Particle-in-cell(localization problem)

I am new in PIC method.
I want to learn this method. I generate few particles in my simulation domain and also have corresponding velocities. For example...X & Y cordinate of each particle and parrale as well as perpendicular velocities of each particle. Now I want to use Boris Scheme for particle advancement.

I have problem in localization. How I can localize my particle w.r.t origin of the simulation domain? I have done like this...

NPART=NO. OF PARTICLE,HZ=SPACE DISCRITIZATION, XP=X CORDINATE OF PARTICLE, WL,WR=WEIGHT FACTOR, L1,L2=PARTICLE DISTANCE FROM NODE AND NODE+1

DO 400 I=1,NPART
KXP=INT(XP(I)/HZ)
L1(I)=((XP(I))-(KXP*HZ))
L2(I)=HZ-L1(I)
WL(I)=L1(I)/HZ
WR(I)=L2(I)/HZ
ENDDO

NOW I WANT TO KNOW HOW I CAN DO LOCALIZATION FOR Y CORDINATE OF THE ELECTRONS?
PLEASE HELP ME...
PLEASE GUIDE ME...
REGARDS