Random Numbers

How do we get a random number?

C provides a function to generate a random number, called rand(). The prototype is in <stdlib.h>. So make sure you have #include<stdlib.h>

     int rand(void);

It is given nothing (that's what void means), and returns a random integer. Unfortunately, the number is in the range 0 to the maximum positive integer we can represent (about 2 billion).

How do we use it?

int x;

x = rand(); 

/* x now holds a random number between 0 and 2 billion */

How do we limit the range to be from 0 to 9?

Well, the modulus (%) operator can help us can help do that by

int x;

x = rand()%10 //this results in random numbers between 0 and 9

Could we use this code above to generate numbers from 10-19?

we can add 10 to all the numbers in the above code to accomplish this.

int x;

x = rand()%10 +10;

Let's break this down.

• We can use modulus to create a random range of numbers and shift those numbers by adding a value to them
• Can we come up with a function that generates random numbers between a certain start and stop value using this method.
• Yes, It's shown below.

int pick (int start, int stop){

int y = (rand()%(stop - start+1))+start;

return y;

}

Seeding Rand()

It turns out that if you use rand() with seeding properly, you're likely to get the same random number over and over. To do this, we have to use the time.h library as well as the srand() function.

To properly seed random, you can use the following code. This only needs to be done once when your code start.

#include<stdlib.h>

#include<time.h>

int main(){

     srand(time(0)); //This line seeds the rand() function

}