Friday, 26 February 2010

R tip: Finding the location of minimum and maximums

I can never remember this R command, so I am going to post it here which probably means I will always remember it and never have to look it up here again.

I sometimes want to find the location of a minimum or maximum value in a vector, so I can look up the corresponding position in another vector, or column or something. You can just use order but that is a bit clunky and more prone to error, so what you really need is which.min and which.max.

x <- sample(1:100,20)
which.min(x)

x <- matrix(sample(1:100,20),nrow=10)
x[which.min(x[,2]),1]

Not exactly earth-shatteringly useful but better than:

x <- matrix(sample(1:100,20),nrow=10)

x[order(x[,2])[1],1]

Or is it? Now I write that, it is actually less characters than which.min. Oh well.

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