rand(）函数在C++中无法正常工作

rand() function does not work properly in C++

本文关键字：常工作 工作 函数 C++ rand      更新时间：2023-10-16

我正在编写一个游戏，其中计算机选择一个用户必须猜测的随机数。然后，用户选择一个数字（他只是猜测一个数字），计算机必须猜测。问题是，我在游戏的第二部分使用的rand（）函数有时会提供一个新范围之外的数字。例如，如果新的范围是低=4，高=10，rand（）提供12，这是不正确的。我试图理解为什么会出现错误，但我找不到。我正在Dev-C++中写作。代码就在下面，后面跟着输出。谢谢你的帮助和时间。

#include<iostream>
#include<conio.h>
#include<stdlib.h>
#include<time.h>


using namespace std;
int main()
{
  int computer_guess, nbr, nbrguesses, count_user_guess=0, count_computer_guess=0; 
  char user_comparison;
  int  low=1, high=99;  //initialize low to the lower boundary and high to the higher     boundary
  srand(time(0));
  nbr=rand()% 99 + 1;  //computer chooses a random number between 1 and 99...
  do
  {
    cout << "guess a number now! " << endl;
    cin >> nbrguesses;     //user enters the number he guessed...
    if ( nbrguesses>nbr)
       cout << "too big" <<endl;
    else if ( nbrguesses<nbr)
       cout << "too small" << endl;
    else if ( nbrguesses==nbr)
       cout << " you found the correct number! " <<endl;
    count_user_guess++;  //count the number of guesses from the user
  }
  while (nbrguesses!=nbr);
  cout << "you tried " << count_user_guess <<" times to find the right number " <<      endl<<endl;
  cout << "----------------------------------------------------------------------" << endl;
  cout << "Now, the computer will guess your number " << endl;
  do
  {
    srand(time(0));
    computer_guess=rand()% high + low;    //computer guesses the number
    cout << "computer guess: " << computer_guess <<endl;
    cout << "is it the correct number?" << endl;
    cin >> user_comparison;     //if user enter 
    if (user_comparison=='>')  // character '>' it means the number guessed by computer is too big
    {
       cout << "too big" <<endl;
       high= computer_guess-1;   //high is given a new boundary 
       cout << "***Current value of high = " << high << " and low = " << low << endl;    //display current boundaries
    }
    else if ( user_comparison=='<')  //the number guessed by computer is too small
    { 
      cout << "too small" << endl;
      low= computer_guess+1;  //low is given a new boundary
      cout << "***Current value of low = " << low << " and high = " << high << endl;  //display current boundaries
    }
    else if ( user_comparison=='=')
       cout << "Computer found the correct number! " <<endl;
    count_computer_guess++;  //count number of guesses from computer
  }
  while (user_comparison!='=');
  cout << "The computer tried " << count_computer_guess <<" times to find the right number " << endl;
  cout << "The Game is over now, have a good day!" << endl;
  getch();
  return 0; 
}
//**************Output******************************************
guess a number now!
50
too big
guess a number now!
25
you found the correct number!
you tried 2 times to find the right number
----------------------------------------------------------------------
Now, the computer will guess your number
computer guess: 11
is it the correct number?
>
too big
***Current value of high = 10 and low = 1
computer guess: 3
is it the correct number?
<
too small
***Current value of low = 4 and high = 10
computer guess: 12
is it the correct number?

您必须更换

computer_guess=rand()% high + low;

通过

computer_guess=rand()%(high- low + 1) + low;

在你的例子中，你的低=4，高=10。但是你想要一个0到6之间的随机数，再加上4。0到10之间没有一个，从那时起，你可以得到14个结果。如果你不想包括上限（Interval[low，high]而不是[low，height]），你必须省略括号中的+1。

为什么

rand（）返回一个相当大的整数。我们想要得到区间[a，b]中的一个随机数。如果我们取rand（）%5，我们得到一个0，1，2，3或4的数字，因此从区间[0,4]。在一般情况下，rand（）%C在区间[0，C-1]中给出一个随机数。

如果我们加上一个常数值，例如Rand（）%C+D，则间隔被移位：[D，C-1+D]。回到问题上来，我们希望区间是[a，b]。因此，我们需要a=D作为下键，C-1+D=b作为上键。我们可以将其转换为C=b-D+1=b-a+1。所以我们想要使用rand（）%（b-a+1）+a。

希望这能解释一下事情是如何运作的。

你很接近，但试试这个：

rand()%(high-low)+low;

srand(time(0));
computer_guess=rand()% high + low;    //computer guesses the number

首先，不要在循环中调用srand。程序启动时只需调用一次即可。

其次，这种逻辑是错误的。应该是：

computer_guess=low + (rand() % (high-low+1));

（这假设高和低是包含的。也就是说，如果高是10，低是1，那么1和10都是可以接受的猜测。）

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