[Tutor] Find Elements in List That Equal A Specific Value

[bob gailer]

On 5/12/2010 1:58 PM, Su Chu wrote:
>
> 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.
>
result = []
for n in list3:
   result.append(sum(list1[x] for x in range(len(list1)) if list2[x] = n)

> 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.
>

-- 
Bob Gailer
919-636-4239
Chapel Hill NC