes each item in turn.

Syntax

for variable in sequence: followed by an indented block.

2.2Looping through a list and a string

Python

pets = ["cats", "goats", "dogs"]
for pet in pets:
    print(pet)

Output

cats
goats
dogs

Python

txt = "Hello"
for x in txt:
    print(x)

Output

H
e
l
l
o

Figure 2. A string is a sequence of characters, so the loop visits one character per round

2.3The range() function

Use range() when you need to repeat a block but have nothing natural to loop over. range(5) makes the numbers 0, 1, 2, 3, 4: five numbers, starting at 0 by default.

Python

for x in range(5):
    print(x)

Output

range(start, stop, step)

start: the first value (default 0).
stop: the end point plus 1; it is never included.
step: the increment (default 1; negative to count down).

Python

for x in range(3, 8, 2):
    print(x)

Output

3
5
7

Python

for x in range(5, -1, -1):
    print(x)

Output

Figure 3. The numbers produced by range(3, 8, 2) and range(5, -1, -1)

End point rule. The stop value must be 1 greater than the last value you want (or, counting down, 1 less). For an expected last value of 9, write 10.

Call	Values	How many
`range(5)`	`[0, 1, 2, 3, 4]`	5
`range(2, 6)`	`[2, 3, 4, 5]`	4
`range(0, 31, 10)`	`[0, 10, 20, 30]`	4
`range(10, 0, -3)`	`[10, 7, 4, 1]`	4
`range(5, 1)`	`[]`	0
`range(0)`	`[]`	0

The last two rows are empty ranges (extra): going from 5 up to 1 with step +1, or from 0 up to 0, produces nothing, so a loop over them never runs. list(...) is used here only to display the values.

2.4Going deeper

for versus while. Use for when you know what to loop over; use while when you repeat until a condition changes.
The variable stays after the loop (extra). After for x in range(3) the name x still holds 2.
Do not change the counter yourself. A for loop updates its variable automatically; no i += 1 is needed.

Python

for x in range(3):
    pass
print(x)

Output

2.5Exam tips and common mistakes

Exam tips

Write the list of values range produces before reading the loop body.
Number of rounds of range(start, stop, step) with a positive step is about (stop - start) / step, rounded up.

Common mistakes

Expecting range(1, 5) to include 5.
Using a positive step to count down (range(5, 0) is empty).
Writing range(5, -1) without the -1 step when counting down.

Practice questions (graded-assignment style)

Q1Which values?

Multiple Choice Question (MCQ): choose the one correct option.

What does the program print?

Python

for x in range(2, 10, 3):
    print(x, end=" ")

2 5 8

2 5 8 11

2 3 4 ... 10

3 6 9

Solution

Idea

Start at 2, add 3, stop before 10.

Python

for x in range(2, 10, 3):
    print(x, end=" ")

Output

2 5 8

The values are 2, 5, 8 (the next would be 11, which is not below 10).
Answer: option 1.

Q2How many rounds

Numerical Answer Type (NAT).

How many numbers does range(1, 10, 2) produce? The program prints the count.

Python

print(len(range(1, 10, 2)))

Solution

Idea

List them: 1, 3, 5, ...

Python

print(len(range(1, 10, 2)))

Output

The values are 1, 3, 5, 7, 9: five numbers.
Answer: 5

Q3Counting down

Multiple Choice Question (MCQ): choose the one correct option.

What does the program print?

Python

for x in range(10, 0, -3):
    print(x, end=" ")

10 7 4 1

10 7 4 1 -2

10 7 4

Nothing

Solution

Idea

The stop value 0 is never reached, and the step is -3.

Python

for x in range(10, 0, -3):
    print(x, end=" ")

Output

10 7 4 1

10, 7, 4, 1 are all above 0; the next, -2, is not.
Answer: option 1.

Q4Empty range

Multiple Choice Question (MCQ): choose the one correct option.

What does the program print?

Python

for x in range(5, 1):
    print(x)
print("done")

5 4 3 2

done only

5 then done

An error

Solution

Idea

A positive step cannot go from 5 down to 1.

Python

for x in range(5, 1):
    print(x)
print("done")

Output

done

range(5, 1) is empty, so the loop body never runs and only the last line prints.
Answer: option 2.

Q5Reading a string

Multiple Select Question (MSQ): select all that apply.

Which statements about the program are true?

Python

for x in "Hello":
    print(x)

It prints 5 lines

Each line holds one character

It prints the word on one line

The loop variable holds a single character each round

Solution

Idea

A string is a sequence of characters.

Python

for x in "Hello":
    print(x)

Output

H
e
l
l
o

"Hello" has 5 characters, each printed on its own line.
Answer: options 1, 2 and 4.

Q6Last value

Numerical Answer Type (NAT).

What is the last number printed?

Python

last = 0
for x in range(0, 31, 3):
    last = x
print(last)

Solution

Idea

The stop value is never reached.

Python

last = 0
for x in range(0, 31, 3):
    last = x
print(last)

Output

range(0, 31, 3) counts 0, 3, ..., 30; 33 would be past 31.
Answer: 30

Revision overview

forfor var in sequence:
visits each item

rangerange(n): 0 to n-1
range(a, b, s): a to b-1

Reverserange(5, -1, -1)
5 down to 0

The for loop and range()Week 3 · Part 2

Sequenceslists, strings, ...

Stopnever included

Emptyrange(5, 1) gives nothing

Revision: repeating over a sequence or a range of numbers

Next · Loopsfor-Loop Practice

Loops

for-Loop Practice

Six small programs: sums, tables, primes, digits, divisibility and factors.

Six small programs that combine for, range(), if and strings. For each: read the program, check the sample runs, and make sure you can explain every line.

3.1Sum of the first n numbers

Python

sum = 0
n = int(input("Enter a no.: "))
for x in range(1, n + 1):
    sum += x
print("Sum of first", n, "numbers is", sum)

Output

Enter a no.: 5
Sum of first 5 numbers is 15

x	sum before	sum after
1	0	1
2	1	3
3	3	6
4	6	10
5	10	15

Extra. The name sum is also a built-in function. Using it as a variable works here but hides the built-in; prefer total.

3.2A multiplication table

Python

for i in range(11):
    print("2 x", i, "=", 2 * i)

Output

2 x 0 = 0
2 x 1 = 2
2 x 2 = 4
2 x 3 = 6
2 x 4 = 8
2 x 5 = 10
2 x 6 = 12
2 x 7 = 14
2 x 8 = 16
2 x 9 = 18
2 x 10 = 20

range(11) gives 0 to 10, so the table starts at 2 x 0 = 0. Use range(1, 11) to start at 1 (extra).

3.3Is the number prime?

A prime number is greater than 1 and divisible only by 1 and itself. The program assumes flag = True and switches it to False as soon as some i in 2 ... x-1 divides x.

Python

x = int(input("Enter a number: "))
if x > 1:
    flag = True
    for i in range(2, x):
        if x % i == 0:
            flag = False
    print("Prime Number" if flag else "Not a prime number")
else:
    print("Not a prime number")

Output

Enter a number: 7
Prime Number

Input	Output	Why
7	`Prime Number`	no divisor among 2..6
9	`Not a prime number`	3 divides 9
2	`Prime Number`	the range 2..1 is empty, so flag stays True
1	`Not a prime number`	not greater than 1
0	`Not a prime number`	not greater than 1

3.4Sum of all digits

Python

x = int(input("Enter a number: "))
sum = 0
for i in str(x):
    sum += int(i)
print("Sum of all digits is", sum)

Output

Enter a number: 1234
Sum of all digits is 10

str(x) turns the number into text, the loop visits each digit character, and int(i) turns it back into a number to add. 1 + 2 + 3 + 4 = 10.

3.5Numbers below x divisible by 3 or 5

Python

x = int(input("Enter a number: "))
for i in range(x):
    if i % 3 == 0 or i % 5 == 0:
        print(i)

Output

Enter a number: 16
0
3
5
6
9
10
12
15

3.6All factors of a number

Python

x = int(input("Enter a no.: "))
for i in range(1, x + 1):
    if x % i == 0:
        print(i)

Output

Enter a no.: 12
1
2
3
4
6
12

Input	Factors
12	`1 2 3 4 6 12`
7	`1 7`
1	`1`
36	`1 2 3 4 6 9 12 18 36`

3.7Going deeper

Early exit (extra). The prime program keeps testing after it finds a divisor. Adding break (Part 6) stops at the first one.
Fewer tests (extra). It is enough to test divisors up to the square root of x.
Flag pattern. Assume True, loop, set False when you find a counter-example: the same idea works for many "is it always...?" checks.

Python

x = 29
flag = True
for i in range(2, x):
    if x % i == 0:
        flag = False
        break
print("Prime" if flag else "Not prime")

Output

Prime

3.8Exam tips and common mistakes

Exam tips

For output questions write out the loop variable values first, then run the body for each.
Check the edge inputs: 0, 1, 2 and a negative number.

Common mistakes

Using range(1, n) for the sum of the first n numbers (it stops at n - 1).
Forgetting int(i) when adding digit characters.
Treating 1 as prime.

Practice questions (graded-assignment style)

Q1Sum

Numerical Answer Type (NAT).

What does the sum program print for the input 10?

Python

sum = 0
n = int(input("Enter a no.: "))
for x in range(1, n + 1):
    sum += x
print("Sum of first", n, "numbers is", sum)

Solution

Idea

Add 1 through 10.

Python

sum = 0
n = int(input("Enter a no.: "))
for x in range(1, n + 1):
    sum += x
print("Sum of first", n, "numbers is", sum)

Output

Enter a no.: 10
Sum of first 10 numbers is 55

1 + 2 + ... + 10 = 55.
Answer: 55

Q2Prime check

Multiple Choice Question (MCQ): choose the one correct option.

What does the prime program print for the input 15?

Python

x = int(input("Enter a number: "))
if x > 1:
    flag = True
    for i in range(2, x):
        if x % i == 0:
            flag = False
    print("Prime Number" if flag else "Not a prime number")
else:
    print("Not a prime number")

Prime Number

Not a prime number

An error

Nothing

Solution

Idea

Does any number from 2 to 14 divide 15?

Python

x = int(input("Enter a number: "))
if x > 1:
    flag = True
    for i in range(2, x):
        if x % i == 0:
            flag = False
    print("Prime Number" if flag else "Not a prime number")
else:
    print("Not a prime number")

Output

Enter a number: 15
Not a prime number

3 and 5 divide 15, so the flag becomes False.
Answer: option 2.

Q3Digit sum

Numerical Answer Type (NAT).

What does the digit-sum program print for the input 9045?

Python

x = int(input("Enter a number: "))
sum = 0
for i in str(x):
    sum += int(i)
print("Sum of all digits is", sum)

Solution

Idea

Add the four digits.

Python

x = int(input("Enter a number: "))
sum = 0
for i in str(x):
    sum += int(i)
print("Sum of all digits is", sum)

Output

Enter a number: 9045
Sum of all digits is 18

9 + 0 + 4 + 5 = 18.
Answer: 18

Q4Divisible by 3 or 5

Multiple Choice Question (MCQ): choose the one correct option.

For the input 11, what is printed (one number per line)?

Python

x = int(input("Enter a number: "))
for i in range(x):
    if i % 3 == 0 or i % 5 == 0:
        print(i)

0 3 5 6 9 10

3 5 6 9 10

0 3 5 6 9

3 5 6 9 10 11

Solution

Idea

range(11) starts at 0 and stops at 10.

Python

x = int(input("Enter a number: "))
for i in range(x):
    if i % 3 == 0 or i % 5 == 0:
        print(i)

Output

Enter a number: 11
0
3
5
6
9
10

The numbers 0 to 10 divisible by 3 or 5 are 0, 3, 5, 6, 9, 10.
Answer: option 1.

Q5Number of factors

Numerical Answer Type (NAT).

How many lines does the factors program print for the input 36?

Python

x = 36
count = 0
for i in range(1, x + 1):
    if x % i == 0:
        count += 1
print(count)

Solution

Idea

Count the divisors of 36.

Python

x = 36
count = 0
for i in range(1, x + 1):
    if x % i == 0:
        count += 1
print(count)

Output

The factors of 36 are 1, 2, 3, 4, 6, 9, 12, 18, 36: nine lines.
Answer: 9

Q6The prime for 2

Multiple Choice Question (MCQ): choose the one correct option.

What does the prime program print for the input 2?

Python

x = int(input("Enter a number: "))
if x > 1:
    flag = True
    for i in range(2, x):
        if x % i == 0:
            flag = False
    print("Prime Number" if flag else "Not a prime number")
else:
    print("Not a prime number")

Prime Number

Not a prime number

An error

Nothing

Solution

Idea

Is 2 > 1? What does range(2, 2) give?

Python

x = int(input("Enter a number: "))
if x > 1:
    flag = True
    for i in range(2, x):
        if x % i == 0:
            flag = False
    print("Prime Number" if flag else "Not a prime number")
else:
    print("Not a prime number")

Output

Enter a number: 2
Prime Number

2 > 1 is true, range(2, 2) is empty, so flag stays True.
Answer: option 1.

Revision overview

Sumrange(1, n + 1)
sum += x

Primeflag = True
test 2 .. x-1
False on a divisor

Digitsfor i in str(x)
int(i)

for-loop practiceWeek 3 · Part 3

Divisiblei % 3 == 0 or i % 5 == 0

Factorsx % i == 0 for 1 .. x

Edge cases0, 1, 2, negatives

Revision: small programs built from for, range and if

Next · LoopsNested Loops and Practice

Loops

Nested Loops and Practice

Loops inside loops, and four problems built on them.

4.1Nested loops

A loop can sit inside another loop. For every round of the outer loop, the inner loop runs completely from its first value to its last.

Python

animal = ["tiger", "cat", "dog"]
sounds = ["roars", "meows", "barks"]
for x in animal:
    for y in sounds:
        print(f"the {x} {y}")

Output

the tiger roars
the tiger meows
the tiger barks
the cat roars
the cat meows
the cat barks
the dog roars
the dog meows
the dog barks

Figure 4. 3 animals × 3 sounds = 9 prints, in this order

Counting rounds

If the outer loop runs m times and the inner loop runs n times each, the inner body runs m × n times in total.

Python

count = 0
for i in range(3):
    for j in range(4):
        count += 1
print(count)

Output

4.2Problem 1: all primes below a number

2 is printed separately; then every i from 3 up to num - 1 is tested by an inner loop over j. The break stops the inner loop at the first divisor; the end=' ' keeps the primes on one line.

Python

num = int(input('Enter a no.: '))
if num >= 2:
    print(2, end=' ')
    for i in range(3, num):
        flag = True
        for j in range(2, i):
            if i % j == 0:
                flag = False
                break
        if flag:
            print(i, end=' ')

Output

Enter a no.: 20
2 3 5 7 11 13 17 19

Input	Output
20	`2 3 5 7 11 13 17 19`
12	`2 3 5 7 11`
3	`2`
2	`2`
1	`(nothing)`

Correction (extra). The task says primes below the entered number, but for the input 2 the program prints 2, which is not below 2. Changing the first test to num > 2 fixes it.

Input	Original	Fixed (num > 2)
2	`2`	`(nothing)`
3	`2`	`2`
10	`2 3 5 7`	`2 3 5 7`

4.3Problem 2: profit or loss per trader

The number of traders and trades is unknown, so while loops are used with sentinel values: employee id -1 ends the outer loop, trade amount 0 ends the inner loop for one trader.

Python

empId = input('Enter employee Id: ')
while empId != '-1':
    trade = int(input('Enter trade amount: '))
    pro_loss = 0
    while trade != 0:
        pro_loss += trade
        trade = int(input('Enter trade amount: '))
    print(f"Profit/loss for emp {empId} is {pro_loss}")
    empId = input('\nEnter emp Id: ')

Output

Enter employee Id: 101
Enter trade amount: 100
Enter trade amount: -30
Enter trade amount: 0
Profit/loss for emp 101 is 70

Enter emp Id: 102
Enter trade amount: 50
Enter trade amount: 0
Profit/loss for emp 102 is 50

Enter emp Id: -1

Trader	Trades	Result
101	100, -30	70
102	50	50

4.4Problem 3: day-wise total rainfall

Python

days = int(input('Enter no. of days: '))
for i in range(1, days + 1):
    total = 0
    rainfall = int(input('Enter the rainfall: '))
    while rainfall != -1:
        total += rainfall
        rainfall = int(input('Enter the rainfall: '))
    print(f"Total rainfall for day {i} is {total}")

Output

Enter no. of days: 2
Enter the rainfall: 10
Enter the rainfall: 20
Enter the rainfall: -1
Total rainfall for day 1 is 30
Enter the rainfall: 5
Enter the rainfall: -1
Total rainfall for day 2 is 5

Here the outer loop is a for (the number of days is known) and the inner loop is a while that stops at the sentinel -1. total = 0 is reset at the start of every day.

4.5Problem 4: the longest word

Python

word = input("Enter a word: ")
maxlen = 0
while word != '-1':
    count = 0
    for letter in word:
        count += 1
    if count > maxlen:
        maxlen = count
    word = input("Enter a word: ")
print("The length of the longest word is:", maxlen)

Output

Enter a word: tree
Enter a word: elephant
Enter a word: sun
Enter a word: -1
The length of the longest word is: 8

The inner for counts the letters of each word; maxlen keeps the largest count seen. (A shorter way is len(word), extra.)

Word	count	maxlen after
tree	4	4
elephant	8	8
sun	3	8

4.6Going deeper

Sentinel loops. When you cannot know how many inputs will come, read one value first, then loop while it is not the sentinel, reading the next value at the end of the body.
Reset inner variables. A running total for each trader or day must be set to 0 inside the outer loop.
Patterns (extra). Nested loops draw grids and triangles: the inner loop prints a row, the outer loop moves to the next row.

Python

for i in range(1, 5):
    for j in range(i):
        print("*", end="")
    print()

Output

*
**
***
****

4.7Exam tips and common mistakes

Exam tips

Write the outer values; for each, list the inner values; then read off the output in order.
For total counts multiply the number of rounds of the loops.

Common mistakes

Forgetting that break leaves only the inner loop.
Not resetting total or pro_loss for each trader or day.
Forgetting to read the next value at the end of a sentinel loop (infinite loop).

Practice questions (graded-assignment style)

Q1Iteration count

Numerical Answer Type (NAT).

What does the program print?

Python

count = 0
for i in range(4):
    for j in range(5):
        count += 1
print(count)

Solution

Idea

Multiply the two loop lengths.

Python

count = 0
for i in range(4):
    for j in range(5):
        count += 1
print(count)

Output

The outer loop has 4 rounds and the inner loop has 5 rounds each: 4 × 5 = 20.
Answer: 20

Q2Nested output

Multiple Choice Question (MCQ): choose the one correct option.

What does the program print?

Python

for i in range(1, 3):
    for j in range(1, 3):
        print(i, j)

1 1 1 2 2 1 2 2 (pairs, in this order)

1 1 2 1 1 2 2 2

1 2 2 1

Only 2 2

Solution

Idea

The inner loop finishes for each outer value.

Python

for i in range(1, 3):
    for j in range(1, 3):
        print(i, j)

Output

For i = 1: j = 1, 2; then for i = 2: j = 1, 2.
Answer: option 1.

Q3Primes below 12

Multiple Choice Question (MCQ): choose the one correct option.

What does the primes-below program print for the input 12?

Python

num = int(input('Enter a no.: '))
if num >= 2:
    print(2, end=' ')
    for i in range(3, num):
        flag = True
        for j in range(2, i):
            if i % j == 0:
                flag = False
                break
        if flag:
            print(i, end=' ')

2 3 5 7 11

2 3 5 7

3 5 7 11

2 3 5 7 9 11

Solution

Idea

Which numbers from 2 to 11 are prime?

Python

num = int(input('Enter a no.: '))
if num >= 2:
    print(2, end=' ')
    for i in range(3, num):
        flag = True
        for j in range(2, i):
            if i % j == 0:
                flag = False
                break
        if flag:
            print(i, end=' ')

Output

Enter a no.: 12
2 3 5 7 11

2, 3, 5, 7 and 11 are prime (11 is below 12).
Answer: option 1.

Q4Trader total

Numerical Answer Type (NAT).

For trader 7 the trades are 20, 20, -5 and then 0. What profit/loss does the program print?

Python

empId = input('Enter employee Id: ')
while empId != '-1':
    trade = int(input('Enter trade amount: '))
    pro_loss = 0
    while trade != 0:
        pro_loss += trade
        trade = int(input('Enter trade amount: '))
    print(f"Profit/loss for emp {empId} is {pro_loss}")
    empId = input('\nEnter emp Id: ')

Solution

Idea

The 0 only stops the loop; it is not added.

Python

empId = input('Enter employee Id: ')
while empId != '-1':
    trade = int(input('Enter trade amount: '))
    pro_loss = 0
    while trade != 0:
        pro_loss += trade
        trade = int(input('Enter trade amount: '))
    print(f"Profit/loss for emp {empId} is {pro_loss}")
    empId = input('\nEnter emp Id: ')

Output

Enter employee Id: 7
Enter trade amount: 20
Enter trade amount: 20
Enter trade amount: -5
Enter trade amount: 0
Profit/loss for emp 7 is 35

Enter emp Id: -1

20 + 20 - 5 = 35.
Answer: 35

Q5Longest word

Numerical Answer Type (NAT).

For the words sky, python, is and then -1, what length is printed?

Python

word = input("Enter a word: ")
maxlen = 0
while word != '-1':
    count = 0
    for letter in word:
        count += 1
    if count > maxlen:
        maxlen = count
    word = input("Enter a word: ")
print("The length of the longest word is:", maxlen)

Solution

Idea

Compare the lengths 3, 6, 2.

Python

word = input("Enter a word: ")
maxlen = 0
while word != '-1':
    count = 0
    for letter in word:
        count += 1
    if count > maxlen:
        maxlen = count
    word = input("Enter a word: ")
print("The length of the longest word is:", maxlen)

Output

Enter a word: sky
Enter a word: python
Enter a word: is
Enter a word: -1
The length of the longest word is: 6

python has 6 letters, the longest word.
Answer: 6

Q6break in a nested loop

Multiple Choice Question (MCQ): choose the one correct option.

What does the program print?

Python

for i in range(1, 4):
    for j in range(1, 4):
        print(i, j)
        break

1 1 only

1 1 then 2 1 then 3 1

1 1, 1 2, 1 3

Nothing

Solution

Idea

break ends only the inner loop.

Python

for i in range(1, 4):
    for j in range(1, 4):
        print(i, j)
        break

Output

1 1
2 1
3 1

The inner loop stops after printing one pair for each i, but the outer loop keeps going.
Answer: option 2.

Revision overview

NestedInner runs fully
for each outer round
m × n rounds

Sentinel-1, 0 end a
while loop of inputs

ResetTotals start at 0
inside the outer loop

Nested loopsWeek 3 · Part 4

Primes below ninner loop + break + flag
fix for n = 2

Longest wordcount letters
keep the maximum

Printend=" " keeps one line

Revision: loops inside loops and sentinel-controlled input

Next · OutputFormatted Printing, end and sep

Output

Formatted Printing, end and sep

Three ways to format output, decimal places, and the end/sep parameters of print.

5.1Why format?

A plain print with commas works, but putting several values into one tidy line is easier with formatted printing. There are three tools: the % operator, the format() method and f-strings. All three give the same output.

Python

num, i = 2, 1
print(num, "x", i, "=", num * i)

Output

2 x 1 = 2

Figure 5. The same line with the three formatting tools

5.2The string modulo operator (%)

Python

num, i = 2, 1
print('%d x %d = %d' % (num, i, num * i))

Output

2 x 1 = 2

Specifier	Meaning	Example
`%d`	integer	`7`
`%f`	float (6 decimals by default)	`3.500000`
`%.2f`	float with 2 decimals (extra)	`3.14`
`%s`	string (extra)	`hi`

The values after % go in a tuple, in the same order as the specifiers.

5.3The format() method

Python

num, i = 2, 1
print('{0} x {1} = {2}'.format(num, i, num * i))

Output

2 x 1 = 2

{0} is replaced by the first argument (num), {1} by the second (i) and {2} by the third (num * i). The numbers are positions, counting from 0.

Python

print('{1} {0} {1}'.format('a', 'b'))

Output

b a b

5.4f-strings

Python

num, i = 2, 1
print(f"{num} x {i} = {num * i}")

Output

2 x 1 = 2

Put an f before the opening quote and write variables (even expressions such as num * i) directly inside {}.

Python

num = 2
for i in range(1, 4):
    print(f"{num} x {i} = {num * i}")

Output

2 x 1 = 2
2 x 2 = 4
2 x 3 = 6

5.5Decimal places

A specifier after a colon controls how a number is shown. .3f means "a float with 3 digits after the decimal point"; the value is rounded.

Figure 6. Reading {x:.3f}

Python

x = 22/7

# string modulo operator
print('Value of pi: %.3f' % (x))

# format() method
print('Value of pi = {0:.2f}'.format(x))

# f-string
print(f"Value of pi: {x:.3f}")

Output

Value of pi: 3.143
Value of pi = 3.14
Value of pi: 3.143

Correction (extra). The source prints Value of pi : 3.141 for the first and last lines. Running the code gives 3.143, because 22/7 = 3.142857... rounds up at the third decimal. The middle line, with 2 decimals, is 3.14, as in the source.

Python

x = 1234.5678
print(f"{x:.2f}")
print(f"{x:.0f}")
print(f"{x:.1f}")

Output

1234.57
1235
1234.6

5.6print() parameters: end and sep

By default print() ends with a newline (\n) and separates its arguments with a space. end and sep change these.

Figure 7. What end and sep change

Python

print("Welcome to", end=" ")
print("Python", end=" ")
print("stuck", end="@")
print("home")

Output

Welcome to Python stuck@home

Python

print("G", "F", "E", sep="")
print("09", "12", "2016", sep="-")

Output

GFE
09-12-2016

end='' keeps the next output on the same line.
sep='' removes the space; any string works as a separator.

5.7Going deeper

Which to use (extra). f-strings are the shortest and easiest to read; know all three because exams use all three.
An empty print() prints a newline (extra). That is how a row of stars ends before the next row starts.
Rounding only changes the display (extra). f"{x:.2f}" shows 2 decimals but x itself is unchanged.

Python

print(1, 2, 3, sep=", ")
x = 3.14159
print(f"{x:.2f}")
print(x)

Output

1, 2, 3
3.14
3.14159

5.8Exam tips and common mistakes

Exam tips

Translate each format string into the final text by replacing placeholders one by one.
For :.nf count n digits after the point and round the last one.

Common mistakes

Forgetting the f before an f-string (the braces then print literally).
Writing % with a single value in a tuple of the wrong length.
Counting format() positions from 1 instead of 0.

Practice questions (graded-assignment style)

Q1Which format?

Multiple Choice Question (MCQ): choose the one correct option.

What does the program print?

Python

print('{1} {0} {1}'.format('a', 'b'))

b a b

a b a

0 1 0

An error

Solution

Idea

Positions count from 0: {1} is "b".

Python

print('{1} {0} {1}'.format('a', 'b'))

Output

b a b

{1} is the second argument b, {0} the first, a.
Answer: option 1.

Q2Decimal places

Numerical Answer Type (NAT).

What does the program print?

Python

x = 1234.5678
print(f"{x:.2f}")

Solution

Idea

Two decimals, rounded.

Python

x = 1234.5678
print(f"{x:.2f}")

Output

1234.57

1234.5678 rounded to 2 decimals is 1234.57.
Answer: 1234.57

Q3Equivalent lines

Multiple Select Question (MSQ): select all that apply.

Which of these print 3 x 4 = 12?

Python

print('%d x %d = %d', 3, 4, 12)

print('%d x %d = %d' % (3, 4, 12))

print('{0} x {1} = {2}'.format(3, 4, 12))

print(f"3 x 4 = {3 * 4}")

print('%d x %d = %d', 3, 4, 12)

Solution

Idea

Option 4 forgets the % operator.

Python

print('%d x %d = %d', 3, 4, 12)

Output

%d x %d = %d 3 4 12

Options 1, 2, 3 each produce 3 x 4 = 12.
Option 4 passes the format string and numbers as separate arguments, so it prints the raw text followed by the numbers.
Answer: options 1, 2 and 3.

Q4end

Multiple Choice Question (MCQ): choose the one correct option.

What does the program print?

Python

print("A", end="-")
print("B")

A-B on one line

A and B on two lines

AB on one line

A B on one line

Solution

Idea

end replaces the newline after the first print.

Python

print("A", end="-")
print("B")

Output

A-B

The first print ends with - instead of a newline, so B follows on the same line.
Answer: option 1.

Q5sep

Numerical Answer Type (NAT).

What does the program print?

Python

print("2026", "09", "18", sep="-")

Solution

Idea

sep goes between the arguments.

Python

print("2026", "09", "18", sep="-")

Output

2026-09-18

The three arguments are joined with - between them.
Answer: 2026-09-18

Q6The pi line

Multiple Choice Question (MCQ): choose the one correct option.

What does the program print?

Python

x = 22/7
print(f"Value of pi: {x:.3f}")

3.141

3.143

3.142

3.14

Solution

Idea

22/7 = 3.142857...; round to 3 decimals.

Python

x = 22/7
print(f"Value of pi: {x:.3f}")

Output

Value of pi: 3.143

The fourth decimal digit is 8, so the third decimal rounds up from 2 to 3: 3.143.
Answer: option 2.

Revision overview

Three tools'%d' % v
'{0}'.format(v)
f'{v}'

Decimals{x:.2f}: 2 decimals,
rounded

Positionsformat() counts from 0

Formatted printingWeek 3 · Part 5

endwhat print ends with
default newline

sepbetween arguments
default space

Source fixpi to 3 places is 3.143

Revision: showing values neatly with print

Next · Controlbreak, continue and pass

Control

break, continue and pass

Three statements that change how a loop runs from the inside.

These three statements change how a loop runs from the inside.

Figure 8. The same loop over 1 to 5 with break, continue and pass at 3

6.1break: leave the loop

break stops the current loop entirely; execution continues after the loop.

Python

x = "Hello @ World"
for i in x:
    if i == "@":
        break
    print(i, end="")

Output

Hello

Python

i = 1
while i <= 5:
    if i == 3:
        break
    print(i)
    i += 1

Output

1
2

The first program prints Hello (with the trailing space) and stops at @. The second prints 1 and 2; at i = 3 the loop ends.

6.2continue: skip to the next round

continue ends only the current round and jumps straight to the next one.

Python

txt = "hello"
for x in txt:
    if x == "l":
        continue
    print(x)

Output

h
e
o

Extra: a trap with while. In a while loop, put the counter update before continue, or the loop can stick forever on the same value.

Python

i = 0
while i < 5:
    i += 1
    if i == 3:
        continue
    print(i)

Output

6.3pass: do nothing

pass is a placeholder. Python needs at least one statement in a block; pass fills the gap and does nothing when executed.

Python

if 5 == 5:
    pass
else:
    pass
print("Hello World")

Output

Hello World

Statement	Effect	Code after it in the round
`break`	leaves the whole loop	skipped
`continue`	jumps to the next round	skipped
`pass`	nothing	runs normally

6.4Going deeper

break and else (extra). A loop's else runs only if the loop was not left by break. That makes it a handy "not found" branch.
Nested loops (extra). break and continue affect only the loop they are directly inside.
Why pass (extra). It lets you write the shape of a program first and fill the blocks in later.

Python

for n in range(2, 7):
    for d in range(2, n):
        if n % d == 0:
            break
    else:
        print(n, "is prime")

Output

2 is prime
3 is prime
5 is prime

6.5Exam tips and common mistakes

Exam tips

Mark the round where the condition first becomes true, then decide whether the later rounds exist (continue) or not (break).
Watch end="" in output questions: the printed text is on one line.

Common mistakes

Mixing up break (stop everything) and continue (skip one round).
Using continue before the counter update in a while loop.
Thinking pass skips the rest of the loop body.

Practice questions (graded-assignment style)

Q1break

Multiple Choice Question (MCQ): choose the one correct option.

What does the program print?

Python

for i in range(1, 6):
    if i == 3:
        break
    print(i, end=" ")

1 2

1 2 3

1 2 4 5

3

Solution

Idea

break ends the loop when i == 3.

Python

for i in range(1, 6):
    if i == 3:
        break
    print(i, end=" ")

Output

1 2

The loop prints 1 and 2, and leaves at 3 before printing it.
Answer: option 1.

Q2continue

Multiple Choice Question (MCQ): choose the one correct option.

What does the program print?

Python

for i in range(1, 6):
    if i == 3:
        continue
    print(i, end=" ")

1 2 4 5

1 2

1 2 3 4 5

3

Solution

Idea

continue skips only the round for 3.

Python

for i in range(1, 6):
    if i == 3:
        continue
    print(i, end=" ")

Output

1 2 4 5

When i == 3 the print is skipped, but rounds 4 and 5 still run.
Answer: option 1.

Q3First multiple of 7

Numerical Answer Type (NAT).

What does the program print?

Python

for i in range(1, 101):
    if i % 7 == 0:
        print(i)
        break

Solution

Idea

The loop stops at the first multiple of 7.

Python

for i in range(1, 101):
    if i % 7 == 0:
        print(i)
        break

Output

range(1, 101) reaches 7 first, prints it and breaks.
Answer: 7

Q4Count with continue

Numerical Answer Type (NAT).

What does the program print?

Python

count = 0
for i in range(1, 11):
    if i % 3 == 0:
        continue
    count += 1
print(count)

Solution

Idea

Count the numbers from 1 to 10 that are not multiples of 3.

Python

count = 0
for i in range(1, 11):
    if i % 3 == 0:
        continue
    count += 1
print(count)

Output

The multiples of 3 are 3, 6, 9, so 10 - 3 = 7 numbers are counted.
Answer: 7

Q5pass

Multiple Choice Question (MCQ): choose the one correct option.

What does the program print?

Python

for i in range(3):
    pass

0 1 2 on three lines

Nothing

pass

An error

Solution

Idea

pass does nothing.

Python

for i in range(3):
    pass

Output

The loop runs three times but each round only executes pass, so nothing is printed.
Answer: option 2.

Q6break versus continue

Multiple Select Question (MSQ): select all that apply.

Choose the correct statements.

break ends the whole loop

continue ends the whole loop

continue skips the rest of the current round

pass skips the rest of the current round

Solution

Idea

Recall the table.

break leaves the loop; continue skips to the next round.
pass does nothing and the rest of the round still runs.
Answer: options 1 and 3.

Revision overview

breakStops the loop
entirely

continueSkips to the
next round

passPlaceholder
does nothing

break, continue, passWeek 3 · Part 6

elseSkipped after break

NestedAffects the inner loop only

Counterupdate before continue

Revision: steering a loop from inside

Next · PracticePractice Zone

Practice

Practice Zone

The source's ten tasks with worked, executed solutions.

The ten practice tasks from the source, each with a worked solution that was actually run. Try each one yourself first.

Example 1Q1 · while loop

Print all even numbers from 1 to 20 using a while loop.

Solution

Idea: Start at 2 and step by 2.

Python

i = 2
while i <= 20:
    print(i, end=" ")
    i += 2

Output

2 4 6 8 10 12 14 16 18 20

Example 2Q2 · while ... else

Print the numbers 1 to 5, then print Done once the loop ends.

Solution

Idea: else runs when the test turns false.

Python

i = 1
while i <= 5:
    print(i, end=" ")
    i += 1
else:
    print("Done")

Output

1 2 3 4 5 Done

Example 3Q3 · for loop

Loop through the string "EngiKode" and print each character.

Solution

Idea: A string is iterable.

Python

for ch in "EngiKode":
    print(ch, end=" ")

Output

E n g i K o d e

Example 4Q4 · range()

Print every 3rd number from 0 to 30 using range().

Solution

Idea: The stop value is 31 so 30 is included.

Python

for x in range(0, 31, 3):
    print(x, end=" ")

Output

0 3 6 9 12 15 18 21 24 27 30

Example 5Q5 · nested loops

Print a 5 × 5 grid of stars using nested for loops.

Solution

Idea: The inner loop prints a row; the empty print() ends it.

Python

for i in range(5):
    for j in range(5):
        print("*", end="")
    print()

Output

*****
*****
*****
*****
*****

Example 6Q6 · formatted printing

Print 3 x 4 = 12 using all three formatting methods.

Solution

Idea: All three give the same text.

Python

a, b = 3, 4
print('%d x %d = %d' % (a, b, a*b))
print('{0} x {1} = {2}'.format(a, b, a*b))
print(f"{a} x {b} = {a*b}")

Output

3 x 4 = 12
3 x 4 = 12
3 x 4 = 12

Example 7Q7 · end / sep

Print 2026-09-18 from three separate values using sep.

Solution

Idea: sep joins the values with a dash.

Python

print('2026', '09', '18', sep='-')

Output

2026-09-18

Example 8Q8 · break

Loop through 1 to 100 and stop as soon as you hit a number divisible by 7.

Solution

Idea: break after the first hit.

Python

for i in range(1, 101):
    if i % 7 == 0:
        print(i)
        break

Output

Example 9Q9 · continue

Print the numbers 1 to 10, skipping multiples of 3.

Solution

Idea: continue jumps over the print for 3, 6, 9.

Python

for i in range(1, 11):
    if i % 3 == 0:
        continue
    print(i, end=" ")

Output

1 2 4 5 7 8 10

Example 10Q10 · combined

Write a program that finds all prime numbers between 1 and 50.

Solution

Idea: Test each number against the smaller ones; break at the first divisor.

Python

for num in range(2, 51):
    flag = True
    for i in range(2, num):
        if num % i == 0:
            flag = False
            break
    if flag:
        print(num, end=" ")

Output

2 3 5 7 11 13 17 19 23 29 31 37 41 43 47

Note on the source. The source lists these tasks and the solution code without printed output. The outputs shown here come from running the code. The solutions above use end=" " to keep numbers on one line; printing one per line works equally well.

7.1Exam tips and common mistakes

Exam tips

Say aloud what the loop variable is in each round before writing the body.
Test with a small input and compare with a hand count.

Common mistakes

Starting range at 0 when the task says 1.
Using range(30) when 30 itself must be included.
Forgetting the empty print() that ends a pattern row.

Practice questions (graded-assignment style)

Q1Even numbers

Multiple Choice Question (MCQ): choose the one correct option.

How many numbers does the Q1 program print?

Python

i = 2
count = 0
while i <= 20:
    count += 1
    i += 2
print(count)

Solution

Idea

List 2, 4, ..., 20.

Python

i = 2
count = 0
while i <= 20:
    count += 1
    i += 2
print(count)

Output

The even numbers from 2 to 20 are ten numbers.
Answer: option 2.

Q2Stars

Numerical Answer Type (NAT).

How many * characters does the grid program print in total?

Python

total = 0
for i in range(5):
    for j in range(5):
        total += 1
print(total)

Solution

Idea

5 rows of 5.

Python

total = 0
for i in range(5):
    for j in range(5):
        total += 1
print(total)

Output

5 outer rounds × 5 inner rounds = 25 stars.
Answer: 25

Q3Number of primes

Numerical Answer Type (NAT).

How many prime numbers are there between 1 and 50?

Python

count = 0
for num in range(2, 51):
    flag = True
    for i in range(2, num):
        if num % i == 0:
            flag = False
            break
    if flag:
        count += 1
print(count)

Solution

Idea

Count the output of Q10.

Python

count = 0
for num in range(2, 51):
    flag = True
    for i in range(2, num):
        if num % i == 0:
            flag = False
            break
    if flag:
        count += 1
print(count)

Output

The primes are 2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 37, 41, 43, 47: fifteen of them.
Answer: 15

Q4Skipping

Multiple Choice Question (MCQ): choose the one correct option.

What is the sum of the numbers the Q9 program prints?

Python

total = 0
for i in range(1, 11):
    if i % 3 == 0:
        continue
    total += i
print(total)

Solution

Idea

Add all of 1 to 10 and subtract the multiples of 3.

Python

total = 0
for i in range(1, 11):
    if i % 3 == 0:
        continue
    total += i
print(total)

Output

55 - (3 + 6 + 9) = 37.
Answer: option 2.

Revision overview

while taskseven numbers
while ... else

for tasksstring, range, stars
formatting, sep

Controlbreak, continue
primes to 50

Practice zoneWeek 3 · Part 7

MethodPredict, run, compare

Edgefirst and last value

Outputone line with end=" "

Revision: turning the Week 3 tools into small programs

Next · All partsWeek 3 Summary Sheet

All parts

Week 3 Summary Sheet

One page to revise all of Week 3: loops, formatting, loop control and the exam checklist.

The big picture

Part 1 · while

Repeats while the test is True; update the counter.
else runs when the loop ends normally.

Part 2 · for and range

for visits each item of a sequence.
range(start, stop, step); stop excluded.

Part 3 · for practice

Sum, table, prime, digit sum, factors.
Flag pattern: assume True, set False.

Part 4 · Nested loops

Inner loop runs fully per outer round.
Sentinel input loops; reset inner totals.

Part 5 · Formatting

%d %f, format(), f-strings, :.nf.
end and sep in print.

Part 6 · break, continue, pass

break stops the loop.
continue skips a round; pass does nothing.

Part 7 · Practice zone

Ten tasks with real outputs.
Primes up to 50: fifteen of them.

Revision sheet

Topic	Key point
while	runs while a condition is True
while...else	else runs once the loop ends normally
for	iterates over a sequence / iterable
range(n)	0 to n-1
`range(a, b, step)`	a to b-1, stepping by step
Reverse range	range(5, -1, -1) gives 5 down to 0
Nested loops	a loop placed inside another loop
`%d %f`	modulo-style format specifiers
`.format()`	{0} {1} {2} positional placeholders
f-strings	variables directly inside {}
`{x:.2f}`	decimal-place format specifier
`end=''`	changes what print() ends a line with
`sep=''`	changes the separator between print() arguments
`break`	stops the loop entirely
continue	skips to the next iteration
pass	does nothing; syntactic placeholder

Which loop when?

Situation	Use
repeat a fixed number of times	`for i in range(n)`
go through every item or character	`for x in sequence`
repeat until something changes or input ends	`while`
stop at the first match	`break`
ignore some items	`continue`
leave a block empty for now	`pass`

Exam checklist (from the source)

while loop syntax and counter increments.
while...else.
for loop over lists and strings.
range() with 1, 2 and 3 arguments.
Reverse iteration with range().
Nested loops.
Factorial, digit count, reverse, palindrome patterns.
Prime-check logic (single and nested-loop versions).
% modulo formatting.
.format() with positional placeholders.
f-strings and decimal-place specifiers.
end and sep parameters.
break versus continue.
pass as a placeholder.

Common mistakes

No counter update

A while without an update runs forever.

range off by one

The stop value is never included.

Inner totals not reset

Set running totals to 0 inside the outer loop.

break vs continue

break leaves the loop; continue skips one round.

f-string without f

Without the f the braces print literally.

Trusting printed pi

22/7 to 3 places is 3.143, not 3.141.

Python,Week 3

Chapters

The while Loop

1.1The while loop

1.2while ... else

1.3Practice: factorial

1.4Practice: count the digits

1.5Practice: reverse the digits

1.6Practice: palindrome check

1.7Going deeper

1.8Loop patterns you will use again and again

A. Accumulator, counter, flag, running maximum

B. Euclid's gcd and the Fibonacci numbers with a while loop

C. A loop that stops on a signal (sentinel) and the loop's else

1.9Exam tips and common mistakes

Practice questions (graded-assignment style)

The for Loop and range()

2.1The for loop

2.2Looping through a list and a string

2.3The range() function

2.4Going deeper

2.5Exam tips and common mistakes

Practice questions (graded-assignment style)

for-Loop Practice

3.1Sum of the first n numbers

3.2A multiplication table

3.3Is the number prime?

3.4Sum of all digits

3.5Numbers below x divisible by 3 or 5

3.6All factors of a number

3.7Going deeper

3.8Exam tips and common mistakes

Practice questions (graded-assignment style)

Nested Loops and Practice

4.1Nested loops

4.2Problem 1: all primes below a number

4.3Problem 2: profit or loss per trader

4.4Problem 3: day-wise total rainfall

4.5Problem 4: the longest word

4.6Going deeper

4.7Exam tips and common mistakes

Practice questions (graded-assignment style)

Formatted Printing, end and sep

5.1Why format?

5.2The string modulo operator (%)

5.3The format() method

5.4f-strings

5.5Decimal places

5.6print() parameters: end and sep

5.7Going deeper

5.8Exam tips and common mistakes

Practice questions (graded-assignment style)

break, continue and pass

6.1break: leave the loop

6.2continue: skip to the next round

6.3pass: do nothing

6.4Going deeper

6.5Exam tips and common mistakes

Practice questions (graded-assignment style)

Practice Zone

7.1Exam tips and common mistakes

Practice questions (graded-assignment style)

Week 3 Summary Sheet

The big picture

Revision sheet

Which loop when?

Exam checklist (from the source)

Common mistakes

Python,
Week 3