[R] A More efficient method?

Gabor Grothendieck ggrothendieck at gmail.com
Wed Jul 4 17:29:12 CEST 2007

```Here are two ways.  The second way is more than 10x faster.

> set.seed(1)
> C <- sample(c("a", "b"), 100000, replace = TRUE)
> system.time(s1 <- ifelse(C == "a", 1, -1))
user  system elapsed
0.37    0.01    0.38
> system.time(s2 <- 2 * (C == "a") - 1)
user  system elapsed
0.02    0.00    0.02
> identical(s1, s2)
[1] TRUE

On 7/4/07, Keith Alan Chamberlain <Keith.Chamberlain at colorado.edu> wrote:
> 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  1
> Cat
> [1] "a" "a" "a" "b" "b" "b" "a" "a" "b"
>
> Sincerely,
> KeithC.
> Psych Undergrad, CU Boulder (US)
> RE McNair Scholar
>
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