亂數函數

rand()

C++ reference-rand

#include <iostream>
#include <cstdlib>

int main() {
    for(int i = 0; i < 5; i++)
        std::cout << rand();
    return 0;
}

怎麼好像每次數字都一樣？

static unsigned long int next = 1;

int rand(void) // RAND_MAX assumed to be 32767
{
    next = next * 1103515245 + 12345;
    return (unsigned int)(next/65536) % 32768;
}

source

#include <iostream>
#include <cstdlib>

int main() {
    srand(7122);
    for(int i = 0; i < 5; i++)
        std::cout << rand() << std::endl;
    return 0;
}

void srand(unsigned int seed)
{
  next = seed;
}

c++ reference - time

#include <iostream>
#include <cstdlib>
#include <ctime>
int main() {
    srand(time(NULL));
    for(int i = 0; i < 5; i++)
        std::cout << rand() << std::endl;
    return 0;
}

如果我想產生0~9之間隨機的數字

#include <iostream>
#include <cstdlib>
#include <ctime>

int main() {
    srand(time(NULL));
    std::cout << rand() % 10;    
    return 0;
}

如果我想產生1~10之間隨機的數字

#include <iostream>
#include <cstdlib>
#include <ctime>

int main() {
    srand(time(NULL));
    std::cout << rand() % 10 + 1;    
    return 0;
}

如果我想產生x~y之間隨機的數字

#include <iostream>
#include <cstdlib>
#include <ctime>

int main() {
    srand(time(NULL));
    int x = 133, y = 999;
    std::cout << rand() % (y - x + 1) + x;    
    return 0;
}

延伸學習

C++11 的 Random library, 你還在用rand()嗎?