Hi,
I'm stuck with a fairly basic re-ordering problem. I want to extract part of
a matrix as a vector, and reorder it to match with an erratic sequence of x,y
coordinates. Here's an example that shows what I want and how close I've
got (but for some reason my mind just can't get the final step at the
moment!):
#Define a grid:
xy100 <- expand.grid(1:10, 1:10)
# Define a matrix ofvalues
mat <- matrix(log(xy100[,1]) * log(xy100[,2])+1, ncol = 10)/2
#Plot some the matrix with image and the same data as xy plot:
par(mfrow = c(2,2))
image(mat)
plot(xy100, cex = c(mat), pch = 20)
#define the subset of coordinates I'm interested in:
shortXY <- expand.grid(1:5, 1:5)
#and shuffle these into an erratic sequence:
rand.seq <- sample(1:25)
shortXY <- shortXY[rand.seq,]
#Identify which coordinates in the full dataset are needed (not doubt better
ways exist!):
needed <- which(paste(xy100[,1],xy100[,2], sep = " ") %in%
paste(shortXY[,1], shortXY[,2], sep = " "))
#Sort these into a sensible order:
sorted <- needed[order(xy100[needed,1], xy100[needed,2])]
# plot showing that when the short-coordinates are ordered (removing the erratic
sequence)
# the sorted subset of the matrix is correctly subsetted:
plot(shortXY[order(shortXY[,1], shortXY[,2]),], cex = c(mat)[sorted], pch = 20)
#but what further ordering do I apply to the
# sorted and subsetted matrix, to mean that I don't have to alter the
# sequence of the shortXY coordinate list? In this example it is obviously the
value of rand.seq:
plot(shortXY, cex = c(mat)[sorted][rand.seq], pch = 20)
# but what is the general value of this: how can I recover rand.seq when it
isn't known?
Obviously I could do some of this (much more elegantly) with merge, but for
various reasons I don't want to (I need to run the process many, many times
on very large datasets). Thanks for any pointers - I'm being very slow
today...
Colin
PS sessionInfo()
R version 2.8.1 (2008-12-22)
i386-pc-mingw32
locale:
LC_COLLATE=English_United Kingdom.1252;LC_CTYPE=English_United
Kingdom.1252;LC_MONETARY=English_United
Kingdom.1252;LC_NUMERIC=C;LC_TIME=English_United Kingdom.1252
attached base packages:
[1] stats graphics grDevices datasets tcltk utils methods base
other attached packages:
[1] debug_1.1.0 mvbutils_1.1.1 svSocket_0.9-5 svIO_0.9-5 R2HTML_1.58
svMisc_0.9-5 svIDE_0.9-5
loaded via a namespace (and not attached):
[1] tools_2.8.1
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