[Resolved] Date Stuff

[The Hobo]

How can I get last monday's date?

Specifically, I want something like:

if (today == friday || today == saturday || today == sunday) {
  //get last monday
} else {
  //if today is monday, tuesday, wednesday, etc. get two mondays ago
}

Any ideas?

[jim mcnamara]

This shows you how to get the day of the week for any date
from Jan 1, 1900 on.

difftime returns the number of seconds between to time_t time values.

You can use that and mktime to make dates in the past.

#include <time.h>
#include <string.h>
/* function to return day of the week using input date format MM-DD-YYYY*/

char * weekday(char *dest, char *date){
	struct tm t;
	int i;
	time_t time_of_day;
	char tmp[20];
	char *buf;
	for(buf=date,i=0;i<6;i++,buf++);
	t.tm_year=1900-atoi(buf);
	strcpy(tmp,date);
	for(buf=tmp,i=0;i<3;i++,buf++);
	*(buf+2)=0x00;
	t.tm_mon=atoi(buf);
	buf=tmp;
	*(buf+2)=0x00;
	t.tm_mday=atoi(buf)-1;
	t.tm_hour=0;
	t.tm_min=0;
	t.tm_sec=1;
	t.tm_isdst=0;
	time_of_day=mktime(&t);
	memset(tmp,0x00,sizeof(tmp));
	strcpy(tmp,ctime(&time_of_day));	
	strcpy(dest,tmp);	
	for(buf=dest;*buf!=0x20;buf++);
	*buf=0x00;
	return dest;
}
char *weekday(char *,char *);
int main(){
     char result[10];
     printf("%s\n",weekday(result,"04-04-2001") );
     return 0;
}

[jim mcnamara]

Oh.

'Today' is:

char *today(char *dest){
	time_t lt;
	struct tm *ptr;
	lt=time(NULL);
	ptr=localtime(&lt);
	strftime(dest,"%d-%m-%Y",ptr);
	return dest;
}

Look up the strftime function to format your dates the way you want.

[The Hobo]

Thanks for your help thus far, Jim.

I know how to get today's date and how to format the date (I actually learned from some of your old posts a few months ago), but how do I get last Monday's date?

I even know how to find out the day (as in Tuesday), and I can even tell how many days ago last Monday was. But where to go from there?

I can't simply subtract that number from a date variable.

Any ideas on this?

[The Hobo]

That first code you provided just bombs out when I try it

It executes up until this line:

strcpy(tmp,ctime(&time_of_day));

[The Hobo]

in the time.h header file, I looked at this:

struct tm {
    int tm_sec;     /* seconds after the minute - [0,59] */
    int tm_min;     /* minutes after the hour - [0,59] */
    int tm_hour;    /* hours since midnight - [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 */
};

tm_wday looks like it might hold the key, but how do I access that?

[The Hobo]

This is what I have so far:

#include <time.h>
#include <iostream>
using namespace std;

void main()
{
    char tmpbuf[128];
    time_t ltime;
    struct tm *today;

    int days, diff;

    time(&ltime);
    today = localtime(&ltime);
    strftime(tmpbuf, 128,"%w", today);
    
    days = atoi(tmpbuf);

    switch (days) {
        case 0: //sunday
            diff = 6; break;
        case 1: //monday
            diff = 7; break;
        case 2: //tuesday
            diff = 8; break;
        case 3: //wednesday
            diff = 9; break;
        case 4: //thursday
            diff = 10;
        case 5: //friday
            diff = 4; break;
        case 6: //saturday
            diff = 5; break;
    }

    cout << diff << endl << endl;

    //need to subtract diff from today to get 
    //last monday's date.
}

Is what I have so far.

[jim mcnamara]

The code runs fine for me... both unix and Borland.
...maybe you need _Ctime or some other MS malapropism.


subtracting days from a date:

#include <stdio.h>
#include <time.h>
#define DAY 24*3600
time_t date_subtract(time_t, int);
int main(){      
     time_t now, then;
     now=time(NULL);
     then = date_subtract(now, 5); /* five days ago */
     printf(ctime(&then));    
     return 0;
} 
time_t date_subtract(time_t now, int days){
	time_t tmp;
	tmp=days * DAY;
	return difftime(now,tmp);
}

[The Hobo]

Got it:

#include <time.h>
#include <iostream>
using namespace std;

void main()
{
    char tmpbuf[128];
    time_t ltime;
    struct tm *today;
    
    struct tm when;
    time_t now, result;

    int days, diff;

    time(&ltime);

    when = *localtime( &ltime );

    today = localtime(&ltime);
    strftime(tmpbuf, 128,"%w", today);
    
    days = atoi(tmpbuf);

    switch (days) {
        case 0: //sunday
            diff = 6; break;
        case 1: //monday
            diff = 7; break;
        case 2: //tuesday
            diff = 8; break;
        case 3: //wednesday
            diff = 9; break;
        case 4: //thursday
            diff = 10;
        case 5: //friday
            diff = 4; break;
        case 6: //saturday
            diff = 5; break;
    }
    
    when.tm_mday = when.tm_mday - diff;
    mktime( &when );

    cout << asctime( &when ) << endl << endl;
}

Edit: Little more simplified:

#include <time.h>
#include <iostream>
using namespace std;

void main()
{
    char tmpbuf[128];
    
    time_t atime;

    struct tm *today;
    struct tm when;

    int diff;

    time(&atime);

    when = *localtime( &atime );
    today = localtime(&atime);

    strftime(tmpbuf, 128,"%w", today);
    diff = atoi(tmpbuf);

    if (diff > 4) {
        diff = diff - 1;
    } else {
        diff = diff + 6;
    }
    
    when.tm_mday = when.tm_mday - diff;
    mktime( &when );

    cout << "Last Monday: " << asctime( &when ) << endl << endl;
}

[jim mcnamara]

Try dummying up your code for Dec 1 (or set your system time forward). It won't work. tm_days will be one, subtract 5, gives minus 4. The results from that are undefined, I believe.

Use difftime.

[The Hobo]

Works fine, unless I did the test wrong:

#include <time.h>
#include <iostream>
using namespace std;

void main()
{
    char tmpbuf[128];
    
    time_t atime;

    struct tm *today;
    struct tm when;

    int diff;

    time(&atime);

    when = *localtime( &atime );
    today = localtime(&atime);

    strftime(tmpbuf, 128,"%w", today);
    diff = atoi(tmpbuf);

    if (diff > 4) {
        diff = diff - 1;
    } else {
        diff = diff + 6;
    }
    
    when.tm_mday = 1; //when.tm_mday - diff;
    when.tm_mon = 11;
    mktime( &when ); //december 1st
    cout << asctime( &when ) << endl << endl;

    when.tm_mday = when.tm_mday - 5;
    mktime( &when ); //december 1st minus 5 days
    cout << asctime( &when ) << endl << endl;
}

Gives output:

Sun Dec 01 15:11:26 2002


Tue Nov 26 15:11:26 2002


Press any key to continue

[The Hobo]

this is my final code, just incase anyone is wondering:

#include <time.h>
#include <iostream>
using namespace std;

tm lastmonday();

int main() {
    tm when;
    char date[128];

    when = lastmonday();
    strftime(date, 128, "%#x", &when);

    cout << "Last Monday: " << date << endl << endl;

    return 0;
}

//function to retrieve the date of last monday
tm lastmonday() {
   
    time_t atime;
    tm when;
    int diff;

    time(&atime);
    when = *localtime(&atime);

    diff = when.tm_wday;

    if (diff > 4) {
        diff = diff - 1;
    } else {
        diff = diff + 6;
    }
    
    when.tm_mday -= diff;
    mktime(&when);

    return when;
}

Thanks for your help, Jim.