Matlab Error "Attempted to access -- ; index out of bounds because numel--=???.

If my code looks like the following;

time=0; %Initialise time

particleno=9

dt=0.01;

tfinal=40;

diagstep=10;

diagcounter=0;

while time<tfinal

tie=time+dt;

   if ((int8(time/diagstep) * diagstep) == int8(time)) 
       [xcounter] = ParticleCounterFun(x0, particleno);   
       diagcounter=diagcounter+1;
   end

 Xtcounter(diagcounter)=xcounter(diagcounter);

end %End while time<tfinal LOOP

the FUNCTION FILE FOR PARTICLECOUNTERFUN IS;

function [ xcounter ] = ParticleCounterFun( x0, particleno )

%Initialise counters used in E field diagnostics

counter01=0;

counter12=0;

counter23=0;

counter34=0;

counter45=0;

for np=1:particleno
     if (0<=x0(np) & x0(np)<1)
         counter01=counter01+1;
     end
     if (1<=x0(np) & x0(np)<2)
         counter12=counter12+1;
     end
     if (2<=x0(np) & x0(np)<3)
         counter23=counter23+1;
     end
     if (3<=x0(np) & x0(np)<4)
         counter34=counter34+1;
     end
     if (4<=x0(np) & x0(np)<5)
         counter45=counter45+1;
     end

end

xcounter=[counter01 counter12 counter23 counter34 counter45];

end

Anyway i get this;

>> ParticlePusher Attempted to access xcounter(6); index out of bounds because numel(xcounter)=5.

Error in ParticlePusher (line 101) Xtcounter(diagcounter)=xcounter(diagcounter);

Help please!