rand() in C

The rand() function in the C programming language is used to generate pseudo-random numbers. It is used in C to generate random numbers in the range 0 to RAND_MAX.

The rand() function is part of the standard C library <stdlib.h> so to use this function, we need to include the <stdlib.h> library.

Syntax of rand()

int rand(void)

Parameters

• This function does not take any parameters.

Return Value

• This function returns the value between 0 and RAND_MAX where RAND_MAX is the macro that is defined in the <stdlib.h> library.

Note: The rand() function by default uses the value 1 as the seed to generate random numbers which leads to the generation of the same sequence of random numbers. To prevent this, we can use srand() function to specify a new seed for rand() function.

Examples of rand() in C

Example 1: Illustration of rand() function

C

// C program to illustrate the use of rand() function
#include <stdio.h>
#include <stdlib.h>

int main()
{
    // Generate a random number using the rand() function
    int value = rand();

    // Print the generated random value
    printf("The Random Value is: %d", value);

    return 0;
}

Output

The Random Value is: 1804289383

Time Complexity: O(1)
Space Complexity: O(1)

Example 2: Generate 10 Random Numbers

C

// C program to generate 10 random number using rand()
// function
#include <stdio.h>
#include <stdlib.h>

int main()
{
    int i = 0;
    // Loop through 10 times
    for (; i < 10; i++) {
        // Generate a random number using the rand()
        // function
        int value = rand();

        // Print the generated random value
        printf("%d ", value);
    }

    return 0;
}

Output

1804289383 846930886 1681692777 1714636915 1957747793 424238335 719885386 1649760492 596516649 1189641421 

But, if we want to generate some random numbers in the range of 0 to N then, we need to modify by printing the modulo of (N+1) of the numbers that are generated by the rand() function.

Example: Generate Random Numbers Smaller than N

C

// C program to generate random numbers smaller than a
// particular number n
#include <stdio.h>
#include <stdlib.h>
int main()
{
    // Set a constant 'N' to 1000
    int N = 1000;

    // Loop through 10 times
    for (int i = 0; i < 10; i++) {
        // Generate a random number between 0 and N using
        // the rand() function
        int value = rand() % (N + 1);
        // Print the generated random value
        printf("%d ", value);
    }

    return 0;
}

Output

897 802 765 992 1 521 220 380 729 969 

In this way, we can easily generate the random number from 0 to N.

Also, if we want a random number that will exist between a lower_bound and upper_bound, we can do this by using simple mathematics on the random number that is generated by rand() function.

Example: Generate Random Numbers Within a Range

C

// C program to illustrate how to generate random number
// withing a range
#include <stdio.h>
#include <stdlib.h>
int main()
{
    // Set the upper bound for random numbers
    int upper_bound = 1000;
    // Set the lower bound for random numbers
    int lower_bound = 100;

    // Loop through 10 times
    for (int i = 0; i < 10; i++) {
        // Generate a random number within the specified
        // bounds
        int value = rand() % (upper_bound - lower_bound + 1)
                    + lower_bound;
        // Print the generated random value
        printf("%d ", value);
    }

    return 0;
}

Output

943 897 704 678 132 783 902 760 689 765