R - Basic understanding of using 'apply' instead of nested loop

Question

So I am new to R (I come from a Python background) and I am still having some issues understanding how/when to implement apply functions (lapply, sapply, rapply, etc) instead of nested loops.

As an example, suppose you wanted to perform some function FUN that compared each element of list to each element of another list. I would write something along the lines of:

n = 1
m = 1
sameList = NULL
for(i in 1:length(list1)){
    for(j in 1:length(list2)){
        if(list1[n]==list2[m]){
            sameList<-c(sameList, list1[n]}
    n = n+1
    }
m = m+1
}

In other words, some nested loop that iterates over every element of each list.

What I am learning is that concatenating a list mid-loop is a very inefficient process in R, which is why apply is used.

So how would apply (or any version of it) be used to replace the above example code?

Answer 1

To use lapply, you would run:

sameList = lapply(list1, function(x) lapply(list2, function(y) if (x==y) x else NULL))