[R] Vector indexing question

[Marc Schwartz]

On Thu, 2007-03-29 at 19:55 -0400, Paul Lynch wrote:
> Suppose you have 4 related vectors:
> 
> a.id<-c(1:25, 1:25, 1:25)
> a.vals <- c(101:175)        # same length as a.id (the values for those IDs)
> a.id.levels <- c(1:25)
> a.id.ratings <- rep(letters[1:5], times=5)    # same length as a.id.levels
> 
> What I would like to do is specify a rating from a.ratings (e.g. "e"),
> get the vector of corresponding IDs from a.id.levels (via
> a.id.levels[a.id.ratings=='e']) and then somehow use those IDs in a.id
> to get the corresponding values from a.vals.
> 
> I think I can probably write a loop to construct of a vector of
> ratings of the same length as a.id so that the ratings match the ID,
> and then go from there.  Is there a better way?  Perhaps using factors
> or levels or something?
> 
> Thanks,
>       --Paul

Is this what you want?

DF <- data.frame(a.id, a.vals, a.id.levels, a.id.ratings)

> DF[DF$a.id.ratings == "e", "a.vals"]
 [1] 105 110 115 120 125 130 135 140 145 150 155 160 165 170 175

or

> subset(DF, a.id.ratings == "e", select = a.vals)
   a.vals
5     105
10    110
15    115
20    120
25    125
30    130
35    135
40    140
45    145
50    150
55    155
60    160
65    165
70    170
75    175

See ?subset

HTH,

Marc Schwartz