For Loops

A for loop runs a block of code once for each item in a sequence. You tell Python what you want to loop over (a list, a string, a range, etc.), and the loop does the stepping for you.

Basic syntax


for variable in sequence:
    # code to run for each item

variable is the name you choose for “the current item”
sequence is anything Python can step through (a list, string, range(), etc.)
The indented block runs once per item in the sequence

Looping with `range()`

range() gives you a sequence of numbers. It’s the most common way to run a loop a set number of times.


for i in range(5):
    print(i)
# Output: 0 1 2 3 4

range() has three common forms:

Form	What it does
`range(stop)`	0 up to (not including) `stop`
`range(start, stop)`	`start` up to (not including) `stop`
`range(start, stop, step)`	`start` up to `stop`, counting by `step`


for i in range(1, 6):
    print(i)
# Output: 1 2 3 4 5
 
for i in range(0, 10, 2):
    print(i)
# Output: 0 2 4 6 8

range(1, 6) gives you 1 through 5, not 1 through 6. This is intentional — it matches how Python indexing works and makes it easy to write range(len(my_list)) to cover every index.

Looping over a list

If you have a list, you usually loop over the items directly (no indexes needed):


fruits = ["apple", "banana", "cherry"]
 
for fruit in fruits:
    print(fruit)
# Output:
# apple
# banana
# cherry

You don’t need len(fruits) here—the loop gives you each item automatically.

To modify items in the list as you go, loop over the indexes with range(len(...)) and use the index to update each item:


grades = [85, 90, 78]
for i in range(len(grades)):
    grades[i] = grades[i] + 5
print(grades)  # [90, 95, 83]

When to use a `for` loop

Use a for loop when you know what you’re iterating over—a list, a range, a string, or any sequence.

If you need to keep going until a condition changes (and you don’t know how many iterations that will take), that’s a while loop. We’ll cover that next.

Summary

A for loop runs once per item in a sequence.
range() is the go-to way to repeat something a specific number of times.

Try it out

main.py

Output