For loop

Learn how to repeat code execution efficiently.

The For Loop

Loops are one of the most important concepts in programming. They allow you to execute a block of code repeatedly — a fixed number of times. Also, you can use for to iterate through a collection of values.

The Idea Behind for

The original purpose of a for loop is to traverse a sequence — go through each element one by one and do something with it. In Python, the for loop is built around this idea:

The most natural sequence to loop over is a list — for example, a list of numbers or names. We will cover lists in detail in the next module. For now, we will use range() — a built-in function that generates a sequence of numbers on the fly. Note that range() does not create a list in memory — it generates values one by one as the loop runs.

The range() Function

range() produces a sequence of integers. You can loop over it directly with for:

Output:

Note that range(5) generates numbers from 0 up to, but not including, 5. This is called a half-open interval — [0, 5) in mathematical notation. Zero-based indexing like this is standard in programming and you will get used to it quickly.

range() with a Start Value

By default range() starts at 0. You can pass a custom start:

The general form is range(start, stop) — generates integers from start up to, but not including, stop.

range() with a Step

The third argument controls the step — how much i increases after each iteration:

The general form is range(start, stop, step).

Step can be any positive integer:

Counting Down with a Negative Step

Passing a negative step makes the loop count downward. In this case start must be greater than stop:

Summary of range() Signatures