Nested while loops in Python - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of nested while loops with counters

What is the output of this Python code using nested while loops?

Python

i = 1
result = []
while i <= 2:
    j = 1
    while j <= 3:
        result.append(f"{i}-{j}")
        j += 1
    i += 1
print(result)

A ['1-1', '1-2', '1-3', '2-1', '2-2', '2-3']

B ['1-1', '2-1', '3-1', '1-2', '2-2', '3-2']

C ['1-1', '1-2', '2-1', '2-2', '3-1', '3-2']

D ['1-1', '1-2', '1-3', '2-1', '3-2', '2-3']

💡 Hint

Think about how the inner loop runs completely for each iteration of the outer loop.

The outer loop runs for i=1 and i=2. For each i, the inner loop runs j=1 to 3, appending strings like 'i-j'. So the list has 6 elements in order.

❓ Predict Output

intermediate

Counting iterations in nested while loops

What is the value of variable count after running this code?

Python

count = 0
x = 0
while x < 3:
    y = 0
    while y < 2:
        count += 1
        y += 1
    x += 1
print(count)

A 2

B 5

C 3

D 6

💡 Hint

Multiply the number of outer loop runs by inner loop runs.

The outer loop runs 3 times (x=0,1,2). The inner loop runs 2 times each outer iteration. So total count increments: 3*2=6.

❓ Predict Output

advanced

Output of nested while loops with break

What is the output of this code snippet?

Python

i = 1
result = []
while i <= 3:
    j = 1
    while j <= 3:
        if i == j:
            break
        result.append((i, j))
        j += 1
    i += 1
print(result)

A [(2, 1), (3, 1), (3, 2)]

B [(1, 1), (1, 2), (2, 1), (2, 2), (3, 1), (3, 2)]

C [(1, 1), (2, 1), (2, 2), (3, 1), (3, 2), (3, 3)]

D [(1, 2), (2, 1), (3, 1), (3, 2)]

💡 Hint

The inner loop breaks when i equals j, so some pairs are skipped.

For i=1, j=1 triggers break immediately, so no pairs added. For i=2, j=1 added, j=2 breaks. For i=3, j=1 and j=2 added, j=3 breaks. So result is [(2,1),(3,1),(3,2)].

❓ Predict Output

advanced

Final value of variable after nested while loops with continue

What is the final value of total after running this code?

Python

total = 0
x = 0
while x < 3:
    y = 0
    while y < 3:
        y += 1
        if y == 2:
            continue
        total += y
    x += 1
print(total)

A 9

B 15

C 12

D 18

💡 Hint

Remember that continue skips the rest of the current inner loop iteration.

The outer loop runs 3 times (x=0,1,2). For each, inner loop: - y=0<3: y=1, !=2, total+=1 - y=1<3: y=2, ==2 continue (skip +=2) - y=2<3: y=3, !=2, total+=3 Per outer: +4. Total: 12.

🧠 Conceptual

expert

Number of iterations in nested while loops with complex conditions

Consider this nested while loop code. How many times does the innermost statement count += 1 execute?

Python

count = 0
x = 1
while x < 5:
    y = x
    while y > 0:
        count += 1
        y -= 2
    x += 1
print(count)

A 8

B 6

C 10

D 9

💡 Hint

Count how many times the inner loop runs for each x, then sum.

x=1: y=1>0 count+=1 y=-1 stop (+1) x=2: y=2>0 count+=1 y=0 stop (+1) x=3: y=3>0 count+=1 y=1>0 count+=1 y=-1 (+2) x=4: y=4>0 count+=1 y=2>0 count+=1 y=0 (+2) Total: 6.