Nested while loops in Python - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a nested while loop?

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

beginner

How does the flow of execution work in nested while loops?

The outer loop runs once, then the inner loop runs fully. After the inner loop finishes, the outer loop runs its next iteration, and the inner loop runs again, repeating this until the outer loop ends.

beginner

Write a simple example of nested while loops that prints a 3x3 grid of stars.

i = 1 while i <= 3: j = 1 while j <= 3: print('*', end=' ') j += 1 print() i += 1

intermediate

Why should you be careful with nested while loops?

Because if the inner or outer loop conditions never become false, the program can get stuck in an infinite loop, which means it keeps running forever.

beginner

How can nested while loops be useful in real life?

They help when you need to repeat tasks inside other repeated tasks, like printing tables, processing grids, or checking combinations of items.

What happens first in nested while loops?

A Both loops run at the same time

B The outer loop runs one iteration, then the inner loop runs fully

C The inner loop runs once, then the outer loop runs fully

D Only the outer loop runs

The outer loop starts and runs one iteration, then the inner loop runs completely before the outer loop continues.

What is a risk when using nested while loops?

A Infinite loops if conditions never become false

B The program runs too fast

C The loops skip iterations automatically

D The loops only run once

If the conditions controlling the loops never become false, the loops will run forever, causing an infinite loop.

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

A 3 times

B 4 times

C 12 times

D 7 times

The inner loop runs 3 times for each of the 4 outer loop iterations, so 4 x 3 = 12 times.

Which of these is a correct way to start a nested while loop?

A i = 0\nwhile i < 2:\n j = 0\n while j < 3:\n print(i, j)\n j += 1\n i += 1

B while i < 2:\n while j < 3:\n print(i, j)\n j += 1\n i += 1

C i = 0\nj = 0\nwhile i < 2 and j < 3:\n print(i, j)\n i += 1\n j += 1

D for i in range(2):\n while j < 3:\n print(i, j)\n j += 1

Option A correctly initializes counters and uses nested while loops with proper increments.

What will this code print? x = 1 while x <= 2: y = 1 while y <= 2: print(x, y) y += 1 x += 1

A 2 2 1 1 2 1 1 2

B 1 1 2 1 1 2 2 2

C 1 1 1 2 1 3 2 1

D 1 1 1 2 2 1 2 2

The outer loop runs for x=1 and x=2. For each x, the inner loop runs y=1 and y=2, printing pairs in order.

Explain how nested while loops work and give a simple example.

Think about loops inside loops and how they repeat.

Describe a real-life situation where nested while loops could be useful.

Imagine doing one task many times, and inside each, doing another task many times.

Practice

1. What does a nested while loop do in Python?
while condition1:
  while condition2:
    # code

easy

A. Runs the inner loop completely for each iteration of the outer loop

B. Runs both loops only once

C. Runs the outer loop only after the inner loop finishes all iterations

D. Runs the inner loop only if the outer loop condition is false

Solution

1. Step 1: Understand nested loops structure

   The outer loop runs while condition1 is true. For each iteration of this outer loop, the inner loop runs fully while condition2 is true.

2. Step 2: Explain the inner loop behavior

   The inner loop completes all its iterations before the outer loop moves to the next iteration.

3. Final Answer:

   Runs the inner loop completely for each iteration of the outer loop -> Option A

4. Quick Check:

   Inner loop runs fully inside outer loop [OK]

Hint: Inner loop finishes all cycles before outer loop continues [OK]

Common Mistakes:

• Thinking both loops run only once
• Confusing order of loop execution
• Assuming inner loop runs only if outer loop is false

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

easy

A. while x < 3:
while y < 2:
print(x, y)
y += 1

B. while x < 3
while y < 2:
print(x, y)
y += 1

C. while x < 3:
while y < 2:
print(x, y)
y += 1

D. while x < 3:
while y < 2:
print(x, y)
y += 1

Solution

1. Step 1: Check indentation and colons

   Python requires a colon after while conditions and proper indentation for nested blocks.

2. Step 2: Identify correct indentation and syntax

   while x < 3: while y < 2: print(x, y) y += 1 uses colons and indents the inner while and print statements correctly.

3. Final Answer:

   while x < 3: while y < 2: print(x, y) y += 1 -> Option D

4. Quick Check:

   Correct colons and indentation = while x < 3: while y < 2: print(x, y) y += 1 [OK]

Hint: Look for colons and consistent indentation [OK]

Common Mistakes:

• Missing colons after while statements
• Incorrect indentation of inner loop
• Mixing indentation levels

3. What is the output of this code?

i = 1
while i <= 2:
    j = 1
    while j <= 3:
        print(i, j)
        j += 1
    i += 1

medium

A. 1 1\n1 2\n2 1\n2 2

B. 1 1\n2 1\n3 1

C. 1 1\n1 2\n1 3\n2 1\n2 2\n2 3

D. 1 1\n1 2\n1 3

Solution

1. Step 1: Trace outer loop iterations

   i starts at 1 and runs while i <= 2, so i = 1 and i = 2.

2. Step 2: Trace inner loop iterations for each i

   For each i, j runs from 1 to 3, printing i and j each time.

3. Final Answer:

   1 1\n1 2\n1 3\n2 1\n2 2\n2 3 -> Option C

4. Quick Check:

   Inner loop prints 3 times per outer loop iteration [OK]

Hint: Inner loop runs fully for each outer loop iteration [OK]

Common Mistakes:

• Only printing inner loop once
• Mixing up i and j values
• Stopping loops too early

4. Find the error in this nested while loop code:

i = 0
while i < 2:
    j = 0
    while j < 2:
    print(i, j)
    j += 1
    i += 1

medium

A. No error, code runs fine

B. Inner loop print and increments are not indented properly

C. Variables i and j are not initialized

D. Outer loop condition is incorrect

Solution

1. Step 1: Check indentation inside inner while loop

   The print and j increment lines must be indented inside the inner while loop to run repeatedly.

2. Step 2: Identify effect of wrong indentation

   Without indentation, print and j += 1 run only once, causing an infinite loop or logic error.

3. Final Answer:

   Inner loop print and increments are not indented properly -> Option B

4. Quick Check:

   Indent inner loop body correctly [OK]

Hint: Indent inner loop body to avoid logic errors [OK]

Common Mistakes:

• Forgetting to indent inner loop code
• Incrementing outer loop variable inside inner loop
• Misplacing loop conditions

5. You want to print a 3x3 grid of numbers where each cell shows the sum of its row and column indexes (starting from 1). Which nested while loop code correctly does this?

hard

A. row = 1
while row <= 3:
col = 1
while col <= 3:
print(row + col, end=' ')
col += 1
print()
row += 1

B. row = 1
while row <= 3:
col = 1
while col <= 3:
print(row * col, end=' ')
col += 1
print()
row += 1

C. row = 0
while row < 3:
col = 0
while col < 3:
print(row + col, end=' ')
col += 1
print()
row += 1

D. row = 1
while row < 3:
col = 1
while col < 3:
print(row + col, end=' ')
col += 1
print()
row += 1

Solution

1. Step 1: Check loop ranges for 1 to 3

   We want rows and columns from 1 to 3 inclusive, so conditions should be <= 3 starting at 1.

2. Step 2: Verify sum calculation and printing format

   row = 1 while row <= 3: col = 1 while col <= 3: print(row + col, end=' ') col += 1 print() row += 1 sums row and col indexes and prints with space, then prints a newline after each row.

3. Final Answer:

   row = 1 while row <= 3: col = 1 while col <= 3: print(row + col, end=' ') col += 1 print() row += 1 -> Option A

4. Quick Check:

   Correct ranges and sum print = row = 1 while row <= 3: col = 1 while col <= 3: print(row + col, end=' ') col += 1 print() row += 1 [OK]

Hint: Use <= 3 and start from 1 for correct grid indexes [OK]

Common Mistakes:

• Starting loops at 0 instead of 1
• Using < 3 instead of <= 3 to miss last row/column
• Printing product instead of sum