Quick way for assigning a value to each member of a group in R

I need to attribute a weight to each member of a group.
I have made the following loop to create the weights_ind vector (vector of individual weights), but my loop is too slow (2 seconds for 2000 individuals) :

# Data example
   n.group = 10
   n.tot = 1000
   groups = sample(1:10, n.tot, replace = TRUE)
   weights_by_group = rexp(n.group, rate = 1)
   weights_ind = rep(NA, n.tot)
     for(i in 1:n.tot){
     for(j in 1:n.group) {
     if(groups[i] == j) 
       weights_ind[i] = weights_by_group[j]
     }}

Which faster function may I use in my case?

As the groups are numbered starting at 1, you can use the groups vector to index the weights_by_groupvector.

weights_ind = weights_by_group[groups]

On my computer, for 2000 individual, this takes about 10 microseconds.

If your groups were not numeric, you could name the weights_by_group vector with the group identifier, and still use the groups to index:

groups = sample(letters[1:n.group], n.tot, replace = TRUE)
weights_by_group = rexp(n.group, rate = 1)
names(weights_by_group) = letters[1:n.group] 
weights_ind = weights_by_group[groups]