# Developers Planet

Capsicum February 2016

### 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)
> M
[,1]  [,2] [,3]
[1,]  TRUE  TRUE TRUE
[2,] FALSE FALSE 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.

akrun February 2016

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)
``````