dim(X) must have a positive length when applying function in data frame

Question

I'm trying to apply this function to a data frame column:

best_recom <- function(x,n=1) {
   y <- result2[x,order(-result2[x,])[n]]
   inds = which(result2[x,] == y, arr.ind=TRUE)
   recom <- names(inds[1])
  return(recom)
}

Like this:

apply(last_visit[,2], 1, best_recom)

But I'm getting this error:

dim(X) must have a positive length

I already tried applying it as a matrix like this:

apply(as.matrix(last_visit)[,2],1,recomenda_n_melhor)

But I get the same error. These are the data frames that are used:

result2 - a similarity matrix - this is just a sample

          X1.0      X1.1      X2.1      X3.1
X1.0     0.0000000 0.5000000 0.3872983 0.3162278
X1.1     0.5000000 0.0000000 0.2581989 0.0000000
X2.1     0.3872983 0.2581989 0.0000000 0.0000000
X3.1     0.3162278 0.0000000 0.0000000 0.0000000

last_visit

  customer  cat
1        1 X5.1
2        2 X6.1
3        3 X1.1
4        4 X2.1

Answer 1

It happens because R coerces last_visit[,2] to a dimensionless vector, whereas apply expects the object to have some dimensions. You can prevent the coercion by adding drop=F to your command, i.e.:

apply(last_visit[,2,drop=F], 1, best_recom)

Another way would be just to use lapply or sapply on the vector:

lapply(last_visit[,2], best_recom)