Re: [Bug-gnubg] Bug in sigmoid?

[Olivier Baur]

Le jeudi, 17 avr 2003, à 22:04 Europe/Paris, Joseph Heled a écrit :

> Hi,
>
> Can you please post the code for the new sigmoid? I did not understand 
> the new formulation.

Sure, here it goes:


#define SIG_Q 10.0f     /* reciprocal of precision step */
#define SIG_MAX 100     /* half number of entries in lookup table */
/* note: the lookup table covers the interval [ -SIG_MAX/SIG_Q  to  
+SIG_MAX/SIG_Q ] */

static float Sig[2*SIG_MAX+1];

static void ComputeSigTable (void)
{
    int i;

    for (i = -SIG_MAX; i < SIG_MAX+1; i++) {
        float x = (float) i / SIG_Q;
        // Sig[SIG_MAX+i] = 1.0f / (1.0f + exp (x)); /* more accurate, 
but fails gnubgtest */
        Sig[SIG_MAX+i] = sigmoid(x);  /* use the current sigmoid 
function instead :-)  */
    }
}


static inline float sigmoid2 (float const x)
{
    float x2 = x * SIG_Q;
    int i = x2;

    if (i > - SIG_MAX) {
        if (i < SIG_MAX) {
            float a = Sig[i+SIG_MAX];
            float b = Sig[i+SIG_MAX+1];
            return a + (b - a) * (x2 - i);
        }
        else
            return 1.0f / 19931.370438230298f;  /* warning: this is 
1.0f/(1.0f+exp(9.9f)) */
    }
    else
        return 19930.370438230298f / 19931.370438230298f;  /* warning: 
this is 1.0f/(1.0f+exp(-9.9f)) */
}



-- Olivier