Random numbers (C)

Repli

I have a simple piece of code to generate 6 random numbers between 0 and 100, however every time I run the program it generates the same numbers:

#include <stdio.h>
#include <stdlib.h>
main()
{
    int i = 0;
    int array[6];
    for (i=0; i<6; i++)
        array[i] = rand()%100;
    for (i=0; i<6; i++)
        printf ("%d ", array[i]);
}

I've tried fflush(stdout) at the beginning of the program but get the same problem - anyone know how to? I know if you use srand() with a different seed it gets past the problem but I'd rather not do that..
Thx in advance

BossArky

I know you said that you didn't want to use srand...but can't you use it with the pc clock time...in c++ anyway...just a thought..

Talliesin

Originally posted by Repli
I know if you use srand() with a different seed it gets past the problem but I'd rather not do that..

That's a bit like saying you want to get rid of your first, but would rather not drink. What's wrong with srand?

daveirl

This post has been deleted.

Repli

Ok but I didn't know how to give srand a different number each time the program is called..
Bossarky gave me a good idea there.. I could pass srand time(0) or something each time the program is called..

daveirl

This post has been deleted.

daveirl

This post has been deleted.

DeadBankClerk

if you are going to be using random numbers just start your program:

int main (int argc, char** argv)
{
    srand(GetTickCount());
    ....

Using the modulus N operator does *not* give a proper random number between 0 and N. My statistics lecturer spent an entire lecture explaining why. Boring as hell.

For true randomness:

int random_int (const int& high, const int& low = 0)
{
    assert (high > low && "Random_int requires High > Low");

    double random = (double)rand() / (double)RAND_MAX;
    double answer = (random * (double)(high - low)) + (double)low;
    return (int)answer;
}

Overkill however

Rev Hellfire

thats not true random, nowhere near it.

if u want a easy sorce of entropy, your best bet is to look at the intel RNG assuming you're using a pentium III+, the athlons also have one. Or dig deep into your pockets and get HSM.

Thorbar

Remember coming accross a set of trig equations which if you fed 3 numbers generated by srand() would return something a little more random. I've noticed myself when using srand that if you run the program serveral times one after the other it will generate the same number a few times as the clock seed is close to the last time it ran and you're rounding it off so much. I can't for the life of me remember where or what those trig equations are but if you have a look at the game development sites you should find something similar. That's where I found em in the first place.

tommycahir

heres some code to generate a ramdom number between 100 and 500 and store it in an array
some tidying up needs to be done but itll give you an idea of how to generate a totally random number

ps half the variables prob arent needed..!!!
i just to lazy to go messing bout with it

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

#define maxarraysize 100
#define maxvalue 500
#define minvalue 100

void main(void)
{
int countletter = 0; // var used to traverse to the end of the array
int random_number = 0; // var to store the random num in the array a temp var
int numarray[maxarraysize]; // array for numbers
int choosen_value = 0; // inputed value by the user
int occurrences = 0; // num of occ of the num in the list
int array_pos[maxarraysize]; // array to store location of the choosen value in the numarray
int index = 0; // counter
int valuepos = 0; // variable to store the value of the location of the choosen value
int rownum = 0,colnum = 0; // row and col of the choosen num
srand(time(NULL)); // used to set a seed value in the rand
//func so that totaolly different everytime that you use it

for(countletter;countletter < maxarraysize;countletter++)
{
random_number = rand() % maxvalue; // used to generate a random value
if ((random_number >= minvalue) && (random_number <= maxvalue)) // check to make sure that the value between 100 and 500
{
numarray[countletter] = random_number;
}
else
{
countletter--;
}
array_pos[countletter] = 0;
}