"lapply" in R does not work for each element - r

Question

test.data <- data.frame(a=seq(10),b=rep(seq(5),times=2),c=rep(seq(5),each=2))
test.data <- data.frame(lapply(test.data, as.character), stringsAsFactors = F)
test.ref <- data.frame(original=seq(10),name=letters[1:10])
test.ref <- data.frame(lapply(test.ref, as.character), stringsAsFactors = F)
test.match <- function (x) {
result = test.ref$name[which(test.ref$original == x)]
return(result)
}
> data.frame(lapply(test.data, test.match))
a b c
1 a a a
2 b b a
3 c c a
4 d d a
5 e e a
6 f a a
7 g b a
8 h c a
9 i d a
10 j e a
> lapply(test.data, test.match)
$a
[1] "a" "b" "c" "d" "e" "f" "g" "h" "i" "j"
$b
[1] "a" "b" "c" "d" "e"
$c
[1] "a"
Hi all,
I am learning to use the apply family in R. However, I am stuck in a rather simple exercise. Above is my code. I am trying to use the "test.match" function to replace all the elements in "test.data" by the reference rule in "test.ref". However, the last column does not work if I turn the final result into data frame. It is even worse if I keep the result as a list.
Many thanks for your help,
Kevin

Answer 1

As mentioned in the comments, you probably want match:
do.test.match.df <- function(df, ref_df = test.ref){
res <- df
res[] <- lapply(df, function(x) ref_df$name[ match(x, ref_df$original) ])
return(res)
}
do.test.match.df(test.data)
which gives
a b c
1 a a a
2 b b a
3 c c b
4 d d b
5 e e c
6 f a c
7 g b d
8 h c d
9 i d e
10 j e e
This is the idiomatic way. lapply will always return a vanilla list. A data.frame is a special kind of list (a list of column vectors). With res[] <- lapply(df, myfun), we're assigning to columns of res.
Since all your columns are the same class, I'd suggest using a matrix instead of a data.frame.
test.mat <- as.matrix(test.data)
do.test.match <- function(mat, ref_df=test.ref){
res <- matrix(, nrow(mat), ncol(mat))
res[] <- ref_df$name[ match( c(mat), ref_df$original ) ]
return(res)
}
do.test.match(test.mat)