C Random Numbers

Random Numbers

In C, you can make random numbers with the rand() function, which is found in the <stdlib.h> library.

By default, rand() gives the same sequence of numbers each time you run the program. To get different results on every run, you can also use srand() to set a "starting point" (called a seed). Since you just learned about date and time, you can now use the current time as the seed (because it is always changing). For this, you include <time.h> in your program.

Basic Random Number

The function rand() returns a random integer.

Example

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

int main() {
  int r = rand();
  printf("%d\n", r);
  return 0;
}

Note: If you run this program several times, you will see the same numbers each time. This is because we have not set a seed yet.

Seeding the Random Generator

To get different numbers every time you run the program, you must give rand() a starting point (seed).

You do this with srand(). A common trick is to use the current time as the seed, because the time is always changing:

Example

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

int main() {
  srand(time(NULL));  // seed with current time

  printf("%d\n", rand());
  printf("%d\n", rand());
  printf("%d\n", rand());
  return 0;
}

Tip: Call srand() only once, at the start of main. Do not call it again inside a loop.

Random Number in a Range

Often you want numbers in a smaller range, like 0 to 9. You can do this by using the modulo operator %:

Example

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

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

  int x = rand() % 10;  // 0..9
  printf("%d\n", x);
  return 0;
}

Real-Life Example: Rolling Dice

A common real-life use of random numbers is rolling a six-sided dice. We can simulate this by using rand() % 6 to get numbers from 0 to 5, and then add 1 to make the range 1 to 6:

Example

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

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

  int dice1 = (rand() % 6) + 1;
  int dice2 = (rand() % 6) + 1;
  printf("You rolled %d and %d (total = %d)\n", dice1, dice2, dice1 + dice2);

  return 0;
}

Each time you run the program, you will get two random numbers between 1 and 6, just like rolling two dice in real life.