Repeating with for and while

range(5) produces 0, 1, 2, 3 and 4: five numbers, and 5 is not one of them.

Run a three-line loop and watch total grow

Copy this program into IDLE and run it before reading the explanation. Then change the list to your own numbers and run it again.

total = 0
for n in [3, 5, 2]:
    total = total + n
    print(n, total)
print("done", total)

3 3
5 8
2 10
done 10

The for line takes the items of the list one at a time. Each time round, the variable n refers to the next item, and the indented lines below run once with that value. One pass through the indented lines is called an iteration. After the last item, Python carries on with the first line that is not indented, so "done" prints once, at the end. A trace table makes the pattern plain.

The variable total is called an accumulator: it starts at 0 before the loop and collects something on every pass. The line total = total + n is so common that Python has a short form, total += n. Counting works the same way with count += 1.

range() makes the numbers for you

To repeat something a fixed number of times, loop over range(). range(5) gives 0, 1, 2, 3, 4: it starts at 0 and stops before 5, just like a slice. range(1, 6) gives 1 to 5. A third number sets the step, so range(0, 10, 2) gives the even numbers 0 to 8. The multiplication table for 7, which every Korean pupil recites in second grade, is a two-line loop.

for i in range(1, 10):
    print(7, "x", i, "=", 7 * i)

7 x 1 = 7
7 x 2 = 14
7 x 3 = 21
7 x 4 = 28
7 x 5 = 35
7 x 6 = 42
7 x 7 = 49
7 x 8 = 56
7 x 9 = 63

A for loop can also walk through the characters of a string, which makes counting letters easy.

count = 0
for ch in "banana":
    if ch == "a":
        count += 1
print(count)

3

while repeats as long as a condition stays True

A while loop has no list to walk through. It checks a condition, runs the indented lines if it is True, and checks again, over and over. That suits jobs where you do not know the number of passes in advance, such as "keep saving until the total reaches 50,000 won". Something inside the loop must change the condition, or the loop never ends. If that happens, stop the program with Ctrl+C.

saved = 0
weeks = 0
while saved < 50000:
    saved += 12000
    weeks += 1
print(weeks, saved)

5 60000

break leaves a loop at once, even in the middle. It is handy when you are searching and can stop as soon as you find what you want. The loop below stops at the first score under 60 and never looks at the 88.

for s in [72, 91, 55, 88]:
    if s < 60:
        print("first below 60:", s)
        break
    print("ok", s)

ok 72
ok 91
first below 60: 55