Loops

In programming, a loop means repeating something multiple times.
There are different kinds of loops:

• While loops repeat something while a condition is true.
• Until loops repeat something while a condition is false.
• For loops repeat something for each element of something.

We’ll talk about all of these in this tutorial.

While loops

Now we know how if statements work.

its_raining = True
if its_raining:
    print("Oh crap, it's raining!")

While loops are really similar to if statements.

its_raining = True
while its_raining:
    print("Oh crap, it's raining!")
    # we'll jump back to the line with the word "while" from here
print("It's not raining anymore.")

If you’re not familiar with while loops, the program’s output may be a bit surprising:

Oh crap, it's raining!
Oh crap, it's raining!
Oh crap, it's raining!
Oh crap, it's raining!
Oh crap, it's raining!
Oh crap, it's raining!
Oh crap, it's raining!
Oh crap, it's raining!
Oh crap, it's raining!
Oh crap, it's raining!
Oh crap, it's raining!
Oh crap, it's raining!
Oh crap, it's raining!
Oh crap, it's raining!
Oh crap, it's raining!
(much more raining)

Again, this program does not break your computer. It just prints the same thing multiple times. We can interrupt it by pressing Ctrl+C.

In this example, its_raining was the condition. If something in the while loop would have set its_raining to False, the loop would have ended and the program would have printed It's not raining anymore.

Let’s actually create a program that does just that:

its_raining = True
while its_raining:
    print("It's raining!")
    answer = input("Or is it? (y=yes, n=no) ")
    if answer == 'y':
        print("Oh well...")
    elif answer == 'n':
        its_raining = False     # end the while loop
    else:
        print("Enter y or n next time.")
print("It's not raining anymore.")

Running the program may look like this:

It's raining!
Or is it? (y=yes, n=no) i dunno
Enter y or n next time.
It's raining!
Or is it? (y=yes, n=no) y
Oh well...
It's raining!
Or is it? (y=yes, n=no) n
It's not raining anymore.

The while loop doesn’t check the condition all the time, it only checks it in the beginning.

>>> its_raining = True
>>> while its_raining:
...     its_raining = False
...     print("It's not raining, but the while loop doesn't know it yet.")
...
It's not raining, but the while loop doesn't know it yet.
>>>

We can also interrupt a loop even if the condition is still true using the break keyword. In this case, we’ll set condition to True and rely on nothing but break to end the loop.

while True:
    answer = input("Is it raining? (y=yes, n=no) ")
    if answer == 'y':
        print("It's raining!")
    elif answer == 'n':
        print("It's not raining anymore.")
        break   # end the loop
    else:
        print("Enter y or n.")

The program works like this:

Is it raining? (y=yes, n=no) who knows
Enter y or n.
Is it raining? (y=yes, n=no) y
It's raining!
Is it raining? (y=yes, n=no) n
It's not raining anymore.

Unlike setting the condition to False, breaking the loop ends it immediately.

>>> while True:
...     break
...     print("This is never printed.")
...
>>>

Until loops

Python doesn’t have until loops. If we need an until loop, we can use
while not:

raining = False
while not raining:
    print("It's not raining.")
    if input("Is it raining? (y/n) ") == 'y':
        raining = True
print("It's raining!")

For loops

Let’s say we have a list of things we want to print. To print each item in it, we could just do a bunch of prints:

stuff = ['hello', 'hi', 'how are you doing', 'im fine', 'how about you']

print(stuff[0])
print(stuff[1])
print(stuff[2])
print(stuff[3])
print(stuff[4])

The output of the program is like this:

hello
hi
how are you doing
im fine
how about you

But this is only going to print five items, so if we add something to stuff, it’s not going to be printed. Or if we remove something from stuff, we’ll get an error saying "list index out of range".

We could also create an index variable, and use a while loop:

>>> stuff = ['hello', 'hi', 'how are you doing', 'im fine', 'how about you']
>>> length_of_stuff = len(stuff)
>>> index = 0
>>> while index < length_of_stuff:
...     print(stuff[index])
...     index += 1
...
hello
hi
how are you doing
im fine
how about you
>>>

But we have len() and an index variable we need to increment and a while loop and many other things to worry about. That’s a lot of work just for printing each item.

This is when for loops come in:

>>> for thing in stuff:
...     # this is repeated for each element of stuff, that is, first
...     # for stuff[0], then for stuff[1], etc.
...     print(thing)
...
hello
hi
how are you doing
im fine
how about you
>>>

Without the comments, that’s only two simple lines, and one variable.

Much better than anything else we tried before.

>>> for thing in stuff:
...     print(thing)
...
hello
hi
how are you doing
im fine
how about you
>>>

Note that for thing in stuff: is not same as for (thing in stuff):.

Here the in keyword is just a part of the for loop and it has a different meaning than it would have if we had thing in stuff without a for. Trying to do for (thing in stuff): creates an error.

Right now the while loop version might seem easier to understand for you, but later you’ll realize that for loops are much easier to work with than while loops and index variables, especially in large projects.

For looping is also a little bit faster than while looping with an index variable.

For loops are not actually limited to lists. We can for loop over many other things also, including strings and
tuples.

For looping over a tuple gives us its items, and for looping over a string gives us its characters as strings of length one.

>>> for short_string in 'abc':
...     print(short_string)
...
a
b
c
>>> for item in (1, 2, 3):
...     print(item)
...
1
2
3
>>>

If we can for loop over something, then that something is iterable.

Lists, tuples and strings are all iterable.

There’s only one big limitation with for looping over lists. We shouldn’t modify the list in the for loop. If we do, the results can be surprising:

>>> stuff = ['hello', 'hi', 'how are you doing', 'im fine', 'how about you']
>>> for thing in stuff:
...     stuff.remove(thing)
...
>>> stuff
['hi', 'im fine']
>>>

Instead, we can create a copy of stuff and loop over it.

>>> stuff = ['hello', 'hi', 'how are you doing', 'im fine', 'how about you']
>>> for thing in stuff.copy():
...     stuff.remove(thing)
...
>>> stuff
[]
>>>

Or if we just want to clear a list, we can use the clear
list method:

>>> stuff = ['hello', 'hi', 'how are you doing', 'im fine', 'how about you']
>>> stuff.clear()
>>> stuff
[]
>>>

Summary

• A loop means repeating something multiple times.
• While loops repeat something as long as a condition is true, and
  they check the condition only in the beginning.
• For loops can be used for repeating something to each item in a list.
• An iterable is something that can be for looped over.
• The break keyword can be used to interrupt the innermost loop at
  any time.

Examples

Repeat something an endless amount of times.

message = input("What do you want me to say? ")
while True:
    print(message, "!!!")

Ask the user to enter five things and print them.

things = []

print("Enter 5 things: ")
while len(things) < 5:
    thing = input("> ")
    things.append(thing)

print("You entered these things:")
for thing in things:
    print(thing)

Ask the user a bunch of questions.

questions_and_answers = [
    # [question, answer], ...
    ["What is 2+4? ", "6"],
    ["What is 2-4? ", "-2"],
    ["What is 2*4? ", "8"],
    ["What is 2/4? ", "0.5"],
    # You could add more questions, but the code that asks them
    # wouldn't need to be changed in any way.
]

for qa in questions_and_answers:
    while True:
        if input(qa[0]) == qa[1]:
            print("Correct!")
            break
        else:
            print("That's not what I was thinking of... Try again.")

Store a list of names and let the user check if the program knows the user.

# You can add names here so the program will know them automatically
# when it starts.
namelist = []

print("Options:")
print("  0      Quit")
print("  1      Check if I know you")
print("  2      Introduce yourself to me")
print("  3      Make me forget you")
print("  4      Print a list of people I know")
print()     # print an empty line

while True:
    option = input("Choose an option: ")

    # Things like option == 0 don't work because option is a string
    # and it needs to be compared with a string:
    #   >>> 0 == 0
    #   True
    #   >>> '0' == '0'
    #   True
    #   >>> 0 == '0'
    #   False
    if option == '0':
        print("Bye!")
        break

    elif option == '1':
        name = input("Enter your name: ")
        if name in namelist:
            print("I know you! :D")
        else:
            print("I don't know you :/")

    elif option == '2':
        name = input("Enter your name: ")
        if name in namelist:
            print("I knew you already.")
        else:
            namelist.append(name)
            print("Now I know you!")

    elif option == '3':
        name = input("Enter your name: ")
        if name in namelist:
            namelist.remove(name)
            print("Now I don't know you.")
        else:
            print("I didn't know you to begin with.")

    elif option == '4':
        if namelist == []:
            print("I don't know anybody yet.")
        else:
            for name in namelist:
                print("I know %s!" % name)

    else:
        print("I don't understand :(")

    print()