DT_COMPRESS Function

Removes previously defined holidays and weekends from the Julian day portion of !DT structures in a date/time variable.

NOTE: Avoid using DT_COMPRESS for normal XY plotting with the PLOT and OPLOT commands. Use the Compress keyword with PLOT and OPLOT to create compressed date/time results.

The value of the system variables !PDT.Exclude_Holiday and/or !PDT.Exclude_Weekend must be set to one (the default) before DT_COMPRESS is called. In addition, the functions CREATE_WEEKENDS and CREATE_HOLIDAYS must be run before you use DT_COMPRESS.

Note that the result of DT_COMPRESS is a double array of Julian days, not another array of !DT structures.

Example 1

This example demonstrates how DT_COMPRESS can be used to compress the weekend days from a date/time variable containing the days in the month of March, 1992. The resulting array of compressed Julian numbers is then processed so that it can be used to create a date/time plot in a specialized plotting application.

march1 = VAR_TO_DT(1992, 3, 1, 11, 30, 0)
PRINT, march1

{ 1992 3 1 11 30 0.00000 87462.479, 0}

; Creates and prints out a variable march1 which is a date/time
; variable. Note the Julian Day carefully, 87462. After 
; compression, this value will be smaller. This is because all the
; weekends from Julian day 1 (September 14, 1752) are
; compressed.

marray = DTGEN(march1,31, /Day)

; Generates a date/time array containing the 31 days of the month of
; March, 1992.

PRINT, marray

{ 1992 3 1 0 0 0.00000 87462.479 0}
.
.
{ 1992 3 31 0 0 0.00000 87492.479 0}

CREATE_WEEKENDS, ['Saturday', 'Sunday']

; Defines Saturday and Sunday as weekend days.

cmarray = DT_COMPRESS(marray)
PRINT, cmarray

62472.5 62473.0 62474.0 62475.0
62476.0 62477.0 62477.5 62477.5
62478.0 62479.0 62480.0 62481.0
62482.0 62482.5 62482.5 62483.0
62484.0 62485.0 62486.0 62487.0
62487.5 62487.5 62488.0 62489.0
62490.0 62491.0 62492.0 62492.5
62492.5 62493.0 62494.0

; Creates and prints out a compressed array for the month of
; March, 1992. Weekend (compressed) days can be identified
; by fraction .5. Note that the values of the weekend days fall
; between the end and beginning of the week. Also note that the
; Julian numbers are smaller than in the original array. This is
; because all of the weekends from Julian day 1 are 
; compressed.

The following block of code must be run before you can use this array of Julian numbers to generate a date/time axis. The DT_COMPRESS function removed the fractional Time portion of each Julian day, leaving date values with .0 or .5 appended to them. A .0 value indicates that the day is a weekday. A .5 value indicates a compressed day (a weekend). To generate a meaningful plot with this data, two things must be done.

First, the compressed days (the ones ending in .5) must be incremented to the value of the next whole day. Second, the Time portion of the Julian numbers representing weekdays must be restored.

The following code accomplishes both of these objectives:

FOR i=0, 30 DO BEGIN $

whole_day = DOUBLE(FIX(cmarray(i))) $
delta_day = cmarray(i) - whole_day $
IF (delta_day GE 0.4) AND $
(delta_day LE 0.6) THEN BEGIN $

; Determine if a date value is a weekend day. If it is, then 
; increment its value to the value of the next whole day.

marray(i) = whole_day + 1.0d $
ENDIF $

ELSE BEGIN $

fract_day = marray(i).julian - $
DOUBLE(FIX(marray(i).julian)) $
cmarray(i) = whole_day + fract_day $

; Restore the fractional portion of weekday Julian values
; from the original date/time array variable.

ENDELSE $

ENDFOR

After this code is run, cmarray can be used to generate a date/time plot for a specialized plotting application

one where the regular PLOT routine is not sufficient. For example, this data could be used to generate a bar chart.

Before using this date data, however, you must first call PLOT or OPLOT with the XType keyword set to 2. This establishes the plot axis and coordinate system, and allows the date/time axis to be generated from an array of Julian numbers.

Example 2

This example defines some holidays for the year 1992 with the CREATE_HOLIDAYS procedure, creates an array with all the days of the year, and then excludes these holidays using the DT_COMPRESS function.

christmas = VAR_TO_DT(1992, 12, 25)
PRINT, christmas

{ 1992 12 31 0 0 0.00000 87761.000 0}

; Create and print out a date/time variable for Christmas. 
; The purpose is to show the Julian day before using
; DT_COMPRESS. Note the Julian Day is 87761.

day1 = VAR_TO_DT(1992, 1, 1)
yarray = DTGEN(day1, 366)

; Create a variable day1 which is used to generate an array that 
; contains all the days of the year (where 366 is used because 
; 1992 is a leap year).

x = ['1-1-92', '5-31-92','7-4-92', $

'-1-92', '11-24-92', '12-25-92']

; Create an array containing date information for the following 
; holidays: New Years, Memorial day, Fourth of July, Labor Day,
; Thanksgiving, and Christmas.

holidays = STR_TO_DT(x, Date_Fmt=1)

; Create a date/time variable for the holidays.

CREATE_HOLIDAYS, holidays

; Define the holidays by setting the !Holiday_List system variable.

cyarray = DT_COMPRESS(yarray)

; Creates an array that excludes the holidays for 1992. The 
; compressed array cyarray appends the .5 decimal to all of the 
; holidays. Non-holidays end in .0. When you print out cyarray, 
; note the Julian day for Christmas is 87755.5. This is six days 
; less than for the yarray, because six holidays were defined 
; and compressed out of the result.

DT_COMPRESS Function

Usage

Input Parameters

Returned Value

Keywords

Discussion

Example 1

Example 2

See Also