[R] Splicing factors without losing levels

[Ken Knoblauch]

Titus von der Malsburg <malsburg <at> gmail.com> writes:

> An operation that I often need is splicing two vectors:
>   > splice(1:3, 4:6)
>   [1] 1 4 2 5 3 6
> For numeric vectors I use this hack:
>   splice <- function(x, y) {
>     xy <- cbind(x, y)
>     xy <- t(xy)
>     dim(xy) <- length(x) * 2
>     return(xy)
>   }
> So far, so good (?).  But I also need splicing for factors and I tried
> this:
> 
>   splice <- function(x, y) {
>     xy <- cbind(x, y)
>     xy <- t(xy)
>     dim(xy) <- length(x) * 2
>     if (is.factor(x) && is.factor(y)) {
>       xy <- as.factor(xy)
>       levels(xy) <- levels(x)
>     }
>     return(xy)
>   }
> This, however, doesn't work because the level name to integer mapping
> gets mixed up when copying the levels from x to xy.
> Thanks!!
>  Titus

How about something like;:

splice.factor <- function(x, y){
	if (!(is.factor(x) & is.factor(y)))
		stop("Both x and y must be factors")
	if (length(x) != length(y)) 
		stop("Both x and y must have same length")
	lx <- levels(x)
	ly <- levels(y)
	lxy <- union(lx, ly)
	xy <- cbind(levels(x)[x], levels(y)[y])
	xy <- t(xy)
	dim(xy) <- NULL
	xy <- factor(xy, levels = lxy)
	xy
	}

> splice.factor(factor(1:3), factor(4:6))
[1] 1 4 2 5 3 6
Levels: 1 2 3 4 5 6

-- 
Ken Knoblauch
Inserm U846
Stem-cell and Brain Research Institute
Department of Integrative Neurosciences
18 avenue du Doyen Lépine
69500 Bron
France
tel: +33 (0)4 72 91 34 77
fax: +33 (0)4 72 91 34 61
portable: +33 (0)6 84 10 64 10
http://www.sbri.fr/members/kenneth-knoblauch.html