C++ rand() why give me the same number How can i have different

Question

if (ii == 3){//车辆只停了一个小时，有3/4的概率离场
    srand((unsigned)time(NULL)*100);
    int a = 1+rand() % 4;
    int b = 1+rand() % 4;
    int c = 1+rand() % 4;
    cout << "++++3 " << a << "++++" << endl;//用作测试判断是否离场函数
    cout << "++++3 " << b << "++++" << endl;
    cout << "++++3 " << c << "++++" << endl;
    cout << "*************" << endl;
    if ((a == 1 && b == 2 && c == 3)||(a==2&&b==1&&c==3)||(a==3&&b==2&&c==1)||(a==3&&b==2&&c==1))
        return true;
    else
        return false;
}

Just like this picture, the program gives me the same number ,but I need different numbers , how can I do it.

#ifndef Head_H
#define Head_H
#include<stdlib.h>
#include<iostream>
#include<time.h>
#define PathSize 5
#define ParkSize 10
#define Price 5
using namespace std;
srand((unsigned)time(NULL));

Bathsheba · Answer 1 · 2015-03-31T15:32:40.503

1

Use srand once, at the start of your program.

If you repeatedly re-seed the generator then you ruin its statistical properties.

Moving on:

Avoid using % to rescale the generator. This will introduce statistical bias (unless by pure luck the modulo is a multiple of RAND_MAX).
Look at the C++11 random number library. This contains better-specified generators than plain old rand, which is awful on some platforms.

edited Mar 31 '15 at 15:32

answered Mar 31 '15 at 15:27

Bathsheba

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use srand at Head.cpp? – user3631068 Mar 31 '15 at 15:29
Yup. Call it once; preferably at the start of `main`. – Bathsheba Mar 31 '15 at 15:32
but the VS tell me error：：Missing type specifier – user3631068 Mar 31 '15 at 15:35
Paste that bit of the code up. You could be missing an `#include`. – Bathsheba Mar 31 '15 at 15:36
@user3631068, That really should be a new question, but it's likely that many others have had the same problem. – chris Mar 31 '15 at 15:42
@Bathsheba.i paste that. – user3631068 Mar 31 '15 at 15:45
Use `srand` inside a function body. – Bathsheba Mar 31 '15 at 15:49
@BathshebaYep.i code it in main function it worked .thaks – user3631068 Mar 31 '15 at 15:50

C++ rand() why give me the same number How can i have different

1 Answers1