Nested for loop execution in Python - Mini Project: Build & Apply

Nested for loop execution

📖 Scenario: You are organizing a small event and want to create a seating chart. You have a list of tables and a list of guests. You want to pair each guest with each table to see all possible seating options.

🎯 Goal: Build a program that uses nested for loops to print every combination of tables and guests.

📋 What You'll Learn

Create a list called tables with the values 'Table 1', 'Table 2', and 'Table 3'

Create a list called guests with the values 'Alice', 'Bob', and 'Charlie'

Use a nested for loop with variables table and guest to iterate over tables and guests

Print each combination in the format: Guest Alice sits at Table 1

💡 Why This Matters

🌍 Real World

Event planners often need to consider all seating options to organize guests efficiently.

💼 Career

Understanding nested loops is essential for tasks like scheduling, pairing data, and generating combinations in programming jobs.

1

Create the tables list

Create a list called tables with these exact values: 'Table 1', 'Table 2', and 'Table 3'

Python

# Create the list tables with 'Table 1', 'Table 2', and 'Table 3'
tables = []  # Your code here

Hint

Use square brackets [] to create a list and separate items with commas.

2

Create the guests list

Create a list called guests with these exact values: 'Alice', 'Bob', and 'Charlie'

Python

tables = ['Table 1', 'Table 2', 'Table 3']
# Create the list guests with 'Alice', 'Bob', and 'Charlie'
guests = []  # Your code here

Hint

Use the same list syntax as before to create the guests list.

3

Write the nested for loops

Use nested for loops with variables table and guest to iterate over tables and guests respectively

Python

tables = ['Table 1', 'Table 2', 'Table 3']
guests = ['Alice', 'Bob', 'Charlie']
# Use nested for loops to go through tables and guests
for table in tables:
    for guest in guests:
        pass  # Your code here

Hint

Write one for loop inside another. The inner loop goes through guests.

4

Print each seating combination

Inside the nested loops, write a print statement to display: Guest {guest} sits at {table} using an f-string

Python

tables = ['Table 1', 'Table 2', 'Table 3']
guests = ['Alice', 'Bob', 'Charlie']
for table in tables:
    for guest in guests:
        # Print the seating arrangement
        # Your code here

Hint

Use print(f"Guest {guest} sits at {table}") inside the 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