# [R] A More efficient method?

S Ellison S.Ellison at lgc.co.uk
Wed Jul 4 17:57:35 CEST 2007

```#Given
Cat=c('a','a','a','b','b','b','a','a','b')	# Categorical variable

#and defining
coding<-array(c(-1,1), dimnames=list(unique(Cat) ))

#(ie an array of values corresponding to your character array levels, and with names set to those levels)

coding[Cat]

#does what you want.

>>> Keith Alan Chamberlain <Keith.Chamberlain at Colorado.EDU> 04/07/2007 14:44:44 >>>
Dear Rhelpers,

Is there a faster way than below to set a vector based on values from
another vector? I'd like to call a pre-existing function for this, but one
which can also handle an arbitrarily large number of categories. Any ideas?

Cat=c('a','a','a','b','b','b','a','a','b')	# Categorical variable
C1=vector(length=length(Cat))	# New vector for numeric values

# Cycle through each column and set C1 to corresponding value of Cat.
for(i in 1:length(C1)){
if(Cat[i]=='a') C1[i]=-1 else C1[i]=1
}

C1
 -1 -1 -1  1  1  1 -1 -1  1
Cat
 "a" "a" "a" "b" "b" "b" "a" "a" "b"

Sincerely,
KeithC.
RE McNair Scholar

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