[英]rand() function does not work properly in C++
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,是用户选择一个数字(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() function 有时会提供新范围之外的数字。 例如,如果新范围是 low=4 和 high=10,则 rand() 提供 12,这是不正确的。 我试图理解为什么会出现该错误,但我找不到它。 我正在用 Dev-C++ 编写。 代码就在下面,后面是 output。感谢您的帮助和时间。
#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?
你必须更换
computer_guess=rand()% high + low;
经过
computer_guess=rand()%(high- low + 1) + low;
在你的例子中你有低= 4和高= 10。但是你想要一个0到6之间的随机数并加4。不是0到10之间的一个,从那时起你可以获得最多14的结果。如果你不想包括上限(Interval [low,high[ 而不是 [low,high] )你必须省略括号中的 +1。
为什么
rand() 返回一个相当大的数字 integer。 我们想从中得到一个区间 [a,b] 中的随机数。 如果我们采用 rand()%5,我们得到一个数字,它是 0、1、2、3 或 4,所以来自区间 [0,4]。 一般rand()%C给出区间[0,C-1]内的随机数。
如果我们加上一个常量,例如 Rand()%C + D,区间就会移动:[D,C-1 +D]。 那么回到问题,我们希望区间为[a,b]。 因此,我们希望下键为 a=D,上键为 C-1+D = b。 这个我们可以转化为C = b-D+1 = b-a+1。 所以我们要使用 rand()%(b-a+1) + a。
希望这能解释一些事情是如何运作的。
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,不要在循环内调用srand
。 程序启动时调用一次即可。
其次,这个逻辑是错误的。 它应该是:
computer_guess=low + (rand() % (high-low+1));
(这假设高和低都包含在内。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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