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pow() problem

Started by shaobohou Feb 22, 2005 at 9:05 AM 3 replies 900+ views

Original Post

Feb 22, 2005 at 9:05 AM

Can anyone tell me why pow(-8.0, 1.0 / 3.0) (cubic root of -8) returns nan (in linux) and negative infinity or something (in devcpp in windows), instead of -2? The problem seems to be that pow can't handle cubic root of negative number, I mean, a calculator could do it or am I using the wrong function here. My current solution is a hack which detects negative input and handle it differently. I suspect it is something to do with the fact the power is represented as a floating point value.

Charles B

Feb 22, 2005 at 9:14 AM

I suppose that it's implemented this way : exp(1/3*log(-8)) thus the NAN error.

shaobohou

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shaobohou

Feb 22, 2005 at 9:15 AM

Any idea why calculators don't have the same problem, how do they implement this sort of thing?

Anon Mike

117

Anon Mike

Feb 22, 2005 at 12:13 PM

A quick google yielded a couple sites that say that for pow(x, y) if x is negative y must be an integer. My guess would be that it has something to do with avoiding getting into the whole complex roots thing.

Robot Guy

Feb 22, 2005 at 3:51 PM

I just checked the C spec and it says :

The pow functions compute x raised to the power y. A domain error occurs if x is finite and negative and y is finite and not an integer value. A domain error may occur if x is zero and y is less than or equal to zero. A range error may occur.

If you really want to compute a cube root (rather than an arbitary root) you should probably use the cbrt function (assuming that you have a compiler that adheres. to the 1999 standard).

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