reordering elements of a list

[greenflame]

I am trying to reorder elements of a list and I am stuck as to what
might be the best way to approach this. I have a (main) list of
elements and another (ordering) list (which is may shorter,  but not
longer than the main list) which contains the order in which I want the
elements of the main list but only as far along as the length of the
ordering list. This may be confusing so I will try to give an example.

Suppose the main list is: mainlist = list('qwertyuiop')

Suppose the ordering list is: orderinglist = [3, 4, 2, 1]

Then I am looking for a function that will take mainlist and
orderinglist as arguments and return the following list:

['e', 'r', 'w', 'q', 't', 'y', 'u', 'i', 'o', 'p']

Also by the way the main list is always going to be a list of strings
and the ordering list will be a list of numbers. Also the largest
number in orderinglist will always be equal to the length of
orderinglist. I hope this makes any sense. Thanks for your help.