Creating a vector of numbers based on letters

So, this is the question:

"Create a function that given one word, return the position of word’s letters on letters vector. For example, if the word is ‘abba’, the function will return 1 2 2 1."

What I have so far is this:

l <- function(word) {
    chr <- c()
    y <- c()
    strsplit(chr,word)
    i<-1
    while(i<length) {
           o<-letters[i]
           x<-chr[i]
           if(o==x) {
                    y[i]<-i
           }
           i+1
    }
    y
}

I have tried running l("hello") and it returns NULL. I'm very lost and would appreciate any help! Thank you!