Loops with for and while · Python tutorial

Computers are good at doing the same thing many times without getting bored. Loops are how you ask them to.

for over a collection#

A for loop takes each item of something in turn:

for fruit in ["apple", "banana", "cherry"]:
    print(fruit)

for letter in "hey":
    print(letter)     # h, e, y on separate lines

The name after for (here fruit) refers to the current item on each round.

range: counting#

range produces a run of whole numbers:

for i in range(5):          # 0, 1, 2, 3, 4
    print(i)

for i in range(2, 6):       # 2, 3, 4, 5
    print(i)

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

Like slicing, the end is not included. range(5) has five numbers, 0 to 4, which are exactly the valid positions of a list of five things.

Adding things up#

The most common loop of all keeps a running total:

numbers = [4, 8, 15, 16, 23, 42]
total = 0
for x in numbers:
    total += x
print(total)    # 108

The same shape finds a largest value, counts matches, or builds a new list. (Python also has sum(numbers) and max(numbers) built in, but you need to know the loop they are hiding.)

Position and value together#

When you need both the index and the item, use enumerate:

for i, name in enumerate(["Asha", "Ravi", "Meera"]):
    print(i, name)     # 0 Asha / 1 Ravi / 2 Meera

That is clearer than for i in range(len(names)): followed by names[i].

while: until something happens#

When you do not know in advance how many rounds there are, loop while a condition holds:

n = 4071
digits = 0
while n > 0:
    n //= 10
    digits += 1
print(digits)    # 4

Something inside a while must move it towards stopping. If nothing changes, the loop runs forever. Press Ctrl+C to stop a runaway program.

break and continue#

break leaves the loop at once. continue skips to the next round.

for x in [3, 8, -1, 5]:
    if x < 0:
        print("found a negative")
        break
    print(x)
# prints 3, 8, then "found a negative"; 5 is never reached

for x in range(10):
    if x % 2 == 1:
        continue
    print(x)       # 0 2 4 6 8

Loops inside loops#

A loop can sit inside another. The inner one runs completely for every round of the outer:

for row in range(1, 4):
    for col in range(1, 4):
        print(row * col, end=" ")
    print()
# 1 2 3
# 2 4 6
# 3 6 9

end=" " tells print to finish with a space instead of a new line.

Be careful with size: two nested loops over n items do n × n steps. For 1,000 items that is a million, which is fine; for 100,000 it is ten billion, which is far too slow. Much of algorithms is finding a way to do one pass instead of two.

Key points#

for x in collection visits each item; range(a, b) counts from a up to, not including, b.
enumerate gives position and item together.
while repeats while a condition is true, and something inside must change it.
break leaves the loop; continue skips to the next round.
Nested loops multiply the work.

Next: packaging code into reusable functions.