Also to find the position of the element "which" can be used as
pop <- c(3,4,5,7,13)
which(pop==13)
and to find the elements which are not contained in the target vector, one may do this:
pop <- c(1,2,4,6,10)
Tset <- c(2,10,7) # Target set
pop[which(!(pop%in%Tset))]