[R] Efficient subsetting

[Jerome Asselin]

Here I have a general solution. x need not be ordered and ranges need not 
be equally spaced.

x <- c(1,1,2,3,4,5)
y <- c(2,3,3,4,5,6)
xcut <- cut(x,breaks=c(1,3,5),right=F)

#If you want the FIRST value of y whose x are in the range
wh <- !duplicated(xcut) & !is.na(xcut)
y[wh]         #   [1] 2 4

#If you want the LAST value of y whose x are in the range
revxcut <- rev(xcut)
wh <- rev(!duplicated(revxcut) & !is.na(revxcut))
y[wh]         #   [1] 3 5

HTH,
Jerome

On May 16, 2003 11:16 am, R A F wrote:
> Content-Length: 1109
> Status: R
> X-Status: N
>
> Hi, I'm facing this problem quite a lot, so it seems worthwhile
> to check to see what the most efficient solution is.
>
> I've two vectors x (values ordered) and y.  I've ranges
> x < x0, x0 <= x < x1, x1 <= x < x2, x2 <= x < x3, x > xn
> and want to construct a subvector yprime of y which consists
> of the first/last value of y whose x values are in the range.
>
> For example,
>
> x   y
> 1   2
> 1   3
> 2   3
> 3   4
> 4   5
> 5   6
>
> and let's say the ranges are 1 <= x < 3 and 3 <= x < 5.  I
> should produce yprime as c( 2, 4 ) (if I ask for the first value
> of y whose x is in the range).  [If there're no x values within
> a given range, output an NA.]
>
> Obviously I can do a loop and use which, etc., but it seems
> like there should be a better way.
>
> Thanks very much.
>
> A general solution would be nice, but if it helps to make the
> algorithm efficient, I'm happy to assume
>
> (a) x values are ordered
> (b) the ranges are always evenly spaced:  for example, x in
> 0 to 10, 10 to 20, 20 to 30, etc.
>
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-- 

Jerome Asselin (Jérôme), Statistical Analyst
British Columbia Centre for Excellence in HIV/AIDS
St. Paul's Hospital, 608 - 1081 Burrard Street
Vancouver, British Columbia, CANADA V6Z 1Y6
Email: jerome at hivnet.ubc.ca
Phone: 604 806-9112   Fax: 604 806-9044