Neat conditional assignment
On Dec 10, 2007 6:52 PM, Gabor Grothendieck <ggrothendieck at gmail.com> wrote:
With this specific example this will work: as.numeric(factor(a)) If that is not the real case but its something else you could try any of these which probably generalize better: (a == "A") + 2 * (a == "B") 1 + (a == "B") ifelse(a == "A", 1, 2)
Just in case this wasn't clear:
a<-rep(c("A","B"),50)
b<-rep(1,100)
c<-rep(2,100)
b * (a == "A") + c * (a == "B")
[1] 1 2 1 2 1 2 1 2 1 2 1 2 1 2 1 2 1 2 1 2 1 2 1 2 1 2 1 2 1 2 1 2 1 2 1 2 1 [38] 2 1 2 1 2 1 2 1 2 1 2 1 2 1 2 1 2 1 2 1 2 1 2 1 2 1 2 1 2 1 2 1 2 1 2 1 2 [75] 1 2 1 2 1 2 1 2 1 2 1 2 1 2 1 2 1 2 1 2 1 2 1 2 1 2
b + (c-b) * (a == "B")
[1] 1 2 1 2 1 2 1 2 1 2 1 2 1 2 1 2 1 2 1 2 1 2 1 2 1 2 1 2 1 2 1 2 1 2 1 2 1 [38] 2 1 2 1 2 1 2 1 2 1 2 1 2 1 2 1 2 1 2 1 2 1 2 1 2 1 2 1 2 1 2 1 2 1 2 1 2 [75] 1 2 1 2 1 2 1 2 1 2 1 2 1 2 1 2 1 2 1 2 1 2 1 2 1 2
ifelse(a == "A", b, c)
[1] 1 2 1 2 1 2 1 2 1 2 1 2 1 2 1 2 1 2 1 2 1 2 1 2 1 2 1 2 1 2 1 2 1 2 1 2 1 [38] 2 1 2 1 2 1 2 1 2 1 2 1 2 1 2 1 2 1 2 1 2 1 2 1 2 1 2 1 2 1 2 1 2 1 2 1 2 [75] 1 2 1 2 1 2 1 2 1 2 1 2 1 2 1 2 1 2 1 2 1 2 1 2 1 2
On Dec 10, 2007 6:41 PM, Neil Stewart <neil.stewart at warwick.ac.uk> 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
Thanks in advance,
Neil
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