You need to parse the output from your C program, or make its output format less complex. The fact that it might not always print a value for k seems like a slightly dubious design, if that's what it's primarily useful for. Also I'd centralize all the printing to one function, and make pline() only calculate and return the desired value (for printing or other use).

The last line of output will be in $s[-1] if that's the only output you are really interested in.