[R] Mapping from one vector to another

[William Dunlap]

One way is to use a vector with names to do the mapping:
> mapVector <- c(A=8, B=11, C=3, D=2)
> mapVector[as.character(d$fac)]
 B  B  D  A  C  D  C  B  B  B
11 11  2  8  3  2  3 11 11 11
> # you may want to wrap this with unname()
> d$mappedFac <- mapVector[as.character(d$fac)]
> d
   x  y fac mappedFac
1  1  1   B        11
2  1  2   B        11
3  1  3   D         2
4  1  4   A         8
5  1  5   C         3
6  1  6   D         2
7  1  7   C         3
8  1  8   B        11
9  1  9   B        11
10 1 10   B        11

Bill Dunlap
TIBCO Software
wdunlap tibco.com


On Thu, Jul 17, 2014 at 8:00 AM, Gang Chen <gangchen6 at gmail.com> wrote:
> Suppose I have the following dataframe:
>
> L4 <- LETTERS[1:4]
> fac <- sample(L4, 10, replace = TRUE)
> (d <- data.frame(x = 1, y = 1:10, fac = fac))
>
>      x  y  fac
> 1  1  1   B
> 2  1  2   B
> 3  1  3   D
> 4  1  4   A
> 5  1  5   C
> 6  1  6   D
> 7  1  7   C
> 8  1  8   B
> 9  1  9   B
> 10 1 10   B
>
> I'd like to add another column 'var' that is defined based on the
> following mapping of column 'fac':
>
> A -> 8
> B -> 11
> C -> 3
> D -> 2
>
> How can I achieve this in an elegant way (with a generic approach for
> any length)?
>
> Thanks,
> Gang
>
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