#include<stdio.h>
#include<stdlib.h>
#include<time.h>/*头文件*/
void main()
{
    int ia,ib;
    long ic;
    char ca,cb,cc;
    time_t beg,end; /*定义时间变量*/
    printf("Do you want to play this game?('y' or 'n')\n");
    ca=getchar();
    while(ca!='n') /*循环*/
    {
        ia=rand(100); /*产生随机数*/
        printf("Now the system have produced a random number between 0 to 99,enter your guess:\n");
        scanf("%d",&ib);
        time(&beg); /*获取当前时间*/
        while(ib!=ia)
        {
            if(ib>ia)
            {
                printf("please input a smaller number!\n");
            }
            if(ib<ia)
            {
                printf("please input a bigger number!\n");
            }
            scanf("%d",&ib);
        }
        time(&end); /*获取结束时间*/
        ic=end-beg; /*计算所花时间*/
        printf("Success!It took you %ld seconds to finish\n",ic);
        {
            if(ic<15)
            {
                printf("you are clever!\n");
            }
            else if(ic<25)
            {
                printf("you are normal!\n");
            }
            else
            {
                printf("you are stupid!\n");
            }
        }
        getchar();
        printf("Do you want to play this game?('y' or 'n')\n");
        scanf("%c",&ca);
    }
}

这是个猜数的程序，可是为什么正确一次后想要再次游戏，不管输入什么数字，程序总是输出“please input a bigger number!”   ???

ia=rand(100); /*产生随机数*/

rand()函数有参数？

这个参数对它是无效的，它产生的伪随机数是在0-RAND_MAX之间，所以才会出现你上面的情况

简单说说：
楼上正解
正是问题所在，极有可能生成一个比较大的数字
应该R=rand()%100;
但是%运算又不是那么的平均分布，所以好的随机程序也要考验程序员的功底