How to Generate Random Number in C

In C, we can use rand() function to generate a random number. It is defined in stdlib.h header file and it generates a random number between 0 and RAND_MAX. In my computer, RAND_MAX returns 532767.

Some posts on the Internet share a way to generate a random number like this:

#include<stdio.h>
#include<stdlib.h>
#include<time.h>
 
int main(){
    srand(time(NULL));
    printf("%d", rand());
}

To generate a random number in a given range from a lower number to an upper number, some people make a function, like this:

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

void printRandoms(int lower, int upper, int count){
    int i;
    for (i = 0; i < count; i++) {
        int num = (rand() %
        (upper - lower + 1)) + lower;
        printf("%d ", num);
    }
}

int main(){
    int lower = 5, upper = 7, count = 1;
    srand(time(0)); 
    printRandoms(lower, upper, count);
}

In my opinion, using rand() is not a secure way of generating random numbers if you are developing a real-world application. To generate a secure random number, here is my solution:

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

int RAND(int a,int b){
    float d=RAND_MAX/(b-a+1);
    return a+round(rand()/d);
}

int main(){
    for(int i=1;i<=10;i++){
        printf("%5d",RAND(2,5));
    }
    printf("%d",RAND_MAX);
}

Instead of using srand(time(0)), I create a function to generate a random number in the [a;b] range. I have split RAND_MAX into $d=b-a+1$ chunks. Then round the division  rand()/d  to integer, this results in a range from $0$ to $b-1$. So I added $a$ to get the desired result.

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