[R] Extract values based on indexes without looping?

[Charles C. Berry]

On Tue, 23 Dec 2008, Sean Zhang wrote:

> Dear R-Helpers:
>
> I am a entry level user of R.
>

You need to read

 	An Introduction to R

particularly this section:

 	2.7 Index vectors; selecting and modifying subsets of a data set

...

 		2. A vector of positive integral quantities

Also, see

 	?Subscript

and run

 	example( Subscript )

HTH,

Chuck



> Have the following question. Many thanks in advance.
>
>
> # value.vec stores values
> value.vec <- c('a','b','c')
> #  which.vec stores the locations/indexs of values in value.vec.
> which.vec <- c(3, 2, 2, 1)
> # How can I obtain the following vector based on the value.vec and which.vec
> mentioned above
> # vector.I.want <- c('c', 'b', 'b', 'a')
> #                          3    2    2    1
>
>
> # I try to avoid using the following loop to achieve the goal because the
> which.vec in reality will be very long
>
> vector.I.want <- rep(NA,length(which.vec))
> for (i in 1:length(which.vec))
> { vector.I.want[i] <- value.vec[which.vec[i]] }
>
> # is there a faster way than looping?
>
> Thanks in advance.
>
> -Sean
>
> 	[[alternative HTML version deleted]]
>
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>

Charles C. Berry                            (858) 534-2098
                                             Dept of Family/Preventive Medicine
E mailto:cberry at tajo.ucsd.edu	            UC San Diego
http://famprevmed.ucsd.edu/faculty/cberry/  La Jolla, San Diego 92093-0901