Is there something wrong with my program?

function diff=poissontest(alpha,n) x=poissrnd(alpha,n); xr=0:ceil(3*alpha); pxsample=hist(x,xr)/n; pxsample=pxsample(:); %pxsample=(countequal(x,xr)/n); px=poissonpmf(alpha,xr); plot(xr,pxsample,xr,px); diff=sum((pxsample-px).^2);