Summing nonsequential elements in a matrix

Sorry if this is a very simple question, but how would I go about finding the sum of a set of predetermined elements in a matrix. So if I was using

a=magic(50),

I found that my idea of using the command

x = sum(a, (1, [5 7 19 33 34 35 36 47 50]))

does not provide the output (the sum of the elements in those positions) that I wanted. I would appreciate any help.

Answer 1

[Cedric]

magic(50) is a square, 50x50 matrix. What do you want to achieve? Is it summing elements [5 7 19 33 34 35 36 47 50] of row 1 of a? If so, you'll want to do

 s = sum( a(1,[5 7 19 33 34 35 36 47 50]) )

If you are unsure, work with smaller objects that can be easily displayed, e.g.

 >> a = magic(5)
 a =
    17    24     1     8    15
    23     5     7    14    16
     4     6    13    20    22
    10    12    19    21     3
    11    18    25     2     9

 >> s = sum( a, (1, [2,4,5]))        % First idea, is it right?
 s = sum( a, (1, [2,4,5]))
              |
 Error: Expression or statement is incorrect--possibly unbalanced (, {, or [.

 >> a(1,[2,4,5])                     % Let's see if we can at least access the 
                                     % correct set of elements before summing.
 ans =
    24     8    15

 >> s = sum( a(1,[2,4,5]) )          % Let's see if we can sum them now.
 s =
    47

[AJ]

Thank you both for your answers. It was a silly error on my part

Answer 2

[Thomas]

works just fine

a=magic(5)
a =
         17.00         24.00          1.00          8.00         15.00
         23.00          5.00          7.00         14.00         16.00
          4.00          6.00         13.00         20.00         22.00
         10.00         12.00         19.00         21.00          3.00
         11.00         18.00         25.00          2.00          9.00

 sum(a(1,[3 5]))
 ans =
         16.00

which is the sum of a(1,3) =1 and a(1,5)=15 , total=16