rand

int rand (void);

Generate random number

Returns a pseudo-random integral number in the range between 0 and RAND_MAX .

This number is generated by an algorithm that returns a sequence of apparently non-related numbers each time it is called. This algorithm uses a seed to generate the series, which should be initialized to some distinctive value using function srand .

RAND_MAX is a constant defined in <cstdlib> .

A typical way to generate trivial pseudo-random numbers in a determined range using rand is to use the modulo of the returned value by the range span and add the initial value of the range:

1
2
3

v1 = rand() % 100; // v1 in the range 0 to 99
v2 = rand() % 100 + 1; // v2 in the range 1 to 100
v3 = rand() % 30 + 1985; // v3 in the range 1985-2014

Notice though that this modulo operation does not generate uniformly distributed random numbers in the span (since in most cases this operation makes lower numbers slightly more likely).

C++ supports a wide range of powerful tools to generate random and pseudo-random numbers (see <random> for more info).

Parameters

(none)

Return Value

An integer value between 0 and RAND_MAX.

Example

/* rand example: guess the number */
#include <stdio.h> /* printf, scanf, puts, NULL */
#include <stdlib.h> /* srand, rand */
#include <time.h> /* time */
int main ()
{
 int iSecret, iGuess;
 /* initialize random seed: */
 srand (time(NULL));
 /* generate secret number between 1 and 10: */
 iSecret = rand() % 10 + 1;
 do {
 printf ("Guess the number (1 to 10): ");
 scanf ("%d",&iGuess);
 if (iSecret<iGuess) puts ("The secret number is lower");
 else if (iSecret>iGuess) puts ("The secret number is higher");
 } while (iSecret!=iGuess);
 puts ("Congratulations!");
 return 0;
}

In this example, the random seed is initialized to a value representing the current time (calling time ) to generate a different value every time the program is run.

Possible output:


Guess the number (1 to 10): 5
The secret number is higher
Guess the number (1 to 10): 8
The secret number is lower
Guess the number (1 to 10): 7
Congratulations!

Compatibility

In C, the generation algorithm used by rand is guaranteed to only be advanced by calls to this function. In C++, this constraint is relaxed, and a library implementation is allowed to advance the generator on other circumstances (such as calls to elements of <random>).

Data races

The function accesses and modifies internal state objects, which may cause data races with concurrent calls to rand or srand .

Some libraries provide an alternative function that explicitly avoids this kind of data race: rand_r (non-portable).

C++ library implementations are allowed to guarantee no data races for calling this function.

Exceptions (C++)

No-throw guarantee: this function never throws exceptions.

functions

functions (non-standard)

types

macro constants