ran3.cpp

此程序为VC++常用数值算法这本书附赠的光盘中包含了本书中全部的源代码

static int inextp,iff,inext;
static double ma[57];
double  ran3(int& idum)
{
	double mk,b,fac,mj;
	int i,ii,k,mbig,mz;
	long mseed;
    mbig = 1000000000; mseed = 161803398; mz = 0; fac = 0.000000001;
    if ((idum < 0) || (iff == 0))
	{
        iff = 1;
        mj = mseed - fabs(idum);
        mj = mj - mbig * int(mj / mbig);
        ma[56] = mj;
        mk = 1;
        for (i = 1; i<=54; i++)
		{
            ii = (21 * i) - 55 * int((21.0 * i)/ 55.0);
            ma[ii] = mk;
            mk = mj - mk;
            if (mk < mz) mk = mk + mbig;
            mj = ma[ii];
        }
        for (k = 1; k<=4; k++) 
		{
            for (i = 1; i<=55; i++)
			{
                ma[i] = ma[i] - ma[1 + i + 30 - 55 * int((i + 30) / 55)];
                if (ma[i] < mz) ma[i] = ma[i] + mbig;
			}
        }
        inext = 0;
        inextp = 31;
        idum = 1;
    }
    inext = inext + 1;
    if (inext == 56 ) inext = 1;
    inextp = inextp + 1;
    if (inextp == 56)  inextp = 1;
    mj = ma[inext] - ma[inextp];
    if (mj < mz ) mj = mj + mbig;
    ma[inext] = mj;
    b = mj * fac;
	return b;
}