[Rd] rounding errors in max.col()

[Philippe Grosjean]

Hello,

I suppose this is a general behavior with external function calls, so I do
not post (yet) a specific bug report. Could someone explain this?

a <- rep(1, 20) + rnorm(20, mean=0.00001, sd=0.0001)
b <- embed(a, 3)
# I want to know where the item in column 2 is greated than both col 1 and 3
(peak)
test1 <- max.col(b) == 2
# ... or I could use a less optimal code
test2 <- apply(b, 1, max) == b[, 2]
any(test1 != test2)	# both are equivalent

# but when numbers are very close
a <- rep(1, 20) + rnorm(20, mean=0.0000001, sd=0.0000001)
b <- embed(a, 3)
test1 <- max.col(b) == 2
test2 <- apply(b, 1, max) == b[, 2]
any(test1 != test2)	# tests are now DIFFERENT!
# Indeed, test2 is correct, and test1 suffers from wrong calculations due
probably to rounding errors in max.col()

Best,

Philippe Grosjean

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