[R] conditional assignment

[Prof Brian Ripley]

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