C Program to Generate Random Number with Output Example

Use rand() to generate a random number in C and srand(time(NULL)) to seed the generator for different results each run, like this: srand(time(NULL)); int num = rand();.

Examples

InputNo input needed

OutputRandom number like 1804289383

InputNo input needed

OutputRandom number like 846930886

InputNo input needed

OutputRandom number like 1681692777

How to Think About It

To generate a random number in C, you first set a starting point called a seed using srand() with the current time so the numbers change each time you run the program. Then you call rand() to get a random number. This number is usually large, so you can use the modulus operator % to limit it to a smaller range if needed.

Algorithm

1

Include necessary headers for input/output and time functions.

2

Seed the random number generator using the current time with <code>srand(time(NULL))</code>.

3

Call <code>rand()</code> to generate a random number.

4

Optionally, use modulus <code>%</code> to limit the random number to a desired range.

5

Print the generated random number.

Code

c

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

int main() {
    srand(time(NULL));
    int random_number = rand();
    printf("Random number: %d\n", random_number);
    return 0;
}

Output

Random number: 1804289383

Dry Run

Let's trace the program generating a random number step-by-step.

1

Seed the generator

Call srand(time(NULL)) with current time, e.g., 1680000000, to set the seed.

2

Generate random number

Call rand(), which uses the seed to produce a pseudo-random number, e.g., 1804289383.

3

Print the number

Print the generated number: 1804289383.

Step	Action	Value
1	Seed with time	1680000000
2	Generate rand()	1804289383
3	Print output	Random number: 1804289383

Why This Works

Step 1: Seeding with srand()

Using srand(time(NULL)) sets a different starting point for the random number generator each time the program runs, so numbers vary.

Step 2: Generating with rand()

rand() produces a pseudo-random integer based on the seed, simulating randomness.

Step 3: Printing the result

The program prints the generated number so you can see the random value.

Alternative Approaches

Generate random number in a range

c

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

int main() {
    srand(time(NULL));
    int min = 1, max = 100;
    int random_number = rand() % (max - min + 1) + min;
    printf("Random number between %d and %d: %d\n", min, max, random_number);
    return 0;
}

This limits the random number to a specific range, which is useful for many applications.

Use arc4random() on BSD systems

c

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

int main() {
    unsigned int random_number = arc4random();
    printf("Random number: %u\n", random_number);
    return 0;
}

arc4random() provides better randomness but is not available on all systems.

Complexity: O(1) time, O(1) space

Time Complexity

Generating a random number with rand() takes constant time because it performs a fixed number of operations.

Space Complexity

The program uses a fixed amount of memory for variables and does not allocate extra space.

Which Approach is Fastest?

Using rand() is fast and simple; alternatives like arc4random() may be slower but offer better randomness.

Approach	Time	Space	Best For
rand() with srand()	O(1)	O(1)	Simple random numbers, portable
rand() with modulus for range	O(1)	O(1)	Random numbers in a specific range
arc4random()	O(1)	O(1)	Better randomness on supported systems