Pearls1

[TOC]

使用常量来设置、清除以及测试位值：

提示：MASK 0x1f 2的SHIFT(5)次方，32位,把int型的变量每一位都当作一个标记。

/* Copyright (C) 1999 Lucent Technologies */
/* From 'Programming Pearls' by Jon Bentley */

/* bitsort.c -- bitmap sort from Column 1
 *   Sort distinct integers in the range [0..N-1]
 */
#include 

#define BITSPERWORD 32
#define SHIFT 5
#define MASK 0x1F
#define N 10000000
int a[1 + N/BITSPERWORD];

void set(int i) {        a[i>>SHIFT] |=  (1<<(i & MASK)); }
void clr(int i) {        a[i>>SHIFT] &= ~(1<<(i & MASK)); }
int  test(int i){ return a[i>>SHIFT] &   (1<<(i & MASK)); }

int main()
{   int i;
    for (i = 0; i < N; i++)
        clr(i);
/*  Replace above 2 lines with below 3 for word-parallel init
    int top = 1 + N/BITSPERWORD;
    for (i = 0; i < top; i++)
        a[i] = 0;
 */
    while (scanf("%d", &i) != EOF)
        set(i);
    for (i = 0; i < N; i++)
        if (test(i))
            printf("%d\n", i);
    return 0;
}

使用qsort快速排序

提示：
void qsort(voidbase,size_t num,size_t width,int(__cdeclcompare)(const void,const void));
各参数：1. 待排序数组首地址 2. 数组中待排序元素数量 3. 各元素的占用空间大小 4. 指向函数的指针

#include 
#include 

int intcomp(int *x, int *y)
{   return *x - *y;
}

int a[1000000];

int main()
{   int i, n=0;
    while (scanf("%d", &a[n]) != EOF)
        n++;
    qsort(a, n, sizeof(int), intcomp);
    for (i = 0; i < n; i++)
        printf("%d\n", a[i]);
    return 0;
}

使用标准库版本的排序

#include 
#include 
using namespace std;

int main()
{   set S;
    int i;
    set::iterator j;
    while (cin >> i)
        S.insert(i);
    for (j = S.begin(); j != S.end(); ++j)
        cout << *j << "\n";
    return 0;
}

1.6 习题

1.4 . 如何生成位于0至n-1之间的k个不同的随机顺序的随机整数? 尽量使你的程序简短且高效。

//生k个成0~n-1之间的随机数(k
#include
#include      /* srand, rand */
using namespace std; 
int const N = 10000;
int a[N];
int main(void)
{
    int i,j,k=10;
    for(i = 0 ;i < N; i++)
        a[i] = i;

    //初始化随机种子，产生随机数 
    srand((unsigned)time(NULL));
    for(i = 0 ; i < k; i ++)
    {
        int tmp = rand() % (N - i); 
        cout << "tmp ->" << tmp << endl;
        swap(a[tmp],a[N-i]);
    }
    for(i = N - 1 ; i > N - k;i--)
    {
        cout<

1.6 增强，生成区间内的随机数，注意生成的随机数各不相同

/* Copyright (C) 1999 Lucent Technologies */
/* From 'Programming Pearls' by Jon Bentley */

/* bitsortgen.c -- gen $1 distinct integers from U[0,$2) */

#include 
#include 
#include 
#define MAXN 2000000
int x[MAXN];

int randint(int a, int b)
{   return a + (RAND_MAX * rand() + rand()) % (b + 1 - a);
}

int main(int argc, char *argv[])
{   int i, k, n, t, p;
    srand((unsigned) time(NULL));
    k = atoi(argv[1]);
    n = atoi(argv[2]);
    for (i = 0; i < n; i++)
        x[i] = i;
    for (i = 0; i < k; i++) {
        p = randint(i, n-1);
        t = x[p]; x[p] = x[i]; x[i] = t;
        printf("%d\n", x[i]);
    }
    return 0;
}