[R] Neat conditional assignment
Achim Zeileis
Achim.Zeileis at wu-wien.ac.at
Tue Dec 11 00:54:03 CET 2007
On Mon, 10 Dec 2007, Neil Stewart wrote:
> I would like to make a new vector with values taken from two existing
> vectors conditional upon a third. At present I am doing this with a for loop
> but wonder if there is not a neater way, given some of the very neat things
> R can do.
>
> a<-rep(c("A","B"),50)
> b<-rep(1,100)
> c<-rep(2,100)
>
> a is thus "A" "B" "A" "B" "A" "B"...
> b is thus 1 1 1 1 1 1 ...
> c is thus 2 2 2 2 2 2 ...
>
> I want d[i] to be b[i] if a[i]=="A" and c[i] if a[i]=="B". I'm current using
> a for loop:
>
> d<-rep(0,100) # initialise d
> for(i in 1:length(a)) {if(a[i]=="A") d[i]<-b[i] else d[i]<-c[i]}
>
> d is thus 1 2 1 2 1 2 1 2 1 ...
>
> Is it possible to do something simpler, say along the lines of the c-style
> ?: conditional statement, or at least avoiding the for loop.
>
> d <- a=="A"?b:c # doesn't work, but you get the idea
d <- ifelse(a == "A", b, c)
is what you want!
Z
> Thanks in advance,
> Neil
>
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