Lists
CMSC 201
Motivation
If we want to average three numbers, we can easily do
the following:
num1 = int(input("Enter a number: "))
num2 = int(input("Enter a number: "))
num3 = int(input("Enter a number: "))
print((num1 + num2 + num3) / 3)
What if we want to average a 100 different numbers? Do
we make a 100 different variables?
Lists
Lists are a way of storing multiple pieces of information in
the same place! They work like this:
myList = [9, 10, 11]
Creates three variables, one that has the value 9, one
with the value 10, and one with the value 11. These are
stored in order!
print(myList[0])
Prints 9, since it is the first thing in the list. myList[1] will
be 10, and myList[2] will be 11.
Lists
Why is this better?
This means that we can refer to items in the list by their
number (or index). The index can be a variable or an
actual number.
a = 10
print(myList[a])
Also, the lists can get as big as we want!
Adding to a List
If you want to add something to the end of a list, simply
call append.
myList = [ 3, 4, 5]
myList.append(6)
print(myList)
prints
[3, 4, 5, 6]
Editing List Contents
If you want to change something in a list, just use the
assignment operator!
myList = [1, 2, 3]
myList[1] = 100
print(myList)
prints
[1, 100, 3]
Remove By Index
If you want to delete something at a certain index, just call
pop(index).
myList = [10, 12, 14]
myList.pop(1)
print(myList)
prints
[10, 14]
Remove By Value
If you want to delete a certain value from a list, use
remove(value)
myList = [10, 12, 14]
myList.remove(14)
print(myList)
prints
[10, 12]
List Contents
Lists can hold any combination of types! This is fine:
myList = [ "hi", True, 10]
We can even put lists inside lists!
myList = [ [1,2,3], [4,5,6], [7,8,9] ]
2D Lists
myList = [ [1,2,3], [4,5,6], [7,8,9] ]
When thinking of 2D lists, think of it as a grid!
1 2 3
4 5 6
7 8 9
To get out the thing at row 1, col 2 you write:
myList[1][2]