If commands in a while loop

Unfortunately, the while loop in the following code doesn't seem to work. It
would be very nice
if someone might help me, thanks in advance:

z = 0*c(1:1000)

 while (sum(z) < 751){
 for(mu in (900:1100)/10000){
 for(i in(1:1000)) {
	z[i] <- V5[i]*exp(5*mu)
 	
	if (z[i] >= 1276281) {z[i] <- 1}
	else{z[i] <- 0}	
	}
	}
 			   }