26、数组随机化与顺序文件随机样本选择算法

数组随机化与顺序文件随机样本选择算法

1. 数组随机化算法

1.1 随机化函数实现

以下是一个用于随机化数组元素顺序的函数模板：

template <class otype>
void randomize (otype a[], int first, int last)
{
    int n = last - first + 1;
    // Number of cells in a[first..last]
    otype temp[n];
    // Need an extra storage array.
    int r = randint(0, n-1);
    // Generate a "shift distance."
    for (int j = 0, k = first + r; k <= last; ++k)
        temp[j++] = a[k];
    // Copy back part of "a" to front of "temp"
    for (int k = first; k <= first + r - 1; ++k)
        temp[j++] = a[k];
    // Copy front part of "a" to back of "temp"
    // Copy all of "temp" back to a[first..last]
    for (int j = 0, int k = first; k <= last; ++j, ++k)
        a[k] = temp[j++];