C++生成随机数

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1，一般情况，生成随机数采用以下方式：

#include <stdio.h>#include <stdlib.h>#include <time.h>int main (){  int iSecret, iGuess;  /* initialize random seed: */  srand ( time(NULL) );  /* generate secret number: */  iSecret = rand() % 10 + 1;  do {    printf ("Guess the number (1 to 10): ");    scanf ("%d",&iGuess);    if (iSecret<iGuess) puts ("The secret number is lower");    else if (iSecret>iGuess) puts ("The secret number is higher");  } while (iSecret!=iGuess);  puts ("Congratulations!");  return 0;}

2，但是实际这种方式生成的随机数并不是很随机，有时候随机变化的幅度很小，而《Accelerated C++》提供了一种不错的方式：

// return a random integer in the range `[0,' `n)'int nrand(int n){if (n <= 0 || n > RAND_MAX)throw domain_error("Argument to nrand is out of range");const int bucket_size = RAND_MAX / n;int r;do r = rand() / bucket_size;while (r >= n);return r;}

3，上面这种方式比较完美，但是稍微有一点不足，那就是只是生成小于RAND_MAX(32767)的随机数，《Accelerated C++》练习题中提供了一种改进的方式：

#include <cmath>#include <cstdlib>#include <iostream>#include <stdexcept>#include <limits.h>using namespace std;#define MY_RAND_MAX 32767int nrand(int n) {  if (n <= 0)    throw domain_error("Argument to nrand is out of range");  int r;  if (n <= MY_RAND_MAX) {           const int bucket_size = MY_RAND_MAX / n;    do {      int bucket = rand() / bucket_size;      r = bucket;    } while (r >= n);  } else {    const int bucket_size = ceil(n / MY_RAND_MAX);    do {      const int bucket = nrand(MY_RAND_MAX);      const int remainder = nrand(bucket_size);      r = (bucket - 1) * bucket_size + remainder;    } while (r >= n);  }  return r;}int main() {  int limit;  while (cin >> limit)    cout << nrand(limit) << endl;  return 0;}

这种方式真的接近完美了，只是算法稍微复杂点。

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