I'd recommend using atan2 instead of dividing. If you had to divide, use element-wise division (the ./ operator) instead of matrix division (the / operator.)
[x,y]=meshgrid(0:0.1:5,0:0.1:5);
u=y;
v1 = atan2(y, x);
v2 = atan(y./x);
The elements of v1 and v2 match except where v2 contains a NaN value (where both x and y are equal to 0.)
[v1(1), v2(1)]
v2(1) = 0;
isequal(v1, v2)
