negative exponentials

[RawRecruit]

Hello All,
I'm playing around with Neural Networks but can't get the sigmoid function 1/(1+ e -var)to work correctly.
That's exponential to the negative variable - can't use superscript in this message box!

log()works fine with a positive number, but as soon as a negative number is applied it falls over!

Can anyone shed any light on the problem? Am I using the correct function?

My code reads along the lines of....
var = 1/(1+(log(0-SubTotArray[0][$OutCount])))
and works OK if the '0-' is removed.

Any ideas?

If anyone needs clarification of the sigmoid function, it can be found at.....

http://mathworld.wolfram.com/SigmoidFunction.html

... just below the diagrams.


Thanks in advance for all your assistance.

 

[sleipnir214]

Two questions...[ol][li]You do know you can't take the logarithm of a negative number, right?[/li][li]Why are you using the log() PHP function when the mathematical equation requires that you calculate a power of e -- wouldn't the exp() PHP function be more appropriate?[/li][/ol]

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