r - Matching a sequence in a larger vector

Question

I'd like to have a function which returns the initial indicies of matching subsequences of a vector. For example:

y <- c("a","a","a","b","c")

multi_match(c("a","a"), y)
# [1] 1 2

multi_match(c("a","b"), y)
# [1] 3

I have a rough implementation, but I feel like I must be reinventing the wheel, and it's a little clunky. Is there a better way to implement this, or is there a pre-existing function somewhere with similar functionality?

multi_match <- function(x, table){
    # returns initial indicies of all substrings in table which match x
    if(length(table) < length(x)){
        return(NA)
    }else{
        check_mat <- matrix(nrow = length(x), ncol = length(table))
        for(i in 1:length(x)){
            check_mat[i,] <- table %in% x[i]
        }
        out <- vector(length = length(table))
        for(i in 1:(length(table)-(length(x)-1))){
            check <- vector(length=length(x))
            for(j in 1:length(x)){
                check[j] <- check_mat[j,(i+(j-1))]
            }
            out[i] <- all(check)
        }
        if(length(which(out))==0){
            return(NA)
        }else{
            return(which(out))
        }
    }
}

Answer 1

Try rollapply in zoo:

library(zoo)
which(rollapply(y, 2, identical, c("a", "a")))
## [1] 1 2
which(rollapply(y, 2, identical, c("a", "b")))
## [1] 3