evaluating per-atom vectors

Dear all,

I want to use the values of per-atom vectors at each time step in "compute property/atom" and check them in an if loop afterwards. I have written this part of the script for this regard:

compute ff all property/atom update_on_run_end yes fx fy fz
variable fmag atom sqrt(c_ff[1]^2+c_ff[2]^2+c_ff[3]^2)

How can I access the vector fmag at each time step in the if loop?

Thanks in advance,
Nazanin