Negative indexing in Python [duplicate]

Answer 1

this is your input

In [73]: bob
Out[73]: ['bob smith', 42, 30000, 'software']

this is what bob[0] gives you

In [74]: bob[0]
Out[74]: 'bob smith'

as you can see bob[0] has only two elements so 1 give you second element and -1 gives you last element which is same

Answer 2

Because in this case the list you are splitting is ['adam', 'smith']. So, bob[0].split()[1] would return the 2nd element (don't forget list indexes are 0-based) and bob[0].split()[-1] would return that last element.

Since the size of the list is 2, the second (index 1) and last (index -1) are the same.

In general if you have a list my_list, then my_list[len(my_list) - 1] == my_list[-1]

Answer 3

Positive indexes count from the start of the list, negative indexes count from the end of the list.

That is:

bob[0].split()[0] == 'bob'
bob[0].split()[1] == 'smith'
bob[0].split()[-1] == 'smith'
bob[0].split()[-2] == 'bob'

Now say you have someone with a middle name:

jane =['jane elizabeth smith', 42, 30000, 'software']

jane[0].split()[1] would give Jane's middle name 'elizabeth', while jane[0].split()[-1] would give her last name 'smith'

Now having said all this;

• do not assume a name is made up of two components
• do not assume a name is of the form <first name> <last name>
• do not assume a first name is one word
• do not assume a last name is one word
• do not assume anything!!!

For a more exhaustive list of things you might be wrong about see Falsehoods Programmers Believe About Names

Answer 4

Python lists can be "back indexed" using negative indices. -1 signifies the last element, -2 signifies the second to last and so on. It just so happens that in your list of length 2, the last element is also the element at index 1.

Your book suggests using -1 because it is more appropriate from a logical standpoint. You don't want the item at index 1 per se, but rather the last element of the list, which is the last name. What if, for example, a middle name was also included? Then using an index of 1 would not work whereas an index of -1 would.