Section 9.2 Lists are mutable
The syntax for accessing the elements of a list is the same as for accessing the characters of a string: the bracket operator. The expression inside the brackets specifies the index. Remember that the indices start at 0:
>>> print(cheeses[0])
Cheddar
Unlike strings, lists are mutable because you can change the order of items in a list or reassign an item in a list. When the bracket operator appears on the left side of an assignment, it identifies the element of the list that will be assigned.
The one-th element of numbers, which used to be 123, is now 5.
You can think of a list as a relationship between indices and elements. This relationship is called a mapping; each index “maps to” one of the elements.
List indices work the same way as string indices:
Any integer expression can be used as an index.
If you try to read or write an element that does not exist, you get an IndexError.
If an index has a negative value, it counts backward from the end of the list.
The in operator also works on lists.
Checkpoint 9.2.1.
alist = [4, 2, 8, 6, 5]
alist[2] = True
print(alist)
[4, 2, True, 8, 6, 5]
Item assignment does not insert the new item into the list.
[4, 2, True, 6, 5]
Yes, the value True is placed in the list at index 2. It replaces 8.
Error, it is illegal to assign
Item assignment is allowed with lists. Lists are mutable.
Checkpoint 9.2.2.
values = [3, 2, 1]
values[0] = values[1]
values[2] = values[2] + 1
print(values)
[3, 2, 1]
That is the original contents of values, but the contents are changed.
[2, 0, 2]
When you set values[0] to values[1] it makes a copy of the value and doesn't zero it out.
[2, 2, 2]
The value at index 0 is set to a copy of the value at index 1 and the value at index 2 is incremented.
[2, 2, 1]
Notice that we do change the value at index 2. It is incremented by 1.
Checkpoint 9.2.3.
alist = [3, 67, "cat", [56, 57, "dog"], [ ], 3.14, False]
print(57 in alist)
True
in returns True for top level items only. 57 is in a sublist.
False
Yes, 57 is not a top level item in alist. It is in a sublist.
