CODEWITHSUNDEEP
c++random
angryInsomniac
angryInsomniac

Reputation: 859

generating Random numbers using rand()

I'm trying to generate a bunch of random numbers using rand() % (range). Here's how my code is setup :

srand(time(NULL));
someClass *obj = new someClass(rand()%range1,rand()%range2... etc ); // i.e. a number of random numbers one after the other

Whenever I run this, it seems all the calls to rand() generate the same number. I tried doing it without the : (edit : all rand() do not generate the same number it seems , read edit at the end)

srand(time(NULL));

then , every execution of the program yields the same results.

Also, since all calls to rand() are in a constructor , I cant really reseed it all the time. I guess I can create all objects sent to the constructor beforehand and reseed the random number generator in between, but it seems like an inelegant solution.

How can I generate a bunch of different random numbers ?

edit: It seems because I was creating a lot of objects in a loop, so every time the loop iterated srand(time(NULL)) was reseeded and the sequence got reset ( as time(NULL) has a resolution of a second) , that's why all subsequent objects had very similar properties.

Upvotes: 3

Views: 2772

Answers (4)

Rohit Vipin Mathews
Rohit Vipin Mathews

Reputation: 11787

check this code at: http://ideone.com/xV0R3#view_edit_box

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

using namespace std;
int main()
{
int i=0;
srand(time(NULL));
while(i<10)
{
cout<<rand()<<endl;
i++;
}
return 0;
}

this produces different random numbers. you need to call srand() only once. rand() generates a different number every time after the srand() call

Upvotes: 1

user1190294
user1190294

Reputation:

Call srand once at the begin of the program. Then call rand()%range any time you want a random number. Here is an example for your situation, that works pretty well

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

using namespace std;

class Test
{
    public:
    Test(int num0,int num1, int num2):num0_(num0),num1_(num1),num2_(num2){}
    int num0_,num1_,num2_;
};

int main()
{
    srand(time(NULL));
    Test *test=new Test(rand()%100,rand()%100,rand()%100);
    cout << test->num0_ << "\n";
    cout << test->num1_ << "\n";
    cout << test->num2_ << "\n";
    delete test;
    return 0;
}

Upvotes: 2

Stack Overflow is garbage
Stack Overflow is garbage

Reputation: 247969

If you call srand once, then all subsequent rand calls will return (different) pseudorandom numbers. If they don't, you're doing it wrong. :)

Apart from this, rand is pretty useless. Boost.Random (or the C++11 standard library <random> header) provides much more powerful random number generators, with nicer, more modern interfaces as well (for example allowing you to have multiple independent generators, unlike rand which uses a single global seed)

Upvotes: 7

Bo Persson
Bo Persson

Reputation: 92261

Unless reseeded with a different starting point, rand() always returns the same sequence. That is actually a feature to make program tests repeatable!

So, you have to call srand if you want a different sequence for different runs. Perhaps you can do that before calling the first constructor?

Upvotes: 2

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