for loops and range()

Looping a known number of times

When you know how many repetitions you need, for is clearer than while — there is no counter to forget to increment.

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

That prints 0, 1, 2, 3, 4. range(5) produces five numbers starting at zero and stopping before five. Almost every off-by-one bug in Python comes from forgetting one half of that sentence.

range takes up to three arguments:

• range(5) → 0, 1, 2, 3, 4
• range(2, 6) → 2, 3, 4, 5
• range(0, 10, 2) → 0, 2, 4, 6, 8

for also walks straight through a string or a list:

for letter in "cat":
    print(letter)

Your turn: put the squares of 1 to 5 in squares, in order — [1, 4, 9, 16, 25].

Accumulating a result

The most common loop shape in any language: start with an empty answer, walk the data, and build the answer up as you go.

total = 0
for price in prices:
    total += price

The variable after for takes each value in turn — you never index by hand.

Your turn: write count_vowels(text) returning how many vowels the text contains. Count both cases, so "Apple" gives 2.