For Loops and Range

What Does a For Loop Do?

A for loop lets you repeat a block of code multiple times, once for each item in a sequence. Instead of writing the same code over and over, you write it once and let the loop handle the repetition. The key insight is that the loop variable (often called i) takes on a different value each time through the loop, and you can use that value inside your code block.

The most common way to create a sequence of numbers to loop over is the range() function. When you write for i in range(1, 5), Python creates a sequence of numbers and your loop visits each one in order, assigning it to i and running your code block.

How Range Generates Numbers

The range() function is a built-in Python tool that generates a sequence of numbers. It does not create all the numbers at once and store them in memory. Instead, it generates one number at a time, on demand, as your for loop requests each one. This is efficient, especially when you need a very long sequence.

To see all the numbers range() would produce, you can convert it to a list using list(range()). For example, list(range(1, 5)) shows you [1, 2, 3, 4]. But inside a for loop, you do not need to do this conversion; the loop automatically gets each number one at a time.

Stepping Through a For Loop

Let us trace through a concrete example to see exactly what happens as a for loop runs. Understanding this step-by-step execution is the key to mastering loops.

Controlling Range with Start, Stop, and Step

The range() function accepts up to three parameters: start, stop, and step. Understanding how each one works gives you precise control over which numbers your loop visits.

The For...Else Clause

Python allows you to attach an else clause to a for loop. The code in the else block runs after the loop completes normally (that is, when the loop finishes all its iterations without hitting a break statement). This is useful when you want to execute code only if the loop ran to completion.

for i in range(1, 5):
    print(i)
else:
    print('The for loop is over')

1
2
3
4
The for loop is over

In this example, the loop runs through all four iterations (1, 2, 3, 4), and after the last iteration completes, the else block executes and prints 'The for loop is over'. If the loop had encountered a break statement, the else block would have been skipped.

For Loops Work with Any Sequence

While range() is the most common way to generate a sequence of numbers, the for loop works with any sequence. You can loop over lists, strings, tuples, and other sequence types. The loop variable takes on each element of the sequence, one at a time, and you can use it in your code block.

This flexibility is one of the most powerful features of for loops in Python. Whether you are iterating over numbers from range(), items in a list, or characters in a string, the same loop structure applies. We will explore this idea in detail in later chapters when we cover data structures like lists and strings.

Common Mistakes with For Loops and Range

• Forgetting that range(stop) excludes the stop value

  The stop parameter is exclusive. range(5) means 'up to but not including 5'.

  Fix: If you want to include 5, use range(6) instead.

• Confusing range(start, stop) with range(stop, start)

  range(5, 1) means 'start at 5, go up to but not including 1'. Since 5 is already past 1, there are no values to generate.

  Fix: Use range(5, 1, -1) to count backwards from 5 to 2.

• Assuming the loop variable i exists before the loop starts

  The loop variable is created by the for statement itself, on the first iteration.

  Fix: Only use the loop variable inside the loop body or after the loop has run at least once.

• Using list(range()) inside a loop when you do not need to

  This wastes memory. range() generates values on demand, which is more efficient.

  Fix: Use for i in range(1000000) directly; the loop will request each value as needed.

Practice: Writing Your Own For Loops

Key Takeaways

1. A for loop repeats a block of code once for each item in a sequence, with the loop variable taking on each value in turn.
2. range() generates a sequence of numbers on demand (one at a time), not all at once. Use list(range()) only if you need to see all values immediately.
3. range(start, stop) includes start but excludes stop. range(start, stop, step) lets you control the increment or decrement.
4. The loop variable persists after the loop ends and retains the value of the last iteration.
5. A for...else clause runs only if the loop completes normally (without hitting a break statement).
6. For loops work with any sequence type, not just range(). This makes them powerful for iterating over lists, strings, and other data structures.