C language rand () function and related time library

rand () function

rand () function prototype int rand (void), it returns an integer of from 0 to RAND_MAX (RAND_MAX C language standard library <stdlib.h> is defined in a macro 0x7fff i.e. 32767)

But only with rand () returns the result is constant, since the rand () function is based on a number (called seed) by a fixed formula computed from, but computer is turned on, the seed value is given up, so the results are unchanged

Providing C srand () had its prototype void srand (int a) function is to initialize rand () function of the initial value

But for a seed, we can not produce every time a random number, then manually enter it once a

So we have to time your computer as a seed, then the seeds will own changed (time () function explanation see below)

#include <bits / STDC ++ H.>
 the using  namespace STD;
 int main () 
{ 
    srand (Time (NULL)); // Time (NULL) is the system time 
    COUT << RAND () << endl;
     return  0 ; 
}

 

How that generates random numbers within a certain range of it, it is easy to think of using "%" and "+" to achieve

Since 0 <= rand ()% (nm + 1) <= nm

So m <= rand () n% (n-m + 1) + m <=

#include <bits / STDC ++ H.>
 the using  namespace STD;
 int main () 
{ 
    int n-, m; 
    CIN >> m >> n-; 
    srand (Time (NULL)); 
    COUT << RAND ()% (n-- m + . 1 ) + m << endl;   // generates a random integer between [m, n-] 
    return  0 ; 
}

time.h library related

time.h header file defines the four types of variables, and various operation functions of two macros date and time.

Four variables

size_t	Is an unsigned integer type, it is the result of sizeof keyword
clock_t	This is a type of memory for processor time, unsigned long type
time_t	This is a type suitable for storing calendar time
struct tm	This is a location to save the time and date of construction

 

 

 

 

 

tm structure is defined as follows:

struct tm

{

　　int tm_sec;/* seconds - [0,59] */

　　int tm_min;/* minutes - [0,59] */

　　int tm_hour;/* hours - [0,23] */

　　int tm_mday;/* day of the month - [1,31] */

　　int tm_mon;/* months since January - [0,11] */

　　int tm_year;/* years since 1900 */

　　int tm_wday;/* days since Sunday - [0,6] */

　　int tm_yday;/* days since January 1 - [0,365] */

　　int tm_isdst;/* daylight savings time flag */

};

 

Two macro

NULL	This macro is a null pointer value constant
CLOCKS_PER_SEC	It represents the number of clock cycles within a second CPU running Results clock () function is used to convert an amount in seconds But the specific value of this amount is related to the operating system, typically 1000

 

 

 

 

 

 

Part of the library functions

difftime ():  to calculate two seconds as the  difference between the target time_t calendar time (time_end - time_beg), if  time_end the representative prior to the  time_beg time point, the result is negative. Function prototype: double difftime (time_t time2, time_t time1)

mktime ():  Function Prototype: time_t mktime (struct tm * timeptr ), the structure pointed to convert timeptr based on a local time zone value time_t

time ():  returns the encoded into  time_t object's current calendar time, and the storage  arg points  time_t object (except  arg for the null pointer, get the current calendar time when the timer = NULL (from 00:00:00 to 1970-01-01 now the number of seconds), the time when the timer = value, to set the calendar time, is an unsigned long type time_t function prototype:. time_t time (time_t * timer )

#include <bits/stdc++.h>
using namespace std;
int main()
{
    time_t seconds;
    seconds = time(NULL);
    printf("Hours since 1970-01-01 = %ld\n", seconds / 3600);
    return 0;
}

 

clock (): returns from performing clock function (usually at the beginning of the program), the clock time of the processor used, typically is used to calculate the certain period of execution time of the program or function prototype: clock_t clock (void) 

#include <bits / STDC ++ H.>
 the using  namespace STD;
 int main () 
{ 
    of clock_t START_TIME, finish_time; 
    START_TIME = Clock (); 

    // running a test program can be run this time 

    finish_time = Clock ();
     Double TOTAL_TIME = ( Double ) (finish_time - START_TIME) / CLOCKS_PER_SEC;   // converts the time in seconds to milliseconds if you want to give to CLOCKS_PER_SEC deleting 
    the printf ( "the Program of the time running iS : F% \ n- " , TOTAL_TIME);
     return  0 ; 
}

 

asctime (): The time variable is defined as tm structure represented by the string representation out function prototype: char * asctime (struct tm * ptr)

#include <bits/stdc++.h>
using namespace std;
int main()
{
    struct tm t;
    t.tm_sec = 10;
    t.tm_min = 50;
    t.tm_hour = 16;
    t.tm_mday = 6;
    t.tm_mon = 7;
    t.tm_year = 119;
    t.tm_wday = 2;

    printf("%s\n", asctime(&t));//2019/8/6 16:50:10 星期二
    return (0);
}

 

localtime ():  function prototype: struct  tm  * localtime (  const  time_t  * Time  ), conversion (at a given time from the start of epoch  time is directed  values time_t) to form struct tm calendar time and the local time expression

#include <bits/stdc++.h>
using namespace std;
int main()
{
    time_t timer;
    struct tm *Now;
    time(&timer);            //获取当前时间
    Now = localtime(&timer); //填充struct tm 结构
    printf("Current local time and date：%s", asctime(Now));
    return (0);
}

 

 ctime ():  Converts the specified time from the start of the era to the cost of calendar time, and then into the text display, as calls  asctime ( localtime ( Time ) ), the function prototype: char * ctime (const time_t * timer)

#include <bits/stdc++.h>
using namespace std;
int main()
{

    time_t curtime;
    time(&curtime);
    printf("current time = %s", ctime(&curtime));
    return (0);
}