st: String comparisons in Mata

["German Rodriguez" <grodri@Princeton.EDU>]

Dear list,

A mata question: Why is "a" converted to uppercase == "A" (and also <= "A")
but NOT >= "A"? 

. mata: strupper("a") >= "A"
  0

. mata: strupper("a") == "A"
  1

I was trying to determine if a string scalar was a letter and the obvious 

scalar function isLetter(string scalar s) {
	S = strupper(s)
	return (S >= "A" & S <= "Z")
}

didn't work with "a" (although it works with "b" because strupper("b") >=
"A" is true). I got around the problem using the slightly more cryptic

scalar function isLetter(string scalar s) {
	n = ascii(strupper(s))
	return (n >= 65 & n <= 90)
}

but I am curious. 

Cheers,
German

P.S. My Stata *is* up-to-date. I am running 9.1 born 20 Jan 2006

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