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Re: Absolute Value |x - y|


On Dec 4, 2004, at 7:11 PM, Frederick Cheung wrote:


On 4 Dec 2004, at 20:56, Gregory Weston wrote:

On Dec 4, 2004, at 2:18 PM, Malte wrote:

i've been looking around trying to find a method that calculates the
|absolute value| of a number.
Without success.
Does anyone know if such a function exists?


They do, but you're probably better off just writing

T abs = ((x<y) ? (y-x) : (x-y));

where T is whatever type you need the result to be.

The difference is probably that fabs has well defined behaviour in case one (or both) of x and y is something like NaN or +/- \infty, whereas this might not be the case for the expression you give. 

If it were me, I'd use __fabs(x-y). Two instructions, branchless. :)

That being said, the result of the - operator and the < operator when given +/-Inf and/or NaN is perfectly well defined. It's part of the IEEE standard and it's really something any serious programmer should know. (Honestly, it surprises me how many talented programmers never bothered to learn the basics of IEEE floating point.) Unless I'm missing something, the ternary expression above would generate the correct result.

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 >Re: Absolute Value |x - y| (From: Gregory Weston <email@hidden>)
 >Re: Absolute Value |x - y| (From: Frederick Cheung <email@hidden>)



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