Next-Gen Coders: Lists

What’s an index?

Every item in your list has a place in line. That place is called its index. We use the index to get that item out of the list or change it.

Indexes start at zero. The first item in the list has an index of zero, the second item has an index of one, and so on.

List : [ 1,  2,  3,  4 ]
         ^   ^   ^   ^
Index :  0   1   2   3

To find out what item is stored at a certain index, you put the index in brackets after the variable name. It looks like this:

colors = ["red", "yellow", "blue"]
print (colors[0])

colors = ["red", "yellow", "blue"]
print (colors[1])

You can also use the index to change what’s been stored at a certain index. Here, we’re going to switch “blue” for “turquoise”.

colors = ["red", "yellow", "blue"]
colors[2] = "turquoise"
print (colors)

Now, instead of holding blue, the last item in colors is turquoise.

Quiz

Given this list:

['apple', 'pear', 'mango', 'grape', 'papaya']

• What’s the index of “mango”?

• What’s in the one index?

• What’s the index of “apple”?

Addding items to a list

We can also add items to a list. This is a good thing, too, since a list isn’t much good if we can’t add to it!

To add an item to a list, we use append(). append() is tacked onto the end of the list we want to add to, and we put the item we want to add in the parenthesis. It looks like this:

colors = ["red", "yellow", "blue"]
colors.append("violet")
colors

When you use append(), the item you want to add gets added to the end of the list. What if you wanted to insert an item into a list, though? Let’s say we want to add “orange” to our list, but we want it to go in its proper place: between “red” and “yellow”. To do that, we use insert().

insert() works almost like append(), but it takes two values:

• The index where you want to insert the item

• The item you want to insert

Using insert() looks like this:

mylist.insert(index, new_value)

In order to use insert(), you have to figure out where you want to insert your item. Here’s our list of colors:

['red', 'yellow', 'blue']

We want “orange” to be after “red”. “red” is at index zero, so “orange” should be at index one.

Now that we’ve sorted out where we want to put orange, we can use insert() to update our list:

colors = ["red", "yellow", "blue"]
colors.insert(1, "orange")
colors

Now, “orange” is between “red” and “yellow”, right where it should be!

Removing values from a list

Sometimes, you want to remove an item from a list completely. Maybe you’re done with that task, or you realize you added an item one too many times. To remove an item from a list, we use pop().

To use pop(), we need to know exactly what item we want Python to remove from our list. In order to be super crystal clear about what we want to do, Python needs the index of the item we want to trash. Using pop() looks like this:

colors = ["red", "yellow", "blue", "apple"]
colors.pop(3)
colors

Above, I accidentally added something that wasn’t a color (“apple”), so I removed it by using pop() and its index (which was three).

Quiz

Look at the following code, and try to figure out what the list will look like after it’s run.

mylist = [2, 1, 8, 7]
mylist.append(5)
mylist.pop(1)
mylist.insert(2, 6)

Lists and loops!

Lists are already pretty great. Grouping items is incredibly useful, making it so that you have fewer variables to keep track of as you code. Where they really shine, though, is when you pair them with for loops.

In the loops lesson, we learned about repeating code a block of code a certain number of times by using range(). If we pair a for loop with a list, though, we can run a block of code for every item in the list.

Using a list with a for loop looks like this:

for item in my_list:
    code
    code
    code

Python starts with the first item in the list, runs the code in the code block. Once that code is done, Python does the same thing with the second item in the list, and so on, until it runs out of items.

Let’s use a list with a for loop to see how this works. We’re going to create a list of animals, then talk about how much we like them.

animals = ['dog', 'cat', 'pig', 'goose', 'emu']

for animal in animals:
    print ("I love my", animal)

Run the code, and watch what’s printed out. Every time the for loop runs, Python takes one of the animals and stores it into the loop variable animal, then runs the code in the block. Once that code is run, Python moves on to the next animal, and so on.

Quiz

How many iterations is this for loop going to go through?

nums = [1, 5, 2, 7]
for num in nums:
    print ("*" * num)