apply function in R - r

Question

I want to apply a user defined function over a matrix object.
I don't have desired results my input is a 4x4 matrix and I want to get as output a 4x4 matrix with the transformation defined in mapfun function
Where is my error?
Thanks in advance
mapfun <- function(val){
if (val == 1){
res <- "A" }
else{
if (val == 2){
res <- "B"
} else
{
if (val == 3){
res <- "C"
} else
{
res <- "D"
}
}
}
return(res)
}
mat1 <- matrix(sample(c(1,2,3,4), 16, replace=T, prob=c(0.25,0.25,0.25,0.25)),
nrow=4, ncol=4)
mat2 <- apply(mat1, 1, FUN=mapfun)`

Answer 1

We can just the numeric values in 'mat1' as index to replace the corresponding 'LETTERS'. The output will be a vector which can be converted back to a matrix by assigning dim.
`dim<-`(LETTERS[mat1], dim(mat1))
Regarding the Warning message in the mapfun, it would be better to use ifelse instead of if/else as we are dealing with a vector of length greater than 1 in each row of 'mat1'.
mapfun <- function(val){
ifelse(val == 1, 'A',
ifelse(val==2, 'B',
ifelse(val==3, 'C', 'D')))
}
apply(mat1, 1, mapfun)

Answer 2

This could probably work:
mat1[] <- mapply(mapfun,mat1)
Note that this will modify mat1, so you could make a copy of mat1 named mat2 and apply the function to mat2:
mat2<- mat1
mat2[] <- mapply(mapfun,mat1)

Answer 3

You could try this (fun) solution as well. First you coerce input value to a character vector and then use repeatedly replace function and return the result as a matrix again. Maybe there is a way to trick replace function so that it would be used only once.
mapfun <- function(val) {
res <- as.character(val)
res <- replace(x = res, list = which(res == "1"), values = "A")
res <- replace(x = res, list = which(res == "2"), values = "B")
res <- replace(x = res, list = which(res == "3"), values = "C")
res <- replace(x = res, list = which(res == "4"), values = "D")
return(matrix(res, ncol = 4)) # ncol(val)
}
mat1 <- matrix(sample(c(1,2,3,4), 16, replace=T, prob=c(0.25,0.25,0.25,0.25)),
nrow=4, ncol=4)
mat1
mapfun(mat1)