rand

Generates a pseudorandom number. A more programmatically secure version of this function is available; see rand_s. Numbers generated by rand aren't cryptographically secure. For more cryptographically secure random number generation, use rand_s or the functions declared in the C++ Standard Library in <random>.

Syntax

int rand(void);

Return value

rand returns a pseudorandom number, as described above. There's no error return.

Remarks

The rand function returns a pseudorandom integer in the range 0 to RAND_MAX (32767). Use the srand function to seed the pseudorandom-number generator before calling rand.

The rand function generates a well-known sequence and isn't appropriate for use as a cryptographic function. For more cryptographically secure random number generation, use rand_s or the functions declared in the C++ Standard Library in <random>.

By default, this function's global state is scoped to the application. To change this behavior, see Global state in the CRT.

Requirements

For more compatibility information, see Compatibility.

Example

// crt_rand.c
// This program seeds the random-number generator
// with a fixed seed, then exercises the rand function
// to demonstrate generating random numbers, and
// random numbers in a specified range.
#include <stdlib.h> // rand(), srand()
#include <stdio.h> // printf()
void SimpleRandDemo(int n)
{
 // Print n random numbers.
 for (int i = 0; i < n; i++)
 {
 printf(" %6d\n", rand());
 }
}
void RangedRandDemo(int range_min, int range_max, int n)
{
 // Generate random numbers in the interval [range_min, range_max], inclusive.
 for (int i = 0; i < n; i++)
 {
 // Note: This method of generating random numbers in a range isn't suitable for
 // applications that require high quality random numbers.
 // rand() has a small output range [0,32767], making it unsuitable for
 // generating random numbers across a large range using the method below.
 // The approach below also may result in a non-uniform distribution.
 // More robust random number functionality is available in the C++ <random> header.
 // See https://learn.microsoft.com/cpp/standard-library/random
 int r = ((double)rand() / RAND_MAX) * (range_max - range_min) + range_min;
 printf(" %6d\n", r);
 }
}
int main(void)
{
 // Seed the random-number generator with a fixed seed so that
 // the numbers will be the same every time we run.
 srand(1792);
 printf("Simple random number demo ====\n\n");
 SimpleRandDemo(10);
 printf("\nRandom number in a range demo ====\n\n");
 RangedRandDemo(-100, 100, 100000);
}```
```Output
Simple random number demo ====
 5890
 1279
 19497
 1207
 11420
 3377
 15317
 29489
 9716
 23323
Random number in a range demo ====
 -82
 -46
 50
 77
 -47
 32
 76
 -13
 -58
 90

See also

Math and floating-point support
srand
rand_s
C++ <random> library