18. Loops

The other way out of repeating yourself is the loop: a block of code that runs over and over. Python has two main loop shapes — while and for — plus break for stopping early. All covered here.

while — repeat as long as the condition is true

while is the most general loop. It checks a condition, runs the block if true, then checks again — repeating until it is false.

count = 10
while count > 0:
    print(count)
    count = count - 1
print("Blast off!")

Run order:

1. Check count > 0. It is 10, so True.
2. Print 10, set count to 9.
3. Back to step 1. count is 9, still True. Print, decrement.
4. Repeat until count is 0. The condition fails, the loop exits, and print("Blast off!") runs once.

for — repeat a known number of times

The for loop is the right tool when you know how many times to repeat, or when you want to step through a sequence. The shape uses range():

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

Output:

1
2
3
4
5

range(start, stop) produces numbers from start up to — but not including — stop. So range(1, 6) gives 1, 2, 3, 4, 5. This is one of the first surprises in Python: the end value is excluded.

To loop from 1 to 10, write range(1, 11).

A step

A third argument to range() is the step — how much the counter changes each round. The default is 1; change it to skip or count backwards:

for i in range(0, 101, 10):    # 0, 10, 20, ... 100
    print(i, end=" ")
print()                        # a blank line at the end

for i in range(10, 0, -1):     # 10, 9, 8, ... 1
    print(i, end=" ")
print()

The step needs the right sign — positive going up, negative going down. A step of 0 raises a ValueError.

while True: with break

Any loop can be cut short with break, which jumps straight to whatever comes after the loop. A while True: loop has no built-in exit; the only way out is break:

n = 1
while True:
    if n * n > 100:
        break
    n = n + 1
print("First number whose square is over 100 is", n)

Use while True: when the exit test is too complicated for a single comparison at the top.

Running at least once with while True:

Sometimes you need the loop body to run at least once before checking the condition. Use while True: and break at the end:

# runs at least once, stops when count > 5
count = 1
while True:
    print(count)
    count = count + 1
    if count > 5:
        break

The break at the bottom exits as soon as the condition is met.

Homework

7 * 1 = 7
7 * 2 = 14
...
7 * 12 = 84

Sum from 1 to N is S

Stuck or finished? Open the homework solutions page.