lubksb.c

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void lubksb(float **a, int n, int *indx, float b[]) {
    int i, ii = 0, ip, j;
    float sum;
 
    for (i = 1; i <= n; i++) {
        ip = indx[i];
        sum = b[ip];
        b[ip] = b[i];
        if (ii)
            for (j = ii; j <= i - 1; j++) sum -= a[i][j] * b[j];
        else if (sum) ii = i;
        b[i] = sum;
    }
    for (i = n; i >= 1; i--) {
        sum = b[i];
        for (j = i + 1; j <= n; j++) sum -= a[i][j] * b[j];
        b[i] = sum / a[i][i];
    }
}