Reference
C Library
IOstream Library
Strings library
STL Containers
STL Algorithms
Miscellaneous

C Library
cassert (assert.h)
cctype (ctype.h)
cerrno (errno.h)
cfloat (float.h)
ciso646 (iso646.h)
climits (limits.h)
clocale (locale.h)
cmath (math.h)
csetjmp (setjmp.h)
csignal (signal.h)
cstdarg (stdarg.h)
cstddef (stddef.h)
cstdio (stdio.h)
cstdlib (stdlib.h)
cstring (string.h)
ctime (time.h)

cstdlib (stdlib.h)
functions:
abort
abs
atexit
atof
atoi
atol
bsearch
calloc
div
exit
free
getenv
labs
ldiv
malloc
mblen
mbstowcs
mbtowc
qsort
rand
realloc
srand
strtod
strtol
strtoul
system
wcstombs
wctomb
functions (non-standard):
itoa
macros:
EXIT_FAILURE
EXIT_SUCCESS
MB_CUR_MAX
NULL
RAND_MAX
types:
div_t
ldiv_t
size_t

rand

function

<cstdlib>

int rand ( void );

Generate random number

Returns a pseudo-random integral number in the range 0 to 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 srand.

RAND_MAX is a constant defined in <cstdlib>. Its default value may vary between implementations but it is granted to be at least 32767.

A typical way to generate 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:

( value % 100 ) is in the range 0 to 99
( value % 100 + 1 ) is in the range 1 to 100
( value % 30 + 1985 ) is in the range 1985 to 2014

Notice though that this modulo operation does not generate a truly uniformly distributed random number in the span (since in most cases lower numbers are slightly more likely), but it is generally a good approximation for short spans.

Parameters

(none)

Return Value

An integer value between 0 and RAND_MAX.

Example

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26

/* rand example: guess the number */
#include <stdio.h>
#include <stdlib.h>
#include <time.h>
int main ()
{
  int iSecret, iGuess;
  /* initialize random seed: */
  srand ( time(NULL) );
  /* generate secret number: */
  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;
}

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!

In this example, the random seed is initialized to a value representing the second in which the program is executed (time is defined in the header <ctime>). This way to initialize the seed is generally a good enough option for most randoming needs.

rand

Parameters

Return Value

Example

See also