Nested for loop execution in Python - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a nested for loop?

A nested for loop is a loop inside another loop. The inner loop runs completely for each step of the outer loop.

beginner

How many times does the inner loop run if the outer loop runs 3 times and the inner loop runs 4 times?

The inner loop runs 4 times for each of the 3 outer loop steps, so 3 × 4 = 12 times in total.

beginner

What is the order of execution in nested for loops?

The outer loop runs one step, then the inner loop runs all its steps. This repeats until the outer loop finishes.

beginner

Write a simple nested for loop that prints pairs of numbers from 1 to 2.

for i in range(1, 3): for j in range(1, 3): print(f"({i}, {j})")

beginner

Why are nested loops useful?

Nested loops help when you want to work with pairs or groups of items, like rows and columns in a table.

How many times will the inner loop run in this code? for i in range(2): for j in range(3): print(i, j)

A 3

B 5

C 2

D 6

The outer loop runs 2 times, and for each time the inner loop runs 3 times, so 2 × 3 = 6 times.

In nested loops, which loop runs first?

A Outer loop

B Inner loop

C Both run at the same time

D Depends on the code

The outer loop runs first, then the inner loop runs completely for each step of the outer loop.

What will this code print? for i in range(1, 3): for j in range(1, 2): print(i, j)

A (1, 1) and (2, 1)

B (1, 1) only

C (2, 1) only

D No output

The outer loop runs for i=1 and i=2, inner loop runs once for j=1 each time, so prints (1, 1) and (2, 1).

What is the total number of iterations for nested loops with outer loop range 4 and inner loop range 5?

A 9

B 5

C 20

D 10

Total iterations = outer loop count × inner loop count = 4 × 5 = 20.

Which of these is a correct nested for loop syntax in Python?

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)

Python requires indentation for nested loops. Option B shows correct indentation.

Explain how nested for loops work and why they are useful.

Think about how you count rows and columns in a table.

Write a nested for loop that prints all pairs of numbers from 1 to 3.

Use range(1, 4) for both loops and print inside inner loop.

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