Repeating With `for`

Repeating With for

Core path: 25 minutes

How can Python repeat something a known number of times?

Instead of writing the same instruction manually, describe the repetition once:

for number in range(5):
    print(number)

Output:

0
1
2
3
4

Interactive model

Follow one for-loop execution cycle

Pause motionReset layout

The loop receives a value, runs the body, advances to the next value, and stops only when the iterable is exhausted.

ITERABLE range(5)

NEXT VALUE number = 0, 1, 2...

LOOP BODY print(number)

MORE VALUES?

NO loop ends

React Flow

Press enter or space to select a node. You can then use the arrow keys to move the node around. Press delete to remove it and escape to cancel.

Press enter or space to select an edge. You can then press delete to remove it or escape to cancel.

Drag nodes to inspect the relationships. Motion shows the active path; the plain background keeps attention on the relationships instead of graph-paper decoration.

View static diagram

Understanding range()

range(5) begins at 0 and stops before 5.

Diagrams open at a readable shape-aware scale. Zoom or expand when you need more detail.

A common failure is assuming the stop value is included. That creates an off-by-one error: the program runs one fewer or one more iteration than the developer intended.

Use the runnable comparison below to inspect several range() shapes side by side:

Run the code to see output.

Do not just read the lists after running. Predict each sequence first, then change exactly one stop value and identify the one boundary effect that changed.

Loop anatomy

for number in range(5):
│      │         │
│      │         └── sequence of values
│      └── loop variable
└── repeat

The indented block is the loop body. It executes once for each value.

Trace the loop

for number in range(3):
    print(f"Number: {number}")

Trace:

iteration 1 → number = 0 → Number: 0
iteration 2 → number = 1 → Number: 1
iteration 3 → number = 2 → Number: 2
stop

Start, stop, and step

for number in range(1, 6):
    print(number)

prints 1 through 5 because 6 is excluded.

for number in range(0, 11, 2):
    print(number)

uses range(start, stop, step).

Guided example: choose the range from the required output

Requirement:

Scan 1
Scan 2
Scan 3

One correct loop is:

for scan_number in range(1, 4):
    print(f"Scan {scan_number}")

Work backward from the evidence. The first required value is 1. The final required value is 3, so the excluded stop must be 4.

Activity: Predict Before Running

Predict, run, and explain:

for x in range(2, 8):
    print(x)

for x in range(2, 8, 2):
    print(x)

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