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Tech Tip: Calculating the first and last day of a Week from a Date
PRODUCT: 4D | VERSION: 11.4 | PLATFORM: Mac & Win
Published On: September 10, 2009
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Sometimes you need to find information about a week and you only have a date within the week, for example, the current date. The following routine allows you to find the first and last day of the week given a date. The following code assumes that the week begins with Sunday and ends with Saturday.

  `...................................................................
  ` Project Method: Util_Calculate_Week
  ` Description: return first and last day of week based on a date
  ` Parameters: $1 - Date - date to calculate week start and end for
  ` Returns: $2 - Pointer to Date - First Day of the Week
  ` $3 - Pointer to Date - Last Day of the Week

  ` Util_Calculate_Week(current date;->$thisweekstart;->$thisweekend)
  `...................................................................

C_DATE($Date_d;$Date1_d;$Date2_d)
C_INTEGER($DayNumber_l)

C_DATE($1)
C_POINTER($2;$3)

$Date_d:=$1
$DayNumber_l:=Day number($1)

Case of
  : ($DayNumber_l=1)
    $Date1_d:=$Date_d
    $Date2_d:=$Date_d+6
  : ($DayNumber_l=2)
    $Date1_d:=$Date_d-1
    $Date2_d:=$Date_d+5
  : ($DayNumber_l=3)
    $Date1_d:=$Date_d-2
    $Date2_d:=$Date_d+4
  : ($DayNumber_l=4)
    $Date1_d:=$Date_d-3
    $Date2_d:=$Date_d+3
  : ($DayNumber_l=5)
    $Date1_d:=$Date_d-4
    $Date2_d:=$Date_d+2
  : ($DayNumber_l=6)
    $Date1_d:=$Date_d-5
    $Date2_d:=$Date_d+1
  : ($DayNumber_l=7)
    $Date1_d:=$Date_d-6
    $Date2_d:=$Date_d
End case

$2->:=$Date1_d
$3->:=$Date2_d

Commented by Designer on September 18, 2009 at 11:20 AM
Thank you Thomas and J. Buchwitz, the method has been updated.
Many thanks for the optimized routine as well.
Commented by Thomas Maul on September 18, 2009 at 1:21 AM
There is a syntax error in this code, it will not work because:
C_POINTER(vThisWeek1_d;vThisWeek2_d)

As the idea of the code is to return the two dates, I guess best is to change the two last lines to:
$2->:=$Date1_d
$3->:=$Date2_d
And call the method such as:
Util_Calculate_Week(current date;->$thisweekfirstday;->$thisweeklastday)

But the code is too complex, it could be drastically reduced with a simple calculation:
C_DATE($1;vThisWeek1_d)
C_POINTER($2;$3)
vThisWeek1_d:=$1-(Day number($1))+1
$2->:=vThisWeek1_d
$3->:=vThisWeek1_d+6

The whole code can be reduced to 5 lines...
Thanks to J. Buchwitz, Germany, for reporting this.