Lists

Often programs need to work with large groups of related data. A list is a useful type for storing a sequence of values. You can think of a list as a sequence of boxes, each storing their own value. Here is an example.

0	1	2	3	4
2	3	5	7	11

In Python, we can create this list by enclosing the elements in brackets, separated by commas. Let's set the variable primes to refer to this list.

primes = [2, 3, 5, 7, 11]

Each “box” in the list has its own index, starting from 0, and we can reference a particular “box” using primes[index]. For example, the statement “print(primes[3])” would display 7.

Look familiar? A list is closely related to a string. The biggest difference is that a string holds characters only, whereas lists can hold any type of value at all. While primes holds a list of integers, you could easily have a list of strings, a list of lists, or even a mixture of different types.

In fact, Python is admirably consistent: Everything that we've done with strings, you can also do with a list.

• The len function returns the list length: len(primes) is 5.

• The + operator can add two lists together to create a new list: primes + [13] leads to the list [2, 3, 5, 7, 11, 13] (though primes doesn't change).

• The * operator can multiply an integer by a list to create a new list with several copies of the original: 3 * [0, 1] results in [0, 1, 0, 1, 0, 1].

• Finally, the brackets can pull out an individual item from the list: primes[2] is 5. And you can include a colon to pull out a sub-list of multiple items primes[1:4] is the list [3, 5, 7].

But the list allows some more things, too. Most notably, a list can change. The following example would replace the 2 with 17.

primes[0] = 17

There are several functions helpful for dealing with lists.

code	result
len(x)	the number of entries in x
max(x)	the maximum value in x
min(x)	the minimum value in x
sorted(x)	a sequence with the same entries as x but in increasing order
sum(x)	the sum of all the numbers in x

The following examples illustrate these functions at work.

print(sum(primes))                     # displays 43 (from 17 + 3 + 5 + 7 + 11)
print(min(primes))                     # displays 3

days = ['Mon', 'Tue', 'Wed', 'Thu', 'Fri', 'Sat', 'Sun']
for i in range(len(days)):
  print(days[i])                       # displays each day name on separate line
print(len(days))                       # displays 7
print(len(days[0]))                    # displays 3
days_sort = sorted(days)
print(days_sort[0])                    # displays Fri