On 05/13/10 03:58, Su Chu wrote: > Hi there, > > I am new to Python. I am attempting to either define a "which" statement or > to find a method that already exists to do this sort of operation. > > My problem is as follows: > I have three lists, one with unique values (list 1), one a sequence of > values that are not necessarily unique (list2), and a shorter list with the > unique values of list 2 (list 3). List 1 and List 2 are of equal lengths. > > > An example: > list1 = [ 1, 2, 3, 4, 5, 6 ] > list2 = [ 2, 2, 2, 5, 6, 6 ] > list3 = [2, 5, 6] > > What I would like to do is find and sum the elements of list 1 given its > corresponding element in list 2 is equal to some element in list 3. > > For example, > the sum of the values in list1 given list2[i]==2 > would be 1 + 2 + 3 = 6. > the sum of the values in list1 given list2[i]==5 > would be 4 > the sum of the values in list1 given list2[i]==6 > would be 5 + 6 = 11 > > and so on. Obtaining these values, I'd like to store them in a vector. > > This seems pretty simple if a 'which' statement exists e.g. (which values in > list 1 == list3[k], and looping through k), but I can't find one. To write > one seems to require a loop.
your proposed 'which' statement, had they existed, is a kind of loop, so I'm wondering why you want to avoid loops. >>> [sum(a for a, b in zip(list1, list2) if b == n) for n in list3] [6, 4, 11] _______________________________________________ Tutor maillist - Tutor@python.org To unsubscribe or change subscription options: http://mail.python.org/mailman/listinfo/tutor
