CODEWITHSUNDEEP
c
cinoxil
cinoxil

Reputation: 11

How can i generate random numbers between 5 and 40000 in C?

Im trying to generate random numbers between 5 and 40000 with using rand() function.And add them to 3x3 matris. I have tried multiply rand with rand and mod 39996 is that correct solution you think ?

Here is the code

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

int main()
{
srand(time(0));

int **matris;

matris=(int **)calloc(3,sizeof(int));   // ilk pointerla 3 kutuya boldum yani 3 satir

for(int i=0;i<3;i++){

    matris[i]=calloc(3,sizeof(int));   // buradada 3 kutunun herbirinin icini 3e boldum
}

for(int i=0;i<3;i++){             // i= matrisin satirini j= matrisin sutununu temsil ediyor
    for(int j=0;j<3;j++){
        matris[i][j]= 5 + ((rand()*rand())%39996); // burada rand() fonksiyonunun karesini alip isleme devam ettim. 5 ile 40000 arasi sayilar olusturuyor.
    }
}

for(int i=0;i<3;i++){
    for(int j=0;j<3;j++){
        printf("%d\t",matris[i][j]);
    }
    printf("\n");

}

return 0;

}

Upvotes: 0

Views: 151

Answers (4)

chux
chux

Reputation: 153447

When RAND_MAX is large, the below is sufficient, although it has a bias.

matris[i][j] = 5 + (rand() % 39996);

As RAND_MAX may only be 32767 (as apperntly in OP's case) , we need more random bits.

if (RAND_MAX == 32767) {
  long r = rand();
  r *= (RAND_MAX + 1L);
  r += rand();
  matris[i][j] = 5 + (r % 39996);
} else {
  matris[i][j] = 5 + (rand() % 39996);
}

rand()*rand() does not form a uniform distribution. In this case we want (RAND_MAX+1)*(RAND_MAX+1) different values.

The range of random numbers generated needs to be at least [0...39996).

Upvotes: 2

pmg
pmg

Reputation: 108978

If you're not worried about bias (*)

randomvalue = rand() % 39996 + 5;

*some values will have a higher probability of being chosen than others*

RAND_MAX is 32767

Hmmm...

call rand() twice and use one single bit from one of the values

// return unbiased random between 5 and 40000
// assuming RAND_MAX == 32767
int unbiased(void) {
    int value;
    do {
        value = rand(); // 0 to 32767
        value = value * 2; // even value from 0 to 65534
        if (rand() & 0x800) value = value + 1; // 0 to 65535
    } while (value > 39995); // ignore biased results
    return value + 5;
}

Upvotes: 0

bjr Gaming
bjr Gaming

Reputation: 1

int random(int min, int max) {
    return (rand() % (max - min + 1)) + min; 
}

Upvotes: 0

0x6261627564
0x6261627564

Reputation: 134

https://www.geeksforgeeks.org/generating-random-number-range-c/

As C does not have an inbuilt function for generating a number in the range, but it does have rand function which generate a random number from 0 to RAND_MAX. With the help of rand() a number in range can be generated as num = (rand() % (upper – lower + 1)) + lower

Upvotes: 1

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