Ok. I suspect you are trying to run this code on Simulink in the MATLAB function block. Since you didn't provide this information, so it is difficult to see what the issue is. Simulink use C/C++ code-generation and then compile your model. Therefore, it is necessary to initialize your variables, even if they will get their values later in code execution. Therefore, at the beginning of your function, initialize x, and y
function [x,y] = fcn(u)
x = 0;
y = 0;
Also, use 'if', instead of while as pointed out by KSSV.