Repeating with for and range

Why this matters

A while loop repeats until a condition changes. Very often, though, you know exactly how many times to repeat, or you want to do something for every item in a collection. That is what for is for, and it is the loop you will use most.

The idea

for letter in "abc":
    print(letter)

This prints a, b, c, one per line. On each pass, the loop variable (letter) holds the next value.

To repeat something a number of times, or to count, use range():

for i in range(3):
    print(i)           # 0, 1, 2

range can take up to three numbers:

Call	Produces
range(5)	0, 1, 2, 3, 4
range(2, 6)	2, 3, 4, 5
range(0, 10, 3)	0, 3, 6, 9
range(5, 0, -1)	5, 4, 3, 2, 1

The stop value is never included. That surprises everyone once: range(1, 4) gives 1, 2, 3. To include n, use range(1, n + 1).

You can combine a for loop with an accumulator, exactly like with while:

total = 0
for price in (5, 10, 20):
    total += price
print(total)           # 35

When to use which loop: use for when you know what you are looping over; use while when you are waiting for something to happen.

Try it

Print the numbers from 10 down to 1 with a for loop, then print only the even numbers from 2 to 20.

Your turn

The starter reads a number n. Write a for loop that

1. prints the square of every whole number from 1 up to and including n, one per line, and
2. after the loop, prints the total of those squares.

For n = 5 the output is:

1
4
9
16
25
Total: 55

Recap

• for item in something: runs once for each item.
• range(start, stop, step) generates numbers; stop is excluded.
• Create accumulators before the loop and update them inside.

Common mistakes

• Forgetting that range(n) starts at 0 and stops before n.
• Changing the loop variable inside the loop and expecting it to affect the next pass. It will not.
• Printing the total inside the loop (you get a total on every line) instead of after it.