Matrix Dimensions Must Agree Error When Adding Polynomials

I have to write a function which adds/subtracts two polynomial coefficient vectors, called

poly_add(p1,p2,operation)

p1 is the first vector

p2 is the second vector

operation is the mode: either 'add' or 'subtract'

The coefficient vectors can be different in size so I have to add zeros accordingly to make them the same length.

This is my code

function p = poly_add( p1, p2, operation )

if length(p1) > length(p2) 

      num_zeros = length(p1) - length(p2);
      p2 = [zeros(1,num_zeros),p2];

elseif length(p2) > length(p1)

      num_zeros = length(p2) - length(p1);
      p1 = [zeros(1,num_zeros),p1];

end

if operation == 'add'

      p = p1 + p2;

elseif operation == 'subtract'

      p = p1 - p2;

end
end

Addition works fine but when I try to subtract I get this error:

??? Error using ==> eq
Matrix dimensions must agree.

Error in ==> poly_add at 4
    if operation == 'add'

I don't see why this happens. When I just try to do it step by step in command window, it works fine.

Answer 1

[Pulkit Goel]

This is happening because while comparing 'subtract' to 'add', MATLAB is looking for same length of vectors to compare. While adding, since your operation is 'add' which is of same length, there was no error thrown.

You can try using isequal to compare

if isequal(operation,'add')
    %your logic
elseif isequal(operation,'subtract')
    %yourlogic
end

Or else you may change 'subtract' to 'sub' which may again error out if operation input is not the same size.