rand

Generates a pseudorandom number by using a well-known and fully-reproducible algorithm. A more programmatically secure version of this function is available; see rand_s. Numbers generated by rand are not 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 is 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 is not 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>. For information about what's wrong with rand and how <random> addresses these shortcomings, see this video entitled rand Considered Harmful.

Requirements

Routine Required header
rand <stdlib.h>

For additional compatibility information, see Compatibility.

Example

// crt_rand.c
// This program seeds the random-number generator
// with the time, then exercises the rand function.
//

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

void SimpleRandDemo( int n )
{
   // Print n random numbers.
   int i;
   for( i = 0; i < n; i++ )
      printf( "  %6d\n", rand() );
}

void RangedRandDemo( int range_min, int range_max, int n )
{
   // Generate random numbers in the half-closed interval
   // [range_min, range_max). In other words,
   // range_min <= random number < range_max
   int i;
   for ( i = 0; i < n; i++ )
   {
      int u = (double)rand() / (RAND_MAX + 1) * (range_max - range_min)
            + range_min;
      printf( "  %6d\n", u);
   }
}

int main( void )
{
   // Seed the random-number generator with the current time so that
   // the numbers will be different every time we run.
   srand( (unsigned)time( NULL ) );

   SimpleRandDemo( 10 );
   printf("\n");
   RangedRandDemo( -100, 100, 10 );
}
22036
18330
11651
27464
18093
3284
11785
14686
11447
11285

   74
   48
   27
   65
   96
   64
   -5
  -42
  -55
   66

See also

Floating-Point Support
srand
rand_s