Nested for loop execution in Python - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of nested loops with range

What is the output of this Python code?

Python

result = []
for i in range(2):
    for j in range(3):
        result.append((i, j))
print(result)

A [(0, 0), (0, 1), (1, 0), (1, 1), (2, 0), (2, 1)]

B [(0, 0), (1, 0), (0, 1), (1, 1), (0, 2), (1, 2)]

C [(0, 0), (0, 1), (0, 2), (1, 0), (1, 1), (1, 2)]

D [(0, 0), (1, 1), (2, 2), (3, 3)]

💡 Hint

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

The outer loop runs for i=0 and i=1. For each i, the inner loop runs j=0,1,2. So pairs are (0,0),(0,1),(0,2),(1,0),(1,1),(1,2).

❓ Predict Output

intermediate

Sum of products in nested loops

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

Python

total = 0
for x in range(1, 4):
    for y in range(1, 3):
        total += x * y
print(total)

A 18

B 20

C 24

D 12

💡 Hint

Calculate the sum of x*y for x=1 to 3 and y=1 to 2.

The pairs are (1,1),(1,2),(2,1),(2,2),(3,1),(3,2). Their products sum to 1+2+2+4+3+6=18.

🔧 Debug

advanced

Identify the error in nested loops

What error does this code raise when run?

Python

for i in range(3):
    for j in range(2):
        print(i, j)

A SyntaxError

B TypeError

C IndentationError

D No error, prints pairs

💡 Hint

Check the syntax of the for loops carefully.

The inner for loop is missing a colon at the end, causing a SyntaxError.

❓ Predict Output

advanced

Output of nested loops with break

What is the output of this code?

Python

result = []
for a in range(3):
    for b in range(3):
        if b == 1:
            break
        result.append((a, b))
print(result)

A [(0, 0), (1, 0), (1, 1), (2, 0), (2, 1)]

B [(0, 0), (1, 0), (2, 0)]

C [(0, 0), (0, 1), (1, 0), (2, 0)]

D [(0, 0), (0, 1), (1, 0), (1, 1), (2, 0), (2, 1)]

💡 Hint

The break stops the inner loop when b equals 1.

For each a, b starts at 0. When b == 1, the inner loop breaks, so only (a,0) is appended for each a.

🧠 Conceptual

expert

Number of iterations in nested loops

How many times will the innermost statement print(i, j, k) execute in this code?

Python

for i in range(2):
    for j in range(3):
        for k in range(4):
            print(i, j, k)

A 6

B 9

C 12

D 24

💡 Hint

Multiply the lengths of all ranges to find total iterations.

The loops run 2 * 3 * 4 = 24 times, so print runs 24 times.

Practice

1. What does a nested for loop do in Python?
for i in range(2):
for j in range(3):
print(i, j)

easy

A. Runs the inner loop fully for each outer loop step

B. Runs both loops only once

C. Runs the outer loop fully for each inner loop step

D. Runs only the outer loop

Solution

1. Step 1: Understand nested loop structure

   The outer loop runs 2 times (i=0,1). For each i, the inner loop runs 3 times (j=0,1,2).

2. Step 2: Observe loop execution order

   For each i, the inner loop completes all its iterations before the outer loop moves to next i.

3. Final Answer:

   Runs the inner loop fully for each outer loop step -> Option A

4. Quick Check:

   Inner loop runs fully per outer loop [OK]

Hint: Inner loop runs completely inside each outer loop step [OK]

Common Mistakes:

• Thinking outer loop runs inside inner loop
• Assuming loops run only once
• Confusing loop order

2. Which of the following is the correct syntax for a nested for loop in Python?

easy

A. for i in range(3): for j in range(2): print(i, j)

B. for i in range(3):
for j in range(2):
print(i, j)

C. for i in range(3):
for j in range(2):
print(i, j)

D. for i in range(3):
for j in range(2):
print(i, j)

Solution

1. Step 1: Check indentation rules

   Python requires consistent indentation to define nested blocks. Inner loop must be indented inside outer loop.

2. Step 2: Identify correct indentation

   for i in range(3): for j in range(2): print(i, j) uses 4 spaces indentation for inner loop and print statement, which is correct.

3. Final Answer:

   for i in range(3):\n for j in range(2):\n print(i, j) -> Option B

4. Quick Check:

   Proper indentation = for i in range(3): for j in range(2): print(i, j) [OK]

Hint: Indent inner loop inside outer loop with spaces [OK]

Common Mistakes:

• Missing indentation for inner loop
• Incorrect indentation for print
• Writing loops on same line without colon

3. What is the output of this code?

for i in range(2):
    for j in range(2):
        print(i + j, end=' ')
    print()

medium

A. 0 1 \n0 1

B. 1 2 \n2 3

C. 0 1 1 2

D. 0 1 \n1 2

Solution

1. Step 1: Calculate inner loop sums for i=0

   j=0: 0+0=0, j=1: 0+1=1 -> prints '0 1 '

2. Step 2: Calculate inner loop sums for i=1

   j=0: 1+0=1, j=1: 1+1=2 -> prints '1 2 '

3. Final Answer:

   0 1 \n1 2 -> Option D

4. Quick Check:

   Sum of i and j per line = 0 1 \n1 2 [OK]

Hint: Add i and j for each inner loop iteration [OK]

Common Mistakes:

• Ignoring print() creates new line
• Adding values incorrectly
• Confusing end=' ' usage

4. Find the error in this nested loop code:

for i in range(3):
for j in range(2):
    print(i, j)

medium

A. print statement should be outside loops

B. Missing colon after inner for loop

C. Inner loop not indented inside outer loop

D. range(2) should be range(3)

Solution

1. Step 1: Check indentation of inner loop

   Inner for loop must be indented inside outer loop to be nested properly.

2. Step 2: Identify correct indentation

   Here, inner loop is at same level as outer loop, causing IndentationError.

3. Final Answer:

   Inner loop not indented inside outer loop -> Option C

4. Quick Check:

   Indent inner loop inside outer loop [OK]

Hint: Indent inner loop under outer loop to fix error [OK]

Common Mistakes:

• Not indenting inner loop
• Misplacing colons
• Wrong range values

5. How many times will the print statement execute in this nested loop?

for i in range(4):
    for j in range(3):
        for k in range(2):
            print(i, j, k)

hard

A. 24

B. 12

C. 36

D. 9

Solution

1. Step 1: Calculate iterations of each loop

   Outer loop: 4 times (i=0..3), middle loop: 3 times (j=0..2), inner loop: 2 times (k=0..1).

2. Step 2: Multiply iterations for total prints

   Total = 4 * 3 * 2 = 24 print executions.

3. Step 3: Re-check options

   36 says 36, but calculation is 24, so correct is 24.

4. Final Answer:

   24 -> Option A

5. Quick Check:

   4*3*2 = 24 prints [OK]

Hint: Multiply all loop ranges for total executions [OK]

Common Mistakes:

• Adding instead of multiplying loop counts
• Counting only outer loops
• Misreading loop ranges