RE: [ferret_users] working with data of a month over the years

[Peter Szabo <szabpet83@xxxxxxxxx>]

Hi Ansley and Jaison,

Thank You for your help.
I used this method to have the January and July data:

let day = l[gt=precip]   
let jan = if (mod(day,360) le 30) and (mod(day,360) ge 1) then 1 else 0
let jul = if (mod(day,360) le 210) and (mod(day,360) ge 181) then 1 else 0
let precip2a = precip * jan
let precip2u = precip * jul

I checked writing out the 30 years for one point and I got 0 for every
December 30th and February 1th data either.

Jaison,I did not get the point of this line:
     let t_jan_base =  T[gt=var] - t[gt=var,t=01-JAN-1971] + 1  ! step 2
But the script worked fine changed to this one:
     let t_jan_base =  T[gt=var] + 1  ! step 2

And I used the compresssl function, thanks for that.

Peter


!---------------example starts here-------------------------------
\cancel mode verify
! Description : Example script for extracting January data from a daily 
!                  30-year time series on 360-day calendar.
!
!-----------------------------------------------------------------

!--create a dummy variable : 360_DAY calendar, daily, 30-years

     define axis/t=01-JAN-1971:30-DEC-2000:1/units=days/calendar=360_DAY\
                   /t0=30-JAN-1971  t30yr
     let var = SIN(t[gt=t30yr]/500)
 
!---This method will result in wrong answer 

     let year_days  =  MOD(t[gt=var],360)
     let daily_mask =  IF year_days LE 30 THEN 1
     let var_jan_all= var * daily_mask
     let var_jan    = COMPRESSL(var_jan_all)
     let jan_pts    = var_jan[l=@NGD]
     list jan_pts

!---The accurate method....

     ! 1.  assume that time units are days
     ! 2.  remove the effects of T0 on T-values
     ! 3.  account for 30-DEC (or multiples of 360)

     let t_jan_base =  T[gt=var] - t[gt=var,t=01-JAN-1971] + 1  ! step 2
     let year_days  =  MOD(t_jan_base,360)
     let daily_mask =  IF year_days GE 1 AND year_days LE 30 THEN 1 ! step 3
     let var_jan_all= var * daily_mask
     let var_jan    = COMPRESSL(var_jan_all)
     let jan_pts    = var_jan[l=@NGD]
     list jan_pts

    !list/l=1:35 daily_mask      
    !list/l=325:365 daily_mask      
    !list/l=715:755 daily_mask      
    !list/l=3595:3635 daily_mask
!---------------example ends   here-------------------------------