pass vectors to function in R

Question

I have a vector of POS tags. And I want to create a global bigram vector of the POS tags. I have created the following function in R:

createbi<-function(a) {
  for(i in 1:(length(a)-1)){
    bigramlist<-c(bigramlist,(paste(a[i], a[i+1], sep=" ")))
  }
}

here in ai want to pass a list of tags :

> a$POStags
 [1] "DT"  "VBZ" "DT"  "JJ"  "NN"  "VBG" "IN"  "DT"  "NNS" ","   "NNS" ","   "CC"  "NNS" "." 

but I am not getting the bigrams in the bigramlist.I think the problem is with parameter passing. I want output as :

> bigramlist
 [1] "DT VBZ" "VBZ DT" "DT JJ"  "JJ NN"  "NN VBG" "VBG IN" "IN DT"  "DT NNS" "NNS ,"  ", NNS"  "NNS ,"  ", CC"   "CC NNS"
[14] "NNS .

can someone help me with this I am new to R

Answer 1

Here are three different approaches to your problem ordered from most to least preferable (in my opinion):

a) paste with head/tail

An alternative, perhaps more R-ish way to do it where x denotes your vector a$POStags , is to use paste directly on two subsets of x :

paste(head(x, -1), tail(x, -1))
#[1] "DT VBZ" "VBZ DT" "DT JJ"  "JJ NN"  "NN VBG" "VBG IN" "IN DT"  "DT NNS" "NNS ,"  ", NNS"  "NNS ," 
#[12] ", CC"   "CC NNS" "NNS ." 

Of course you can put that in a function too:

createbi <- function(a) paste(head(a, -1), tail(a, -1))

b) sapply

Alternatively you could use an sapply loop as follows (but I would prefer the first approach):

createbi2 <- function(a) sapply(1:(length(a)-1), function(i) paste(a[i], a[i+1]))

c) for loop

If you insist on a for loop, I'd do it like this (notice preallocation of bigramlist ):

createbi <- function(a) {
  n <- length(a)-1
  bigramlist <- character(n)
  for(i in 1:n){
    bigramlist[i] <- paste(a[i], a[i+1])
  }
  bigramlist
}

But you see by now how much simpler approach a) is than writing the for loop.