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, take this specific action.”

Basic Syntax

The syntax of a for loop is clean and reads like plain English:

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 (for example: item, num, char).
• in: The keyword that connects the variable to the collection.
• 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 us see how it works with different types of data:

The range() Function: Your Best Friend for Loops

Often, you do not 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 starting at 0 up to (but not including) stop.
2. range(start, stop): Generates numbers starting at start up to (but not including) stop.
3. range(start, stop, step): Generates numbers starting at start up to stop, incrementing by step each time.

Examples with range()

Example 1: Simple Repetition

To run a block of code exactly 5 times, you can use range(5):

Example 2: Custom Start and Stop

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

Example 3: Using a Step

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

Example 4: Counting Downwards

To count backwards, you need a negative step:

Nested for Loops

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

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

Example: Generating a Multiplication Table

Let us generate a multiplication table for numbers 1 through 3 using nested loops:

The for-else Clause

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

It is perfect for code that needs to run only when a search or process finishes “naturally.”

Example: Searching for an Item in a List

Let us try searching for a number that does not exist in our list:

Now, let us try a case where the search number is successfully found:

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