Exercise 32: Loops and Lists

You should now be able to do some programs that are much more interesting. If you have been keeping up, you should realize that now you can combine all the other things you have learned with if-statements and boolean expressions to make your programs do smart things.

However, programs also need to do repetitive things very quickly. We are going to use a for-loop in this exercise to build and print various lists. When you do the exercise, you will start to figure out what they are. I won't tell you right now. You have to figure it out.

Before you can use a for-loop, you need a way to store the results of loops somewhere. The best way to do this is with lists. Lists are exactly what their name says: a container of things that are organized in order from first to last. It's not complicated; you just have to learn a new syntax. First, there's how you make lists:

hairs = ['brown', 'blond', 'red']
eyes = ['brown', 'blue', 'green']
weights = [1, 2, 3, 4]

You start the list with the [ (left bracket) which "opens" the list. Then you put each item you want in the list separated by commas, similar to function arguments. Lastly, end the list with a ] (right bracket) to indicate that it's over. Python then takes this list and all its contents and assigns them to the variable.

Warning

This is where things get tricky for people who can't code. Your brain has been taught that the world is flat. Remember in the last exercise where you put if-statements inside if-statements? That probably made your brain hurt because most people do not ponder how to "nest" things inside things. In programming nested structures are all over the place. You will find functions that call other functions that have if-statements that have lists with lists inside lists. If you see a structure like this that you can't figure out, take out a pencil and paper and break it down manually bit by bit until you understand it.

We now will build some lists using some for-loops and print them out:

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the_count = [1, 2, 3, 4, 5]
fruits = ['apples', 'oranges', 'pears', 'apricots']
change = [1, 'pennies', 2, 'dimes', 3, 'quarters']

# this first kind of for-loop goes through a list
for number in the_count:
    print "This is count %d" % number

# same as above
for fruit in fruits:
    print "A fruit of type: %s" % fruit

# also we can go through mixed lists too
# notice we have to use %r since we don't know what's in it
for i in change:
    print "I got %r" % i

# we can also build lists, first start with an empty one
elements = []

# then use the range function to do 0 to 5 counts
for i in range(0, 6):
    print "Adding %d to the list." % i
    # append is a function that lists understand
    elements.append(i)

# now we can print them out too
for i in elements:
    print "Element was: %d" % i

What You Should See

$ python ex32.py
This is count 1
This is count 2
This is count 3
This is count 4
This is count 5
A fruit of type: apples
A fruit of type: oranges
A fruit of type: pears
A fruit of type: apricots
I got 1
I got 'pennies'
I got 2
I got 'dimes'
I got 3
I got 'quarters'
Adding 0 to the list.
Adding 1 to the list.
Adding 2 to the list.
Adding 3 to the list.
Adding 4 to the list.
Adding 5 to the list.
Element was: 0
Element was: 1
Element was: 2
Element was: 3
Element was: 4
Element was: 5

Study Drills

  1. Take a look at how you used range. Look up the range function to understand it.
  2. Could you have avoided that for-loop entirely on line 22 and just assigned range(0,6) directly to elements?
  3. Find the Python documentation on lists and read about them. What other operations can you do to lists besides append?

Common Student Questions

How do you make a 2-dimensional (2D) list?
That's a list in a list like this: [[1,2,3],[4,5,6]]
Aren't lists and arrays the same thing?
Depends on the language and the implementation. In classic terms a list is very different from an array because of how they're implemented. In Ruby though they call these arrays. In Python they call them lists. Just call these lists for now since that's what Python calls them.
Why is a for-loop able to use a variable that isn't defined yet?
The variable is defined by the for-loop when it starts, initializing it to the current element of the loop iteration, each time through.
Why does for i in range(1, 3): only loop two times instead of three times?
The range() function only does numbers from the first to the last, not including the last. So it stops at two, not three in the above. This turns out to be the most common way to do this kind of loop.
What does elements.append() do?
It simply appends to the end of the list. Open up the Python shell and try a few examples with a list you make. Any time you run into things like this, always try to play with them interactively in the Python shell.

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