Loop and Lists

Loop 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 a list. A list is exactly what its name says—a container of things that are organized in order. It’s not complicated; you just have to learn a new syntax. First, there’s how you make a list:

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

What you do is start the list with the [ (left bracket), which “opens” the list. Then you put each item you want in the list separated by commas, just like when you did function arguments. Lastly you 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.

1 the_count = [1, 2, 3, 4, 5]
2 fruits = ['apples', 'oranges', 'pears', 'apricots']
3 change = [1, 'pennies', 2, 'dimes', 3, 'quarters']
4
5 # this first kind of for- loop goes through a list
6 for number in the_count:
7 print "This is count %d" % number
8
9  # same as above
10 for fruit in fruits:
11 print "A fruit of type: %s" % fruit
12
13 # also we can go through mixed lists too
14 # notice we have to use %r since we don't know what's in it
15 for i in change:
16 print "I got %r" % i
17
18 # we can also build lists, first start with an empty one
19 elements = []
20
21 # then use the range function to do 0 to 5 counts
22 for i in range(0, 6):
23 print "Adding %d to the list." % i
24 # append is a function that lists understand
25 elements.append(i)
26
27 # now we can print them out too
28 for i in elements:
29 print "Element was: %d" % i

Output

$ 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.

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 Question

How do you make a two- 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?

It depends on the language and the implementation. Just call these lists for now, since that’s what Python calls them.

How come a for- loop can use variables that aren’t defined yet?

It defines that variable, 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.