Will output a matrix A obtained by modifying an input matrix B in the following way: 1. If an entry in B is greater than or equal to zero then multiply it by 4 2. If an entry in B is less than 0 then add 4 to it.

This is what I have came up with so far.

function [a] = modify_matrix(B)  
  if B < 0
      a = B + 4; 
  else
      a = B * 4;
  end
  end

The issue I run into is, it just multiplies by 4. If there is anything I need to add or fix to make this equation work? Please help.

Answer 1

[Star Strider]

This is straightforward with ‘logical indexing’:

B = randi([-1 9], 5, 4)                 % Create Matrix To Test Code
a = B;
a(a>=0) = a(a>=0)*4;
a(a<0)  = a(a<0)+4;

and that could be made even more efficient:

lt0 = a<0;
a = a(lt0)+4 + a(~lt0)*4

If you have to use if...else blocks, you also have to use nested loops, and test each element:

a = zeros(size(B));
for k1 = 1:size(B,1)
    for k2 = 1:size(B,2)
        if B(k1,k2) >= 0
            a(k1,k2) = B(k1,k2)*4;
        else
            a(k1,k2) = B(k1,k2)+4;
        end
    end
end

So, it depends on how you want to approach it.

Answer 2

[Image Analyst]

A way similar to the first way Star Strider showed, but slightly different:

function a = modify_matrix(B)  
  a = B + 4; % Initialize.  So far all elements have 4 added.
  % Find indexes >=  0
  positiveIndexes = B >= 0; % A logical vector.
  % Multiply those indexes by 4
  a(positiveIndexes) = 4 * B(positiveIndexes);