Nested while loops in Python - Mini Project: Build & Apply

Nested while loops

📖 Scenario: You are helping a small bakery organize their daily baking schedule. They bake different types of bread in batches. Each batch has a fixed number of loaves. You want to print out the baking plan showing each batch and the loaves inside it.

🎯 Goal: Build a program that uses nested while loops to print the batch number and loaf number for each loaf baked.

📋 What You'll Learn

Create a variable batches with the value 3 representing the number of batches.

Create a variable loaves_per_batch with the value 4 representing loaves in each batch.

Use a nested while loop: the outer loop counts batches, the inner loop counts loaves.

Print the batch number and loaf number in the format: Batch 1, Loaf 1.

💡 Why This Matters

🌍 Real World

Bakers and factory workers often need to organize tasks in batches and steps, so nested loops help automate schedules and reports.

💼 Career

Understanding nested loops is essential for programming tasks that involve multi-level data or repeated actions inside other repeated actions, common in software development and automation.

1

Set up batch and loaf counts

Create a variable called batches and set it to 3. Create another variable called loaves_per_batch and set it to 4.

Python

# Create variables batches and loaves_per_batch
# Your code here

Hint

Use simple assignment like batches = 3 and loaves_per_batch = 4.

2

Initialize batch counter

Create a variable called batch and set it to 1 to start counting batches.

Python

batches = 3
loaves_per_batch = 4
# Initialize batch counter
# Your code here

Hint

Start counting batches from 1 using batch = 1.

3

Write nested while loops for batches and loaves

Write a while loop that runs while batch <= batches. Inside it, create a variable loaf set to 1. Then write another while loop inside that runs while loaf <= loaves_per_batch. Inside the inner loop, print Batch {batch}, Loaf {loaf} using an f-string, then increase loaf by 1. After the inner loop ends, increase batch by 1.

Python

batches = 3
loaves_per_batch = 4
batch = 1
# Write nested while loops to print batches and loaves
# Your code here

Hint

Remember to reset loaf to 1 inside the outer loop before the inner loop starts.

4

Run the program to see the baking plan

Run the program to print the baking plan showing each batch and loaf number as Batch 1, Loaf 1 up to Batch 3, Loaf 4.

Python

batches = 3
loaves_per_batch = 4
batch = 1
while batch <= batches:
    loaf = 1
    while loaf <= loaves_per_batch:
        print(f"Batch {batch}, Loaf {loaf}")
        loaf += 1
    batch += 1
# Run the program to see the output

Hint

Check that the output lists all batches and loaves correctly.

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