Cookbooks
C
Chapter 3. Numbers
Generating random numbers

C:
Generating random numbers

How to:

In C, random numbers can be generated using the rand() function, which is part of the C standard library <stdlib.h>. By default, rand() produces pseudo-random numbers in the range from 0 to RAND_MAX (a constant defined in <stdlib.h>). For more control over the range, programmers can manipulate the output of rand().

Here’s a simple example of generating a random number between 0 and 99:

#include <stdio.h>
#include <stdlib.h> // For rand() and srand()
#include <time.h>   // For time()

int main() {
    // Seed the random number generator
    srand((unsigned) time(NULL));

    // Generate a random number between 0 and 99
    int randomNumber = rand() % 100;

    printf("Random Number: %d\n", randomNumber);

    return 0;
}

Sample output could vary each time you run this program:

Random Number: 42

To generate random numbers within a different range, you can adjust the modulus operator (%) accordingly. For instance, rand() % 10 generates numbers from 0 to 9.

It is important to note that seeding the pseudo-random number generator (srand() call) with the current time (time(NULL)) ensures different sequences of random numbers across program executions. Without seeding (srand()), rand() would produce the same sequence of numbers every time the program is run.

Deep Dive

The rand() function and its seeding counterpart srand() have been part of the C standard library for decades. They are based on algorithms that generate sequences of numbers that only appear to be random—hence the term “pseudo-random.” The underlying algorithm in rand() is typically a linear congruential generator (LCG).

While rand() and srand() are sufficient for many applications, they have known limitations, especially concerning the quality of randomness and potential predictability. For applications requiring high-quality randomness, such as cryptographic operations, alternatives like /dev/random or /dev/urandom (on Unix-like systems), or APIs provided by cryptographic libraries, should be considered.

With the introduction of C11, the ISO C standard included a new header, <stdatomic.h>, offering a more refined control for concurrent operations, but not directly concerning randomness. For true randomness in C, developers often turn to platform-specific or external libraries that offer better algorithms or make use of hardware entropy sources.

Remember, while rand() serves as a simple and accessible means to generate pseudo-random numbers, its uses in modern applications are limited by the quality and predictability of its output. When more robust solutions are required, especially for security-conscious applications, exploring beyond the standard library is highly recommended.

Rounding numbers