I want the program to calculate dV(r)/dLnr= V(r)dN(r)/dLnr, where dN/dr = number distribution and I have the "N" in ".txt" files.. How can I write that code? (I know how to define the pathname, in order to open the folder with the txt files).How can I define the "ln" and the "d/dr"? and then I must plot dV(r)/dLnr= V(r)dN(r)/dLnr -Yaxis, radius-Xaxis, x-axis has just numbers from 0.0 to 18.0 um. I wasn't very clear, was I? I'm sorry, but I m realy confused.:( if anyone can give any Ideas, it woyld be very helpful.