Part 3: Python Programming

7. Loops

Loops

Computers excel at repetition. A loop runs a block of code many times without you writing it out again and again. Python has two loops: the for loop, which repeats once for each item in a sequence, and the while loop, which repeats as long as a condition stays true.

The for loop and range()

A for loop walks through the items of a sequence, giving you one item per pass. The range() function generates a sequence of numbers, which is the usual way to repeat a fixed number of times. range(5) produces 0, 1, 2, 3, 4 (it stops before 5):

PYTHON

for i in range(5):
    print(i)

▸ OUTPUT

range can take a start, stop, and step: range(2, 11, 2) gives 2, 4, 6, 8, 10.

Iterating over sequences

You can loop directly over the characters of a string or, as you will see later, the items of a list:

PYTHON

for letter in "cat":
    print(letter.upper())

▸ OUTPUT

C
A
T

The while loop

A while loop keeps going as long as its condition is true. You must make sure something inside the loop eventually makes the condition false, or the loop will run forever:

PYTHON

count = 3
while count > 0:
    print(count)
    count = count - 1
print("Lift off!")

▸ OUTPUT

3
2
1
Lift off!

break and continue

Two keywords give you finer control. break leaves the loop immediately. continue skips the rest of the current pass and jumps to the next one:

PYTHON

for n in range(1, 10):
    if n == 5:
        break
    if n % 2 == 0:
        continue
    print(n)

▸ OUTPUT

1
3

Odd numbers 1 and 3 print. Even numbers 2 and 4 are skipped by continue, and when n reaches 5 the break stops the loop entirely.

The loop else clause

Both loops may have an else block that runs only if the loop finished normally without hitting a break. It is handy for search loops:

PYTHON

for n in [4, 6, 8]:
    if n % 2 != 0:
        print("Found an odd number")
        break
else:
    print("All numbers were even")

▸ OUTPUT

All numbers were even

Choosing between for and while

Use a for loop when...	Use a while loop when...
You know how many times to repeat	You repeat until a condition changes
You are walking through a sequence	The number of repeats is unknown in advance
Example: process each student	Example: keep asking until input is valid

Loops turn a few lines of code into enormous amounts of work. Practice by printing a multiplication table for a number using a for loop and range.