for and range

for: do something for each item

Loops are the reason computers beat hand work. Check 10,000 rows, resize every photo in a folder, send a message to every subscriber: each is a few lines repeated automatically. From here on, nearly every useful program you write in this course, and on the job, has a loop in the middle.

A for loop runs its indented body once per item in a sequence. A string is a sequence, so this visits every character:

for ch in "abc":
    print(ch)

ch is the loop variable: Python assigns it the next character before each pass through the body.

range: a sequence of numbers on demand

When you want to repeat something N times or count, use range():

• range(5) produces 0, 1, 2, 3, 4 (starts at 0, stops before 5)
• range(2, 6) produces 2, 3, 4, 5
• range(0, 10, 2) produces 0, 2, 4, 6, 8 (the third number is the step)

The stop value is excluded, exactly like string slices in lesson 2-1. range(5) has 5 items, which makes repeat 5 times read naturally.