3.2. Indexing

We can access the items of a list using indexing. This means that we can give Python a number, and it will retrieve the item sitting at that location.

Here is the syntax used for indexing:

list[index].

Now unlike us humans who start counting at 1,

Python indexing starts at 0!

This isn’t unique to Python. Many other languages use 0 indexing.

Below we show you the index corresponding to each item in the list months.

# index:    0      1      2      3      4      5      6      7      8       9     10      11
months = ['Jan', 'Feb', 'Mar', 'Apr', 'May', 'Jun', 'Jul', 'Aug', 'Sept', 'Oct', 'Nov', 'Dec']
print(months[1])

Feb

Since Python starts indexing at 0, months[0] will give us the first month and month[1] will gives us the second month and so on.

We can also use indexing to extract a range of items. We use the square brackets [] as before and we also specify the start index followed by the end index.

list[start:end]

Note:

• The start index is inclusive

• The end index is exclusive

Here is an example. We start at month 2 (March) and go up to, but not including, month 7 (August).

# index:    0      1      2      3      4      5      6      7      8       9     10      11
months = ['Jan', 'Feb', 'Mar', 'Apr', 'May', 'Jun', 'Jul', 'Aug', 'Sept', 'Oct', 'Nov', 'Dec']
print(months[2:7])

['Mar', 'Apr', 'May', 'Jun', 'Jul']

If you omit the start index, you will automatically start at index 0.

# index:    0      1      2      3      4      5      6      7      8       9     10      11
months = ['Jan', 'Feb', 'Mar', 'Apr', 'May', 'Jun', 'Jul', 'Aug', 'Sept', 'Oct', 'Nov', 'Dec']
print(months[:7])

['Jan', 'Feb', 'Mar', 'Apr', 'May', 'Jun', 'Jul']

If you omit the end index, you will automatically end at the last item in the list. Here is an example.

# index:    0      1      2      3      4      5      6      7      8       9     10      11
months = ['Jan', 'Feb', 'Mar', 'Apr', 'May', 'Jun', 'Jul', 'Aug', 'Sept', 'Oct', 'Nov', 'Dec']
print(months[5:])

['Jun', 'Jul', 'Aug', 'Sept', 'Oct', 'Nov', 'Dec']

You can also index from the end of the list by using negative indices. Here is an example.

# negative index:
#          -12    -11    -10    -9     -8     -7     -6     -5     -4      -3     -2     -1
months = ['Jan', 'Feb', 'Mar', 'Apr', 'May', 'Jun', 'Jul', 'Aug', 'Sept', 'Oct', 'Nov', 'Dec']
print(months[-3])

Oct

Note: the last item in the list starts at -1 whereas the first item in the list starts at 0.

3.2.1. Index Errors

If you try to access an index that does not exist you will get an IndexError. See what happens when you run the following code.

# index:    0      1      2      3      4      5      6      7      8       9     10      11
months = ['Jan', 'Feb', 'Mar', 'Apr', 'May', 'Jun', 'Jul', 'Aug', 'Sept', 'Oct', 'Nov', 'Dec']
print(months[12])

Traceback (most recent call last):
  File "/home/main.py", line 3, in <module>
      print(months[12])
            ~~~~~~^^^^
  IndexError: list index out of range

This is a common error as often we forget to account for the fact that Python indexing starts at 0.

Question 1

What do you think the output of the following code will be?

days = ['Mon', 'Tue', 'Wed', 'Thur', 'Fri', 'Sat', 'Sun']
print(days[3])

Solution

Thur

days[3] will print the fourth item in the list, which is 'Thur'.

Question 2

Consider the following list.

days = ['Mon', 'Tue', 'Wed', 'Thur', 'Fri', 'Sat', 'Sun']

Which would give the following output:

['Tue', 'Wed']

1. print(days[1:2])
   

2. print(days[1:3])
   

3. print(days[:2])
   

4. print(days[2])
   

5. print(days[-2])
   

Solution

Solution is locked

Question 3

What do you think the output of the following code will be?

days = ['Mon', 'Tue', 'Wed', 'Thur', 'Fri', 'Sat', 'Sun']
print(days[:3])

Solution

Solution is locked

Question 4

What do you think the output of the following code will be?

days = ['Mon', 'Tue', 'Wed', 'Thur', 'Fri', 'Sat', 'Sun']
print(days[-4:])

olution

Solution is locked

Question 5

Consider the following list.

days = ['Mon', 'Tue', 'Wed', 'Thur', 'Fri', 'Sat', 'Sun']

Which of the following will correspond to Saturday? Select all that apply.

days = ['Mon', 'Tue', 'Wed', 'Thur', 'Fri', 'Sat', 'Sun']
print(days[-4:])

1. days[-2]
   

2. days[5:6]
   

3. days[-2:-1]
   

4. days[5]
   

Solution

Solution is locked

Code challenge: Item Number

You have been provided with a shopping list.

shopping_list = ['carrots', 'avocado', 'chocolate', 'toothpaste', 'tomatoes']

Write a program that allows the user to check an item at a specified index in the shopping list.

Here are some examples of how your code should run.

Example 1

Enter an index: 0
You need to buy carrots

Example 2

Enter an index: 2
You need to buy chocolate

Solution

Solution is locked

Code challenge: What Month Is It?

You have been given the following list to start with.

months = ['January', 'February', 'March', 'April', 'May', 'June', 'July', 'August', 'September', 'October', 'November', 'December']

Write a program that asks the user for a month as an integer and then displays the name of the month.

Example 1

Enter a month: 1
January

Example 2

Enter a month: 6
June

Solution

Solution is locked