The for Loop and range()

15 min · Beginner

In Python, a for loop does not work like a C-style for (int i=0; ...) loop. Instead, it is an iterator loop that draws items directly from any sequence.

Direct iteration over text

For loop is a statement that takes the items of a sequence one at a time and runs its body once for each item. The loop variable holds the current item.

A Python for loop visits each item of a sequence one by one, binding it to the loop variable. A string is a sequence of characters, so a loop can visit them.

Python

word = "Code"for ch in word:    print(ch)

Example 1 · Checking each character of a product code

Python

product_code = "PX-204"for ch in product_code:    print(f"Character: {ch}")

Output

Character: P
Character: X
Character: -
Character: 2
Character: 0
Character: 4

What this code does

The loop variable ch holds one character per round.
The code has six characters, so the loop body runs six times.

There is no index variable, bounds checking, or increment line. Python hands over each character in turn. Looping over lists and other collections works the same way and comes in later modules.

Generating numbers with range()

range() is a built-in that produces whole numbers on demand. range(stop) starts at 0 and ends just before stop, so range(3) gives 0, 1 and 2.

Use the built-in range() function to generate a sequence of integers efficiently without building an entire list in memory.

Python

for i in range(3):    print(i)

Example 2 · Packing orders 1 to 5

Python

for order in range(1, 6):    print(f"Order #{order} packed")

Output

Order #1 packed
Order #2 packed
Order #3 packed
Order #4 packed
Order #5 packed

What this code does

range(1, 6) starts at 1 and stops before 6, so order is 1, 2, 3, 4, 5.
The stop value 6 is not included, which is why it is one more than the last order.

range(3) produces numbers 0, 1, and 2. Notice that the stop value 3 is exclusive.

Start, stop, and step parameters

Step is the amount added in range(start, stop, step) to move from one number to the next. A negative step counts down, and the default step is 1.

range(start, stop, step) gives full control over the generated sequence, including skipping numbers or counting in reverse.

Python

for val in range(10, 0, -2):    print(val)

Example 3 · Every second day

Python

for day in range(1, 10, 2):    print(f"Day {day}")

Output

Day 1
Day 3
Day 5
Day 7
Day 9

What this code does

The step 2 adds 2 each time: 1, 3, 5, 7, 9.
The next number would be 11, which is past the stop value 10, so the loop ends.

Example 4 · Sum of cubes

Python

1total_cubes = 02for n in range(1, 4):3    cube = n ** 34    total_cubes += cube5    print(f"{n} cubed = {cube}")6
7print(f"Sum of cubes: {total_cubes}")

Output

1 cubed = 1
2 cubed = 8
3 cubed = 27
Sum of cubes: 36

What this code does

Iterates through 1, 2, and 3 using range(1, 4).
Calculates cubes (1, 8, 27) and sums them to 36.

Using a negative step -2 counts backwards from 10 down to 2.

Next step

Check your understanding

10 quick questions

Direct iteration over text

For loop is a statement that takes the items of a sequence one at a time and runs its body once for each item. The loop variable holds the current item.

A Python for loop visits each item of a sequence one by one, binding it to the loop variable. A string is a sequence of characters, so a loop can visit them.