Idiomatic matrix type conversion, say convert Integer (0/1) matrix to boolean matrix

Question

I have:

 B <- matrix(c(1, 0, 0, 1, 1, 1), 
             nrow=3, 
             ncol=2)

and I want:

 B
      [,1] [,2]
[1,]           


        

Answer 1

A matrix is a vector with dim attributes. When we apply as.logical, the dim attributes are lost. It can be assigned with dim<-

`dim<-`(as.logical(B), dim(B))
#      [,1] [,2]
#[1,]  TRUE TRUE
#[2,] FALSE TRUE
#[3,] FALSE TRUE

---

Or another option is create a logical condition that preserves the attribute structure

B != 0

Or

!!B

Answer 2

mode(B) <- "logical" or "mode<-"(B, "logical"). We can also use storage.mode function.

This workaround is good for two reasons:

1. the code is much readable;
2. the logic works in general (see examples below).

I learnt this trick when reading source code of some packages with compiled code. When passing R data structures to C or FORTRAN functions, some type coercion may be required and they often use mode or storage.mode for this purpose. Both functions preserve attributes of R objects, like "dim" and "dimnames" of matrices.

## an integer matrix
A <- matrix(1:4, nrow = 2, dimnames = list(letters[1:2], LETTERS[1:2]))
#  A B
#a 1 3
#b 2 4

"mode<-"(A, "numeric")
#  A B
#a 1 3
#b 2 4

"mode<-"(A, "logical")
#     A    B
#a TRUE TRUE
#b TRUE TRUE

"mode<-"(A, "chracter")
#  A   B  
#a "1" "3"
#b "2" "4"

"mode<-"(A, "complex")
#     A    B
#a 1+0i 3+0i
#b 2+0i 4+0i

str("mode<-"(A, "list"))  ## matrix list
#List of 4
# $ : int 1
# $ : int 2
# $ : int 3
# $ : int 4
# - attr(*, "dim")= int [1:2] 2 2
# - attr(*, "dimnames")=List of 2
#  ..$ : chr [1:2] "a" "b"
#  ..$ : chr [1:2] "A" "B"

Note that mode changing is only possible between legitimate modes of vectors (see ?vector). There are many modes in R, but only some are allowed for a vector. I covered this in my this answer.

In addition, "factor" is not a vector (see ?vector), so you can't do mode change on a factor variable.

f <- factor(c("a", "b"))

## this "surprisingly" doesn't work
mode(f) <- "character"
#Error in `mode<-`(`*tmp*`, value = "character") : 
#  invalid to change the storage mode of a factor

## this also doesn't work
mode(f) <- "numeric"
#Error in `mode<-`(`*tmp*`, value = "numeric") : 
#  invalid to change the storage mode of a factor

## this does not give any error but also does not change anything
## because a factor variable is internally coded as integer with "factor" class
mode(f) <- "integer"
f
#[1] a b
#Levels: a b