Singular

friends,
I'm looking for a singular procedure to return a given monomial's coefficient
of a polynomial.

Example poly f = 2 x ^ 3 + 5z^3;
test (f, x ^ 3) // --> 2
test (f, y ^ 3) // --> 0
who can help me?

There is 'coeffs' command for this.
ring r = 0,(x,y,z),dp;
poly f = 2 x ^ 3 + 5z^3;
> coeffs(f,x);
_[1,1]=-5z3 //coefficient of x^0
_[2,1]=0 // coeff of x^1
_[3,1]=0 // coeff of x^2
_[4,1]=2 // coeff of x^3

If you need to extract a specific entry from this matrix, directly specify the position,
> coeffs(f,x)[4,1]; //For x^3
2

Similarly for other ring variables. It shouldn't be too difficult to convert it into a function.

Another possibility is 'coeffs'
(http://www.singular.uni-kl.de/Manual/4-0-3/sing_271.htm)
and a third one: