Capsicum Capsicum - 11 days ago 5
R Question

R: Index matrix row-wise in vectors

I have a vector containing only character elements, say

v <- c("A","B","C")


and a matrix containing only logical elements, with width equal to the length of
v
and arbitrary length:

> M <- matrix(c(TRUE,FALSE,TRUE,FALSE,TRUE,TRUE),ncol=3,byrow=TRUE)
> M
[,1] [,2] [,3]
[1,] TRUE FALSE TRUE
[2,] FALSE TRUE TRUE


Now I would like to index each row of
M
into
v
, collapse, and obtain a vector
r
each element of which contains the corresponding row of
M
in one character expression. In the example given, the elements of
r
would be

> r
"A C" "B C"


I can do this for each row separately (or within a loop), using

r[i] <- paste(v[as.logical(M[i,])], collapse="")


but hoped there would be a more efficient solution that deals with the full matrix at once.

Answer

We can use apply with MARGIN=1

apply(M, 1, function(x) paste(v[x], collapse=' '))
#[1] "A C" "B C"

data

 M <- matrix(c(TRUE,FALSE,TRUE,FALSE,TRUE,TRUE),ncol=3, byrow=TRUE)
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