What is Iteration?

When we talk about iteration in Python, we're talking about repeating actions repeatedly. Let's explore two main ways to do this: `for` loops and `while` loops.

Baisc Loop Types

Loop Type Description Example
For Loop A standard loop used to iterate over a sequence (like a list, tuple, or range) or numbers.
Important Note: The range starts at 0 and stops at 5 (does not include 5).		 
for i in range(5):
    print(i)
Enhanced For Loop A type of for loop used to simplify iterating over elements in a list 

fruits = ["apple", "banana", "cherry"]
for fruit in fruits:
    print(fruit)
While Loop A loop that runs as long as a specified condition is true. 
count = 0
while count < 5:
    print(count)
    count += 1

When to Use a For Loop vs. While Loop

  • Use a for loop when:

    • You know how many times you need to repeat an action.

    • You want to iterate over a list (Ex. fruits = [“apple“, “banana”, “cherry“]

  • Use a while loop when:

    • The number of iterations is unknown and depends on a condition

    • Examples:

      • Continuously asking the user for input until they provide a valid answer.

Common Errors in Loops

Below are a couple mistakes you might encounter while working with for and while loops, along with code examples to illustrate them.

Infinite Loop in a While Loop

One of the most common errors when using a while loop is creating an infinite loop. This happens when the condition of the loop never becomes false, causing the loop to run indefinitely.

# Infinite loop because the condition is always true
count = 0
while count < 5:
    print(count)
    # Forgot to update the count variable, so the condition is never false

Mini Challenge

Download the starter file here.

Write a program that:

  1. Uses a for loop to iterate through the numbers 1 to 10 and prints whether each number is even or odd.

  2. Uses an enhanced for loop to iterate through a list of names and prints each name

  3. Uses a while loop to continuously ask the user to guess a secret number (between 1 and 5). If the user guesses incorrectly, the loop should continue until the user guesses the correct number.

Off-by-One Error in For Loop

Another common mistake with for loops is the "off-by-one" error, where the loop runs one extra or one fewer time than expected. This typically happens when you're working with ranges or lists and miscalculate the bounds.

# Off-by-one error because range(5) gives numbers from 1 to 4, but the user expects 1 to 5
for i in range(1, 5):
    print(i)