For loop execution model in Python - Mini Project: Build & Apply

Understanding For Loop Execution Model

📖 Scenario: Imagine you are organizing a small party and you have a list of guests. You want to greet each guest by name.

🎯 Goal: You will create a list of guest names, set up a greeting message, use a for loop to greet each guest, and finally print all greetings.

📋 What You'll Learn

Create a list called guests with the exact names: 'Alice', 'Bob', 'Charlie'

Create a variable called greeting with the exact value 'Hello'

Use a for loop with the variable guest to iterate over guests

Before the loop, create a list called messages that stores each greeting message combining greeting and guest

Print each greeting message stored in messages after the loop

💡 Why This Matters

🌍 Real World

Greeting guests or customers by name is common in apps, websites, and events to make people feel welcome.

💼 Career

Understanding how to use for loops to process lists is a fundamental skill for many programming jobs, including web development, data processing, and automation.

1

Create the guest list

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

Python

# Create the list guests with 'Alice', 'Bob', and 'Charlie'
guests = []  # Your code here

Hint

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

2

Set up the greeting message

Create a variable called greeting and set it to the string 'Hello'

Python

guests = ['Alice', 'Bob', 'Charlie']
# Create a variable greeting with the value 'Hello'
greeting = ''  # Your code here

Hint

Use quotes to create a string. For example: greeting = 'Hello'

3

Use a for loop to create greeting messages

Before the loop, create a list called messages. Use a for loop with the variable guest to iterate over guests. Inside the loop, append each greeting message combining greeting and guest separated by a space to messages.

Python

guests = ['Alice', 'Bob', 'Charlie']
greeting = 'Hello'
# Create an empty list messages before the loop
messages = []
# Use a for loop to add greeting messages to messages
for guest in guests:
    # Your code here

Hint

Use messages.append(f"{greeting} {guest}") inside the loop to add each message.

4

Print all greeting messages

Write a for loop with the variable message to iterate over messages and print each message.

Python

guests = ['Alice', 'Bob', 'Charlie']
greeting = 'Hello'
messages = []
for guest in guests:
    messages.append(f"{greeting} {guest}")

# Print each message in messages
for message in messages:
    # Your code here

Hint

Use print(message) inside the loop to show each greeting.

Practice

1. What does a for loop do in Python?

easy

A. Repeats a block of code for each item in a sequence

B. Runs code only once

C. Stops the program immediately

D. Creates a new variable

Solution

1. Step 1: Understand the purpose of a for loop

   A for loop runs code repeatedly for each item in a list, string, or range. 'Repeats a block of code for each item in a sequence' matches this; others do not.

2. Final Answer:

   Repeats a block of code for each item in a sequence -> Option A

3. Quick Check:

   For loop = repeat for each item [OK]

Hint: For loops repeat actions for every item in a list [OK]

Common Mistakes:

• Thinking for loops run only once
• Confusing for loops with if statements
• Believing for loops stop the program

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

easy

A. for (i=0; i<5; i++) print(i)

B. for i to range(5) print(i)

C. for i in range(5): print(i)

D. foreach i in range(5): print(i)

Solution

1. Step 1: Recall Python for loop syntax

   Python uses 'for variable in sequence:' followed by indented code. 'for i in range(5): print(i)' matches; others use wrong keywords or styles.

2. Final Answer:

   for i in range(5): print(i) -> Option C

3. Quick Check:

   Python for loop = for variable in sequence: [OK]

Hint: Use 'for variable in sequence:' syntax in Python [OK]

Common Mistakes:

• Using 'to' instead of 'in'
• Using C-style for loop syntax
• Using 'foreach' which is not Python

3. What is the output of this code?

for i in range(3):
    print(i * 2)

medium

A. 2 4 6

B. 0 1 2

C. 1 2 3

D. 0 2 4

Solution

1. Step 1: Trace the loop range and output

   range(3) gives i=0,1,2. Prints i*2: 0, 2, 4.

2. Final Answer:

   0 2 4 -> Option D

3. Quick Check:

   Multiply each i by 2 = 0 2 4 [OK]

Hint: Multiply loop variable by 2 for each iteration [OK]

Common Mistakes:

• Printing i instead of i*2
• Starting count from 1 instead of 0
• Confusing range(3) with range(1,3)

4. Find the error in this code:

for i in range(5)
    print(i)

medium

A. Indentation error on print line

B. Missing colon ':' after range(5)

C. range(5) is invalid syntax

D. Variable i is not defined

Solution

1. Step 1: Check for syntax errors in for loop

   Python requires ':' after the for statement. The code misses ':' after range(5).

2. Final Answer:

   Missing colon ':' after range(5) -> Option B

3. Quick Check:

   For loop needs ':' after header [OK]

Hint: Always add ':' after for loop header [OK]

Common Mistakes:

• Forgetting colon ':' after for statement
• Incorrect indentation
• Assuming range(5) is invalid

5. You want to create a list of squares for numbers 0 to 4 using a for loop. Which code correctly does this?

hard

A. squares = []
for i in range(5):
squares.append(i*i)
print(squares)

B. squares = [i*i for i in range(5)]
print(squares)

C. squares = []
for i in range(5):
squares = i*i
print(squares)

D. for i in range(5):
squares.append(i*i)
print(squares)

Solution

1. Step 1: Identify correct code for list of squares using for loop

   Initialize squares = [], then for i in range(5): append i*i, then print. squares = [] for i in range(5): squares.append(i*i) print(squares) works; others miss init, use comp, or overwrite.

2. Final Answer:

   squares = []\nfor i in range(5):\n squares.append(i*i)\nprint(squares) -> Option A

3. Quick Check:

   Initialize list, append squares in loop [OK]

Hint: Initialize list before loop, append inside loop [OK]

Common Mistakes:

• Not initializing list before appending
• Overwriting list variable inside loop
• Using list comprehension but question asks for for loop