# [R] Creating a factor from a combination of vectors

Gabor Grothendieck ggrothendieck at myway.com
Tue Nov 30 12:55:15 CET 2004

```Yves Brostaux <brostaux.y <at> fsagx.ac.be> writes:

:
: Dear list,
:
: Here's a little problem I already solved with my own coding style, but I
: feel there is a more efficient and cleaner way to write it, but had no
: success finding the "clever" solution.
:
: I want to produce a factor from a subset of the combination of two
: vectors. I have the vectors a et b in a data-frame :
:
:  > df <- expand.grid(a=c(0, 5, 10, 25, 50), b=c(0, 25, 50, 100, 200))
:  > fac.df
:     a   b
: 1   0   0
: 2   5   0
: 3  10   0
: 4  25   0
: 5  50   0
: 6   0  25
: 7   5  25
: <snip>
:
: and want to create a factor which levels correspond to particular
: combinations of a and b (let's say Low for a=0 & b=0, Medium for a=10 &
: b=50, High for a=50 & b=200, others levels set to NA), reading them from
: a data-frame which describes the desired subset and corresponding levels.
:
: Here's my own solution (inputs are data-frames df and cas, output is the
: sub factor):
:
:  > cas <- as.data.frame(matrix(c(0, 10,50, 0, 50, 200), 3,
: 2,dimnames=list(c("Low", "Medium", "High"), c("a", "b"))))
:  > cas
:         a   b
: Low     0   0
: Medium 10  50
: High   50 200
:
:  > sub <- character(length(df\$a))
:  > for (i in 1:length(df\$a)) {
: +   temp <- rownames(cas)[cas\$a==df\$a[i] & cas\$b==df\$b[i]]
: +   sub[i] <- ifelse(length(temp)>0, temp, NA)
: + }
:  > sub <- ordered(sub, levels=c("Low", "Medium", "High"))
:  > sub
:  [1] Low    <NA>   <NA>   <NA>   <NA>   <NA>   <NA>   <NA>   <NA>
: <NA>   <NA>   <NA>   Medium <NA>   <NA>   <NA>   <NA>
: [18] <NA>   <NA>   <NA>   <NA>   <NA>   <NA>   <NA>   High
: Levels: Low < Medium < High
:
: I was looking for a vectorized solution (apply style) binding
: data-frames df and cas, but didn't succeed avoiding the for loop. Could
: anybody bring me the ligths over the darkness of my ignorance ? Thank
: you very much in advance.
:

Use interaction() and factor() like this:

factor( interaction(df), lev = c("0.0", "10.50", "50.200"),
lab = c("Low", "Medium", "High"), ordered = TRUE)

```