MATLAB Coder Error : Output argument 'b' is not assigned on some execution paths? Help!

Hello guys! I am trying to convert the following code into C using MATLAB Coder and keep getting the same error which makes no sense to me because the code works fine on MATLAB.It keeps telling me 'Output argument 'b' is not assigned on some execution paths'.. What does it mean? Here's my code:-

function [ b ] = dct2C( M,N )

a = [1 2 3 4; 2 3 4 5; 3 4 5 6; 4 5 6 7];
c = [1 2; 3 4]
q_mtx = [16 11; 12 12];

for x = 1:2:M
    for y = 1:2:N

          b = a(x:x+1, y:y+1); 

          b = c*b*c';

          b = b./q_mtx ;

          a(x:x+1, y:y+1) = b; 
      end
  end

end

[Geoff Hayes]

Hiroshi - why do you the following assignment

 a(x:x+1, y:y+1) = b;

but then don't return a?

[Hiroshi Nakamura]

OMG you are right Geoff! I should've returned A~ Nice call there! Thanks!

Answer 1

[Image Analyst]

It's saying that because it doesn't know what you're going to pass in for M and N. What if you passed in -30 and -5000? Would b get assigned? No, because the loop would never execute. Assign b to something just before the loop, like an error condition value like -1 or something.

b = -1;  % Default if the loops never get entered.

[Hiroshi Nakamura]

Ahhhh that makes a lot of sense now.. Thank you! I really appreciate it! :)