R help - Dec 2008 - Extract values based on indexes without looping?

Dear R-Helpers:

I am a entry level user of R.

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]]

> sapply(which.vec, function(x){ value.vec[x] } )
[1] "c" "b" "b" "a"


On Dec 23, 2008, at 9:52 PM, Sean Zhang wrote:
> # 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')
> vector.I.want <- rep(NA,length(which.vec))
> for (i in 1:length(which.vec))
> { vector.I.want[i] <- value.vec[which.vec[i]] }

How about this?
value.vec[which.vec]

HTH,

Jorge


On Tue, Dec 23, 2008 at 9:52 PM, Sean Zhang <seanecon@gmail.com> wrote:
> Dear R-Helpers:
>
> I am a entry level user of R.
>
> 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]]
>
> ______________________________________________
> R-help@r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide
> http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.
>
	[[alternative HTML version deleted]]

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]]
>
> ______________________________________________
> R-help at r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide
http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.
>
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