realft.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

void realft(double data[], int &n, int &isign)
{
	int n2p3,i,i1,i2,i3,i4;
	double theta,c1,c2,wpr,wpi,wr,wi,wrs,wis,h1r,h1i,h2r;
	double h2i,wtemp,wri;
	wri=0.0;
    theta = 6.28318530717959 / 2.0 / n;
    c1 = 0.5;
    if (isign == 1)
	{
        c2 = -0.5;
        four1(data, n, 1);
	}
    else
	{
        c2 = 0.5;
        theta = -theta;
	}
    wpr = -2.0 * sin(0.5 * theta) * sin(0.5 * theta);
    wpi = sin(theta);
    wr = 1.0 + wpr;
    wi = wpi;
    n2p3 = 2 * n + 3;
    for (i = 2; i<=n / 2 + 1; i++)
	{
        i1 = 2 * i - 1;
        i2 = i1 + 1;
        i3 = n2p3 - i2;
        i4 = i3 + 1;
        wrs = float(wr);
        wis = float(wi);
        h1r = c1 * (data[i1] + data[i3]);
        h1i = c1 * (data[i2] - data[i4]);
        h2r = -c2 * (data[i2] + data[i4]);
        h2i = c2 * (data[i1] - data[i3]);
        data[i1] = h1r + wrs * h2r - wis * h2i;
        data[i2] = h1i + wrs * h2i + wis * h2r;
        data[i3] = h1r - wrs * h2r + wis * h2i;
        data[i4] = -h1i + wrs * h2i + wis * h2r;
        wtemp = wr;
        wr = wr * wpr - wi * wri + wr;
        wi = wi * wpr + wtemp * wpi + wi;
    }
    if( isign == 1 )
	{
        h1r = data[1];
        data[1] = h1r + data[2];
        data[2] = h1r - data[2];
	}
    else
	{
        h1r = data[1];
        data[1] = c1 * (h1r + data[2]);
        data[2] = c1 * (h1r - data[2]);
        four1(data, n, -1);
    }
}