conditional assignment
On Mon, 26 Jan 2004, Simon Cullen wrote:
On Mon, 26 Jan 2004 20:15:51 +0100, <uaca at alumni.uv.es> wrote:
I want to conditionally operate on certain elements of a matrix, let me explain it with a simple vector example
z<- c(1, 2, 3) zz <- c(0,0,0) null <- (z > 2) & ( zz <- z) zz
[1] 1 2 3 why zz is not (0, 0, 3) ?????
<snip>
in the other hand, it curious that null has reasonable values
null
[1] FALSE FALSE TRUE
What you have done there is create a boolean vector, null, of the same length as z (and zz). For instance: (z > 2) & (zz <- z) =(F F T) & (T T T) (as assignment - presumably - returns T)
assignment returns the new value of zz, which as it is all non-zero coerces to the logical vector T T T
=(F F T). What will work is: z <- c(1, 2, 3) index <- z>2 zz <- z * index
Rather better I think is zz <- ifelse(z > 2, z, zz) or even ind <- z > 2 zz[ind] <- z[ind]
Brian D. Ripley, ripley at stats.ox.ac.uk Professor of Applied Statistics, http://www.stats.ox.ac.uk/~ripley/ University of Oxford, Tel: +44 1865 272861 (self) 1 South Parks Road, +44 1865 272866 (PA) Oxford OX1 3TG, UK Fax: +44 1865 272595