for loop vector multiplication

i have this assigment:

Create a function that computes the cost of each item and returns it as a vector. Find an elegant and simple way to write the code using what you have learned about matrix and vector operations.

here's what i have come up with so far which is a not so elegant solution and only works for a specific size of matrix. in my script it has have more than 1 column and less than 4 columns:

function itemCost=computeItemCost(resourceItemMatrix,resourceCost)
if length(resourceItemMatrix)>1
    A=sum(resourceItemMatrix(:,1).*reshape(resourceCost,length(resourceCost),1));
    itemCost=A;
    if length(resourceItemMatrix)>1
        B=sum(resourceItemMatrix(:,2).*reshape(resourceCost,length(resourceCost),1));
        C=sum(resourceItemMatrix(:,3).*reshape(resourceCost,length(resourceCost),1));
        itemCost=[A,B,C];
        if length(resourceItemMatrix)>3
            D=sum(resourceItemMatrix(:,4).*reshape(resourceCost,length(resourceCost),1));
            itemCost=[A,B,C,D];
        end
    end
end
end

I know there is a much simpler way to do this(perhaps a for-loop?), but just keep running my head against a wall when i try. help would be much appreciated.

[Jan]

"less than 4 columns"? 4 columns are accepted also, but 2 columns are not.

Answer 1

[Jan]

Do you mean:

function itemCost = computeItemCost(resourceItemMatrix, resourceCost)
itemCost = resourceCost(:).' * resourceItemMatrix;
end

You check for length(resourceItemMatrix)>1 twice in your code. If resourceItemMatrix is a matrix, length() might not do what you expect. Prefer:

if size(resourceItemMatrix, 2) > 1

A simplification of your original code:

function itemCost = computeItemCost(resourceItemMatrix, resourceCost)
switch size(resourceItemMatrix, 2)
   case {0, 2}
      error('Unexpected input.'); % ???
   case {1, 3, 4}
      itemCost = resourceCost(:).' * resourceItemMatrix;
   otherwise
      itemCost = resourceCost(:).' * resourceItemMatrix(:, 1:4);
end
end

Or according to the text of the question:

function itemCost = computeItemCost(resourceItemMatrix, resourceCost)
n = size(resourceItemMatrix, 2);
if n >= 1 && n <= 4
  itemCost = resourceCost(:).' * resourceItemMatrix;
else
  error('Unexpected input.');  % Or whatever you need.
end
end