builtins.TypeError: unsupported operand type(s) for -: 'list' and 'int'

Question

builtins.TypeError: unsupported operand type(s) for -: 'list' and 'int'

Why does it give this error? I have a list of list b and I want to subtract c from the last element of the first list of b.

sofar = abs(b[len(b[0])-1] - c)

Answer 1

The last element of the first list in the list-of-lists b is (no need for len(LIST) - 1 , -1 will get the last element just fine):

b[0][-1]

You can see that from:

>>> x = [1,2,3]         # Three lists.
>>> y = [4,5,6]
>>> z = [7,8,9,10,11]
>>> b = [x,y,z]         # List of lists, [[1,2,3],[4,5,6],[7,8,9,10,11]].
>>> b[0][-1]            # Voilà ...
3

And you can modify it with a simple:

>>> b[0][-1] += 10
>>> b
[[1, 2, 13], [4, 5, 6], [7, 8, 9, 10, 11]]
#       ^^
#       Has had 10 added to it.

What you have, b[len(b[0])-1] , is the list b[something] , where something is the index of the last item in b[0] . That's why it's complaining about trying to subtract an integer from a list.