The Python for Loop

The for loop in Python is a fundamental control structure used for definite iteration. This means you use it when you want to repeat a block of code a specific number of times, typically by iterating over a sequence like a list, tuple, string, or a range of numbers.

Think of it as a way to say, “For each item in this collection, do this action.”

Basic Syntax

The syntax of a for loop is clean and readable:

for variable_name in iterable:
    # Code block to be executed for each item
    # This block is indented

• for: The keyword that starts the loop.
• variable_name: A temporary variable that holds the current item from the iterable in each iteration. You can name it anything you like (e.g., item, num, char).
• in: The keyword that separates the variable from the iterable.
• iterable: An object that can be looped over, such as a list, string, tuple, dictionary, or a range object.
• :: The colon that marks the end of the loop declaration.
• Indented Block: The code to be executed in each iteration. Indentation (usually 4 spaces) is how Python knows this code belongs to the loop.

Iterating Over Different Sequences

The for loop is incredibly versatile. Let’s see how it works with different types of data.

fruits = ["Apple", "Banana", "Cherry"]
 
print("Our fruit basket contains:")
for fruit in fruits:
    print(f"- {fruit}")

Our fruit basket contains:
- Apple
- Banana
- Cherry

word = "PYTHON"
 
for character in word:
    print(character, end=' ') # The 'end' parameter prints spaces instead of newlines

P Y T H O N 

coordinates = (10, 20, 30)
 
for coord in coordinates:
    print(f"Coordinate value: {coord}")

Coordinate value: 10
Coordinate value: 20
Coordinate value: 30

The range() Function: Your Best Friend for Loops

Often, you don’t have a sequence to loop over, but you want to repeat an action a certain number of times. The range() function is perfect for this. It generates a sequence of numbers on the fly.

Syntax of range()

The range() function can take one, two, or three arguments:

1. range(stop): Generates numbers from 0 up to (but not including) stop.
2. range(start, stop): Generates numbers from start up to (but not including) stop.
3. range(start, stop, step): Generates numbers from start up to stop, incrementing by step.

Examples with range()

Example 1: Simple Repetition

To run a block of code 5 times, you use range(5).

Repetition #1: Hello, World!
Repetition #2: Hello, World!
Repetition #3: Hello, World!
Repetition #4: Hello, World!
Repetition #5: Hello, World!

Example 2: Custom Start and Stop

To generate numbers from 1 to 10 (inclusive), you need range(1, 11).

1
2
3
4
5
6
7
8
9
10

Example 3: Using a Step

To print only the even numbers from 2 to 10, you can use a step of 2.

2
4
6
8
10

Example 4: Counting Downwards

To count backward, you need a negative step.

5
4
3
2
1
Liftoff!

Nested for Loops

You can put one for loop inside another. This is called a nested loop. It’s useful for working with 2D data structures (like a grid or matrix) or for creating combinations.

The inner loop will complete all its iterations for each single iteration of the outer loop.

Example: Printing a Multiplication Table

1 * 1 = 1
1 * 2 = 2
1 * 3 = 3
---
2 * 1 = 2
2 * 2 = 4
2 * 3 = 6
---
3 * 1 = 3
3 * 2 = 6
3 * 3 = 9
---

The for-else Clause

This is a lesser-known but useful feature of Python loops. The else block in a for loop is executed only if the loop completes all its iterations without being terminated by a break statement.

It’s useful for code that needs to run only when a search or process finishes “naturally.”

Example: Searching for an Item

5 was not found in the list.

Now, let’s try a case where the number is found:

Found it! The number is 4.

Practice Problems

Problem 1: Sum of Numbers

Write a program to calculate the sum of all numbers from 1 to 100.

The sum of numbers from 1 to 100 is: 5050

Problem 2: Count Vowels in a String

Write a program that counts the number of vowels (a, e, i, o, u) in a given string.

The number of vowels is: 3