From: Marc Schwartz <marc_schwartz_at_me.com>

Date: Thu, 14 Apr 2011 15:37:29 -0500

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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. Received on Thu 14 Apr 2011 - 20:40:05 GMT

Date: Thu, 14 Apr 2011 15:37:29 -0500

On Apr 14, 2011, at 2:34 PM, Kevin Ummel wrote:

> Take vector x and a subset y:

*>
**> x=1:10
**>
**> y=c(4,5,7,9)
**>
**> For each value in 'x', I want to know how many elements in 'y' are less than 'x'.
**>
**> An example would be:
**>
**> sapply(x,FUN=function(i) {length(which(y<i))})
**> [1] 0 0 0 0 1 2 2 3 3 4
**>
**> But this solution is far too slow when x and y have lengths in the millions.
**>
**> I'm certain an elegant (and computationally efficient) solution exists, but I'm in the weeds at this point.
**>
**> Any help is much appreciated.
**>
**> Kevin
**>
**> University of Manchester
**>
*

I started working on a solution to your problem above and then noted the one below.

Here is one approach to the above:

> colSums(outer(y, x, "<"))

[1] 0 0 0 0 1 2 2 3 3 4

*>
*

> Take two vectors x and y, where y is a subset of x:

*>
**> x=1:10
**>
**> y=c(2,5,6,9)
**>
**> If y is removed from x, the original x values now have a new placement (index) in the resulting vector (new):
**>
**> new=x[-y]
**>
**> index=1:length(new)
**>
**> The challenge is: How can I *quickly* and *efficiently* deduce the new 'index' value directly from the original 'x' value -- using only 'y' as an input?
**>
**> In practice, I have very large matrices containing the 'x' values, and I need to convert them to the corresponding 'index' if the 'y' values are removed.
*

Something like the following might work, if I correctly understand the problem:

> match(x, x[-y])

[1] 1 NA 2 3 NA NA 4 5 NA 6

**HTH,
**
Marc Schwartz

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