Loops: for and while

A loop repeats a block of code. Python has two kinds: a for loop that walks through items, and a while loop that repeats as long as a condition stays True.

The for loop

A for loop takes each item from a sequence one at a time:

for letter in 'abc':
    print(letter)

This prints a, then b, then c.

Using range()

range() creates a series of numbers. range(n) counts from 0 up to but not including n:

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

You can also give a start and stop: range(2, 5) gives 2, 3, 4.

The while loop

A while loop keeps going while its condition is True. You must change something inside so it eventually stops:

count = 0
while count < 3:
    print(count)
    count += 1

If the condition never becomes False you get an infinite loop.

break and continue

• break stops the loop immediately.
• continue skips to the next repetition.

for i in range(5):
    if i == 3:
        break
    print(i)   # 0 1 2

Looping over a list

A for loop works naturally with lists, visiting each item in turn:

names = ['Ali', 'Sara', 'Wei']
for name in names:
    print('Hello', name)

Building up a total

A common pattern is to start at zero and add inside the loop:

total = 0
for n in [10, 20, 30]:
    total += n
print(total)   # 60

Counting with range and a step

range can take a third number, the step, to skip values:

for i in range(0, 10, 2):
    print(i)   # 0 2 4 6 8

Choose a for loop when you know the items or how many times to repeat, and a while loop when you repeat until some condition changes.

Looping with a position number

Sometimes you want both the item and its position. The enumerate() helper gives you a counter alongside each item:

for i, name in enumerate(['a', 'b']):
    print(i, name)   # 0 a then 1 b

Without enumerate you would keep a separate counter and add one each round, which is easy to get wrong.