How do I get a random number?

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1.00/5 (2 votes)

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hello everyone

ive been trying to get a random number between 1 and 9 999 999
using C language and it doesnt seem to work

What I have tried:

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

int main(void){
	int i = 0;
	int nombre_aleatoire = 0;
	srand(time(NULL)); 
	for(i=0; i<1; i++){
		nombre_aleatoire = rand()*RAND_MAX/10 - rand()%10000;  
		printf("%d ",nombreMystere);
		
	}
	return 0;
}

this returns big values while the vallues need to range between 1 and 9999999

could anyone help ? thanks !

Posted 26-Mar-17 5:38am

Member 13079820

Updated 26-Mar-17 6:20am

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Dave Kreskowiak · Answer 1 · 2017-03-26T06:20:00

Solution 2

This is easily searchable in Google: "C++ random number between 0 and 100[^]"

C

num = (int)(rand() * 9999999) + 1;

Using the "%" operator to limit the range has the side effect of generating lower values slightly more often than higher ones.

Posted 26-Mar-17 6:20am

Dave Kreskowiak

Comments

Member 13079820 26-Mar-17 12:49pm

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

int main(void){
int i = 0;
int nombre_aleatoire = 0;
srand(time(NULL));
for(i=0; i<88; i++){
nombre_aleatoire = (int)(rand() * 9999999) + 1;
printf("%d ",nombre_aleatoire);

}
return 0;
}

i tried to change the i value (to see more random values) and it generates negative numbers. why?

Dave Kreskowiak 26-Mar-17 14:23pm

It's been a LONG time since I've done random in C/C++. Things have changed a bit...

Rand() returns an integer from 0 to 32767, not from 0 to 1. Multiply 32767 by 999999 and you get a number that overflows an int, thus giving the strange results.

The new code has a couple of methods of returning a range of integers, one biased and one not:

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

// Define two methods of generating random numbers;
int GetRandomModulo(int min, int max);
int GetRandomModern(int min, int max);

int main()
{
	int i = 0;
	int num = 0;
	srand(time(NULL));

	for (i = 0; i < 88; i++)
	{
		// use

		num = GetRandomModulo(1, 9999999);

		// or 

		num = GetRandomModern(1, 9999999);

		printf("%d\r\n", num);
	}

    return 0;
}

int GetRandomModulo(int min, int max)
{
	int range = max - min + 1;
	int num = rand() % range + min;

	return num;
}

int GetRandomModern(int min, int max)
{
	// This code is should really be wrapped in its own class to
	// prevent constantly reseeding the RNG engine every time it's
	// called.

	// Seed a Mersenne Twister RNG engine
	static std::random_device seed;
	static std::mt19937 engine(seed());
	static std::uniform_int_distribution<int> ditribution(min, max);

	return ditribution(engine);
}

Patrice T · Answer 2 · 2017-03-26T06:17:00

Solution 1

Quote:
this returns big values while the vallues need to range between 1 and 9999999

No, this code return nothing because it does not compile. Show real code.
Looks like you need to study the rand() function too.

<French>

C++

rand()*RAND_MAX/10 - rand()%10000

L'utilisation de plusieurs rand() ne rendra pas le resultat plus aléatoire, c'est inutile.
nombreMystere n'es tpas declaré dans ton code, ça provoque une erreur de compilation.
</;French>

Posted 26-Mar-17 6:17am

Patrice T

Comments

Member 13079820 26-Mar-17 12:53pm

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

int main(void){
int i = 0;
int nombre_aleatoire = 0;
srand(time(NULL));
for(i=0; i<88; i++){
nombre_aleatoire = rand()*RAND_MAX/10 - rand()%10000;
printf("%d ",nombre_aleatoire);

}
return 0;
} * i tried something and forgot to delete it.

we stuudied (rand()%1000 for example) but not with big values
and it doesnt work with vaalues larger than 32747 (rand_max)

Patrice T 26-Mar-17 13:18pm

Use Improve question to update your question.
So that everyone can pay attention to this information.