How to declare a date variable in C?

Question

I need to create an abstract type of data that represents a person, containing name, date of birth and CPF, and create a variable which is a pointer to this TAD (no core program).

To a certain extent I know, but I don’t know which is the right kind to put in place of the "?".

struct pessoa
{
    char nome;
    double cpf;
    ?   Data_Nasc;
}x,*ptr;
ptr = &x;

Answer 1

You don’t do this. C is not a language that makes abstractions so easy. Of course nothing prevents you from creating a type to save and manipulate dates, as you did in your previous question. Although the name, probably not adequate structure and does not have auxiliary functions to manipulate the structure, this is basically what should be done: create a structure, which is a type composed of members, and use this within another structure when necessary.

Outside this may use some kind of library ready, but for learning purposes do not think appropriate.

A base without wanting to be complete, optimized to the maximum, would be:

typedef struct {
    int ano;
    short mes;
    short dia;
} Data
struct pessoa {
    char nome;
    double cpf;
    Data data_nasc;
} x,*ptr;

I put in the Github for future reference.

I didn’t quite understand why use double as CPF. The correct would be a char * or char[12], but at worst, if you haven’t learned to play with it yet, be a long. The name is also wrong, or it would have to be a pointer or a array. The way it is only allows 1 character in the name.

Answer 2

The solution pointed out by colleague @Maniero, from the point of view of data abstraction is valid, however, he reinvented the wheel, because the standard library already has all the abstractions of date and time and all the functions necessary to manipulate them.

Most suitable to replace the ? would be the kind time_t defined by the standard library time.h. This type represents the amount of seconds that has passed since the date of January 1, 1970 at 00:00:00 (UTC), is considered the zero milestone of the calendar system used by computers.

Although the Gregorian calendar facilitates chronological reasoning for humans, when logical comparisons are desired, or calculations with dates on computers, this kind of calendar ends hindering the work.

For example, for us, to know what happened first, if it was something in 10/04/1977 12:45:15 or something at 10/03/1976 13:09:12 is something almost but to solve this on a computer all 6 fields would have to be analysed independently, although performed almost instantaneously does not cease to be a job extra that the processor could avoid if it made use of another format date. Let’s imagine a database with thousands of records and the processor receiving a command to put everything in order chronological, if we can make the comparisons with a single operation by registration instead of 6 operations/record the final time also will tend to be 6 times smaller.

The following example illustrates the conversion of a value of the type time_t for a date and time humanly readable:

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


int main( int argc, char * argv[] )
{
    time_t agora;
    char datahora[100];

    /* Recupera a quantidade de segundos desde 01/01/1970 */
    agora = time(NULL);

    /* Formata a data e a hora da forma desejada */
    strftime( datahora, sizeof(datahora), "%d.%m.%Y - %H:%M:%S", localtime( &agora ) );

    printf( "Data/Hora: %s\n", datahora );

    return 0;
}

Already another, illustrates how to read a date and time in human format for the amount of seconds from ground zero:

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

char datahora[] = "01.01.2016 - 12:00:00";  

int main( int argc, char * argv[] )
{
    struct tm tm; 
    int ano, mes; 

    sscanf( datahora, "%d.%d.%d - %d:%d:%d", &tm.tm_mday, &mes, &ano, &tm.tm_hour, &tm.tm_min, &tm.tm_sec );

    tm.tm_year = ano - 1900;
    tm.tm_mon = mes - 1;

    printf( "Segundos desde 01/01/1970: %ld\n", mktime( &tm ) );

    return 0;
}

The best part is when we need to compare dates, we can use comparison operators, the same way we compare integers:

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

char datahora1[] = "12.01.2000 - 12:00:00"; 
char datahora2[] = "01.01.2015 - 13:10:20";  

int main( int argc, char * argv[] )
{
    time_t t1;
    time_t t2;

    struct tm tm; 
    int ano, mes; 

    sscanf( datahora1, "%d.%d.%d - %d:%d:%d", &tm.tm_mday, &mes, &ano, &tm.tm_hour, &tm.tm_min, &tm.tm_sec );

    tm.tm_year = ano - 1900;
    tm.tm_mon = mes - 1;

    t1 = mktime( &tm );

    sscanf( datahora2, "%d.%d.%d - %d:%d:%d", &tm.tm_mday, &mes, &ano, &tm.tm_hour, &tm.tm_min, &tm.tm_sec );

    tm.tm_year = ano - 1900;
    tm.tm_mon = mes - 1;

    t2 = mktime( &tm );

    if( t1 < t2 )
    {
        printf( "Data1 esta antes de Data2!\n");
    }
    else if( t1 > t2 )
    {
        printf( "Data1 esta depois Data2!\n");
    }
    else
    {
        printf( "Data1 eh iguaal a Data2!\n");
    }

    return 0;
}

I hope I’ve helped!

Answer 3

Note that it is not necessary for each data to be mapped to a variable in its structure. With this in mind, a simple solution is to have one variable for day, another for month plus one for year of birth.

struct pessoa
{
    char nome;
    double cpf;
    int dia_de_nascimento;
    int mes_de_nascimento;
    int ano_de_nascimento;
}

One remark: the field type nome of its structure is probably wrong, unless its program accepts only one-character names.

Answer 4

C has a type of data that stores dates. Uilque mentioned the library in his answer (time.h). It follows the official documentation in Portuguese:

http://pt.cppreference.com/w/c/chrono/tm

Here’s an example of how to get a current date on original in English:

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

int main(void)
{
    struct tm start = {.tm_mday=1};
    mktime(&start);
    printf("%s\n", asctime(&start));
}

If you’re doing this to learn on your own, I suggest starting with an easier language like C#, Java or Javascript (unless you already know them and want to really dive into C). Now, whether this is for a college or college assignment... your teacher is a madman or executioner. Continue to study to pass, but try to learn an easier language like the ones I mentioned. The date and time structures in these languages are much easier to use for those at the beginning.

Answer 5

According to this example in Wikipedia, would be something like:

#include <time.h>

int main(void)
{
    time_t dataEHorario;

    // Você ainda pode setar a data para o atual do sistema
    current_time = time(NULL);

    exit(EXIT_SUCCESS);
}