Scheme add columns in a matrix

Question

I am trying to write a function that takes a matrix (represented as a list of lists) and adds the elements down the columns and returns a vector (represented as a list):

Example:

    (define sample
      '((2 6 0 4)
        (7 5 1 4)
        (6 0 2 2)))

should return '(15 11 3 10).

I was trying to use the (list-ref) function twice to obtain the first element of each column with no luck. I am trying something like:

(map (lambda (matrix) ((list-ref (list-ref matrix 0) 0)) (+ matrix))

Obligatory reference to [Transpose a matrix in racket (list of lists](http://stackoverflow.com/q/30775032/1281433). Summing the columns just uses **+** instead of **list**. — Joshua Taylor, Oct 10 '15 at 01:58

score 3 · Accepted Answer · answered Oct 10 '15 at 01:35

The solution is simple if we forget about the indexes and think about higher-order procedures, try this:

(define sample
  '((2 6 0 4)
    (7 5 1 4)
    (6 0 2 2)))

(apply map + sample)
=> '(15 11 3 10)

Explanation: map can take multiple lists as arguments. If we apply it to sample (which is a list of lists) and pass + as the procedure to do the mapping, it'll take one element from each list in turn and add them, producing a list with the results - effectively, adding all the columns in the matrix.

Thanks a lot for the help. Was able to reuse this to my needs. — Jordan.J.D, Oct 10 '15 at 02:43

Scheme add columns in a matrix

1 Answers1