[R] Selecting values based on two criteria

[Boris Steipe]

Try this:

d1 <- c(135631,136950,137952,138787,139623,142231,143067,144762,
       145601,146441)

d2 <- c(135882,136954,137956,138792,139630,140569,141398,142237,
       143078,143907,144771,145611,146446,147285,148128)

len <- length(d1)

# concatenate the two vectors
mrg <- c(d1,d2)

# order the concatenated vector
ord <- order(mrg)


# Look at the result: the indices > len (10) are numbers from d2.
ord   # [1]  1 11  2 12  3 13  4 14  5 15 16 17  6 18  7 19 20  8 21  9 22 10 23 24 25

# Thus you are looking for values in ord that are > len,
# and immediately follow a value that is <= len.

# Initialize a result vector (you could also overwrite d1).
d4 <- rep(NULL, len)

# A counting index ...
ind <- 0

# Slowly, element by element. If your vectors are very large you might
# use apply.

for (i in 1:length(ord)) {
    if (ord[i] > len && ord[i-1] <= len) {
        ind <- ind + 1
        d4[ind] <- mrg[ord[i]]
    }
}

# your d3:
d3 <- c(135882,136954,137956,138792,139630,142237,143078,144771,
       145611,146446)

identical(d3, d4)
# TRUE

# special cases you may need to consider:
# - what if there are values in d1 that are equal to values in d2
# - negative values?
# - what if min(d2) is smaller than than min(d1)?




Cheers,
B.





On Dec 26, 2014, at 4:16 PM, Morway, Eric <emorway at usgs.gov> wrote:

> I'm in need of help selecting from d2 those values that come after a value
> in d1.  For example, d2[1] is both greater than d1[1] and is the value that
> is closest to d2[1].  Similarly, d2[2] is both greater than d1[2] and is
> the next "highest" number in d2.  Every value in d1 has a corresponding
> value in d2 based on this criteria, however, many of the values in d2 will
> be discarded.  The final result I'm looking for is a subset of d2 as given
> in d3.  Notice for example that 140569 is discarded and not contained in
> d3.  This small example is of course a much smaller example of a much
> larger problem.  Example R script of how to whittle down d2 to look like d3
> based on the criteria above
> 
> d1 <- c(135631,136950,137952,138787,139623,142231,143067,144762,
>        145601,146441)
> 
> d2 <- c(135882,136954,137956,138792,139630,140569,141398,142237,
>        143078,143907,144771,145611,146446,147285,148128)
> 
> d3 <- c(135882,136954,137956,138792,139630,142237,143078,144771,
>        145611,146446)
> 
> 	[[alternative HTML version deleted]]
> 
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