Re: [ferret_users] A request for variable construction

["Ansley C. Manke" <ansley.b.manke@xxxxxxxx>]

    Hi Olivier,
    
    The construction
    
    i[i=1:5]
    
    uses the "abstract" axis in the I direction, a pre-defined axis
    which can be referred to in any direction.  In a new session,
    
    yes? show axis/all
     name       axis              # pts   start               
      end
     MONTH_IRREG TIME              12mi   16-JAN 12:00        
      15-DEC 17:49
     MONTH_REG TIME                12mr   16-JAN 06:00        
      16-DEC 01:20
     SEASONAL_REG TIME              4mr   15-FEB 15:43        
      15-NOV 14:05
     MONTH_GREGORIAN TIME          12mi   16-JAN 12:00        
      15-DEC 17:49
     MONTH_NOLEAP TIME             12mi   16-JAN 12:00        
      16-DEC 12:00
     MONTH_360_DAY TIME            12mr   16-JAN 00:00        
      16-DEC 00:00
     MONTH_ALL_LEAP TIME           12mi   16-JAN 12:00        
      16-DEC 12:00
     MONTH_JULIAN TIME             12mi   16-JAN 12:00        
      15-DEC 18:00
     ABSTRACT                99999999 r   1                   
      1.E+08
     EZ                         20480 r   1                   
      20480
    
    
    It's like any axis; it starts when it's defined to start at index 1,
    but the syntax is shortened.  i[i=1:5] is equivalent to
    i[gx=abstract,i=1:5].
    
    
    One alternative would be to define an axis of any length, making it
    modulo:
    
    yes? define axis/modulo=100/x=1:100:1 all_i
    yes? list i[gx=all_i,i=-10:10]
    
    
    and as to skipping values, it there's a subtlety here.  Looking at
    it with an axis that isn't just index values:
    
    yes? use etopo20
    
    yes? list x[gx=rose,i=1:5]
                 VARIABLE : X
                            axis ETOPO20X1_1081
                 FILENAME : etopo20.cdf
                 FILEPATH :
      /home/users/tmap/ferret/linux/fer_dsets/data/
                 SUBSET   : 5 points (LONGITUDE)
     20.17E / 1:  20.17
     20.5E  / 2:  20.50
     20.83E / 3:  20.83
     21.17E / 4:  21.17
     21.5E  / 5:  21.50
    
    yes? list i[gx=rose,i=1:5]
                 VARIABLE : I
                            axis ETOPO20X1_1081
                 FILENAME : etopo20.cdf
                 FILEPATH :
      /home/users/tmap/ferret/linux/fer_dsets/data/
                 SUBSET   : 5 points (LONGITUDE)
     20.17E / 1:  1.000
     20.5E  / 2:  2.000
     20.83E / 3:  3.000
     21.17E / 4:  4.000
     21.5E  / 5:  5.000
    
    yes? list x[gx=rose,i=1:10:2]
                 VARIABLE : X
                            axis (AX007)
                 FILENAME : etopo20.cdf
                 FILEPATH :
      /home/users/tmap/ferret/linux/fer_dsets/data/
                 SUBSET   : 5 points (LONGITUDE)
     20.17E / 1:  20.17
     20.83E / 2:  20.83
     21.5E  / 3:  21.50
     22.17E / 4:  22.17
     22.83E / 5:  22.83
    
    yes? list i[gx=rose,i=1:10:2]
                 VARIABLE : I
                            axis (AX006)
                 FILENAME : etopo20.cdf
                 FILEPATH :
      /home/users/tmap/ferret/linux/fer_dsets/data/
                 SUBSET   : 5 points (LONGITUDE)
     20.17E / 1:  1.000
     20.83E / 2:  2.000
     21.5E  / 3:  3.000
     22.17E / 4:  4.000
     22.83E / 5:  5.000
    
    In the last listing there, apparently the thinking in designing
    this is that when asking for i, we're getting the index of the
    _expression_ being listed, that is, every-second-gridpoint of the x
    axis of rose.  So it's counting the elements listed.  It seems a bit
    strange to me as well, but that looks like what is happening.
    
    
    -Ansley

On 8/26/2020 1:51 AM, Olivier Marti wrote:

Hi to all.

In ferret, you can can construct a variable variable from scratch with something like A=I[I=1:5]  :

yes? LIST I[I=1:5]

             VARIABLE : I

                        axis ABSTRACT

             SUBSET   : 5 points (X)

 1   / 1:  1.000

 2   / 2:  2.000

 3   / 3:  3.000

 4   / 4:  4.000

 5   / 5:  5.000

You can limit the range :

yes? LIST I[I=3:5]

             VARIABLE : I

                        axis ABSTRACT

             SUBSET   : 3 points (X)

 3   / 3:  3.000

 4   / 4:  4.000

 5   / 5:  5.000

But you can not extend before zero : 

yes? LIST I[I=0:5]

             VARIABLE : I

                        axis ABSTRACT

             SUBSET   : 5 points (X)

 1   / 1:  1.000

 2   / 2:  2.000

 3   / 3:  3.000

 4   / 4:  4.000

 5   / 5:  5.000

Neither can you skip values : 

yes? LIST I[I=1:6:2]

             VARIABLE : I

                        axis (AX004)

             SUBSET   : 3 points (X)

 1   / 1:  1.000

 3   / 2:  2.000

 5   / 3:  3.000

Of course, we got an obvious and simple solution to construct these variables :

yes? LIST 2*I[I=1:4]-2

             VARIABLE : 2*I[I=1:4]-2

             SUBSET   : 4 points (X)

 1   / 1:  0.000

 2   / 2:  2.000

 3   / 3:  4.000

 4   / 4:  6.000

But we could have simpler and clearer scripts if the _expression_ I[0:6:2] could give directly this result.

Best regards,

Olivier

— 

Olivier Marti  - LSCE Bât 714 p. 1049

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-- 
Ansley Manke
Science Data Integration Group
NOAA Pacific Marine Environmental Laboratory
7600 Sand Point Way NE
Seattle WA 98115

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