Loops

Loops are used to do something multiple times in a row. There are two types of loops in Python, for and while.

The "for" loop

For loops go over a given sequence (they "iterate" over the sequence) and do things once for each element of the sequence. Here is an example:

primes = [2, 3, 5, 7]
for prime in primes:
    print(prime)

Our sequence here is a list of numbers called primes. prime is a name that is used for each element, one after another.

We can generate a sequence of numbers using the "range" function. This starts at 0 and generates all numbers lower than the given limit.

# Prints out the numbers 0,1,2,3,4
for x in range(5):
    print(x)

You can also define where to start:

# Prints out 3,4,5
for x in range(3, 6):
    print(x)

If you don't need every number but just every second, third etc. you can specify a "step length":

# Prints out 3,5,7
for x in range(3, 8, 2):
    print(x)

"while" loops

While loops repeat as long as a certain boolean condition is met. For example:

# Prints out 0,1,2,3,4

count = 0
while count < 5:
    print(count)
    count += 1  # This is the same as count = count + 1

"break" and "continue" statements

break is used to exit a for loop or a while loop, whereas continue is used to skip the current block, and return to the "for" or "while" statement. A few examples:

# Prints out 0,1,2,3,4

count = 0
while True:
    print(count)
    count += 1
    if count >= 5:
        break

# Prints out only odd numbers - 1,3,5,7,9
for x in range(10):
    # Check if x is even
    if x % 2 == 0:
        continue
    print(x)

Exercise

Loop through and print out all even numbers from the numbers list in the same order they are received. Don't print any numbers that come after 237 in the sequence.