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（以…为模）operation does not generate a truly uniformly distributed random number in the span（跨度,跨距,范围） (since in most cases lower numbers are slightly （些微地，轻微地；纤细地）more likely), but it is generally a good approximation （接近；近似值；近似法）for short spans.
Parameters
(none)
Return Value
An integer value between 0 and RAND_MAX.
Example
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/* 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 higherGuess the number (1 to 10): 8 The secret number is lowerGuess 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.
