[gradsusr] variables in .gs scripts
Yi-Chih Huang
dscpln at gmail.com
Tue Nov 15 15:18:43 EST 2011
Hello,
After testing the script, I got the errors below. If the second
argument must be a constant, then a combination of const and maskout would
not be as powerful as I thought. My question remains: what would be the
command in GrADS for the if statement?
################
Error from CONST: 2nd argument must be a constant
Operation Error: Error from const function
Operation Error: Error from const function
Error ocurred at column 1
DEFINE error: Invalid expression.
################
if (rh=100) then
"mai=elr-malr"
else
"mai=0"
endif
################
Thanks much,
Yi-Chih
On Tue, Nov 15, 2011 at 2:59 PM, Yi-Chih Huang <dscpln at gmail.com> wrote:
> Hello,
>
> Thanks much for the elucidation. I think the command below has the
> same functions as the if statement. It is good to learn the expression of
> the if statement in GrADS. But if the calculations are very complicate and
> include several commands, probably a combination of const and maskout would
> not be able to handle such complicate situations.
>
> ################
> mai = const(const(maskout(rh,rh-100),elr-malr),0,-u)
>
> ################
>
> if (rh=100) then
> "mai=elr-malr"
> else
> "mai=0"
> endif
> ################
>
> Thanks,
>
> Yi-Chih
>
>
> On Mon, Nov 14, 2011 at 10:49 PM, Yaqiang Wang <yaqiang.wang at gmail.com>wrote:
>
>> 1. maskout the rh values less than 100 to missing: rh=maskout(rh,rh-100)
>> 2. get a new rh with the negtive values which are larger than 100
>> before (100 is now 0): rh=100-rh
>> 3. maskout the negtive values: rh=maskout(rh,rh)
>> 4. get mai values: mai=elr-malr
>> 5. maskout mai: mai=maskout(mai,rh)
>> 6. const mai missing values to 0: mai=const(mai,0,-u)
>>
>> On Tue, Nov 15, 2011 at 10:58 AM, Yi-Chih Huang <dscpln at gmail.com> wrote:
>> > Hello,
>> >
>> > I had read the documents about const and maskout online. However,
>> > probably my understanding about const and maskout is so shallow that I
>> can't
>> > use a combination of const and maskout to accomplish the function of
>> the if
>> > statement. Thus, I posed the question.
>> >
>> > Thanks,
>> >
>> > Yi-Chih
>> >
>> >
>> > On Mon, Nov 14, 2011 at 9:41 PM, Yaqiang Wang <yaqiang.wang at gmail.com>
>> > wrote:
>> >>
>> >> You can find the usage of const and maskout from online help document.
>> >> http://www.iges.org/grads/gadoc/gradfuncconst.html
>> >> http://www.iges.org/grads/gadoc/gradfuncmaskout.html
>> >>
>> >> On Tue, Nov 15, 2011 at 10:28 AM, Yi-Chih Huang <dscpln at gmail.com>
>> wrote:
>> >> > Hello,
>> >> >
>> >> > I work on regular grads. I have searched online for the
>> examples to
>> >> > use
>> >> > const and maskout instead of if statement. But I can't find any
>> >> > example.
>> >> > Could anyone show me an example to use const and maskout instead of
>> if
>> >> > statement?
>> >> >
>> >> > Thanks much for your help,
>> >> >
>> >> > Yi-Chih
>> >> >
>> >> >
>> >> > On Mon, Nov 14, 2011 at 4:25 PM, Arlindo da Silva <
>> dasilva at alum.mit.edu>
>> >> > wrote:
>> >> >>
>> >> >> On Mon, Nov 14, 2011 at 3:40 PM, Yi-Chih Huang <dscpln at gmail.com>
>> >> >> wrote:
>> >> >>>
>> >> >>> Hello,
>> >> >>>
>> >> >>> I want to calculate variable 2 based on the value of some
>> variable
>> >> >>> as
>> >> >>> follows.
>> >> >>>
>> >> >>> When var1 = 100, then var2 = (calculation 1)
>> >> >>> When var1 != 100, then var2 = (calculation 2)
>> >> >>>
>> >> >>> I wrote a .gs script as follows. But the condition rh=100 did not
>> >> >>> effect. Could anyone reveal what would be the right command in .gs
>> >> >>> script
>> >> >>> to give the condition in which variables are in the .ctl file?
>> >> >>>
>> >> >>>
>> >> >>>
>> >> >>>
>> "malr=dalr*(1+2500000*q/287/tk)/(1+0.622*pow(2500000,2)*q/1004/287/tk/tk)"
>> >> >>> "elr=-(tk(z+1)-tk(z-1))/(height(z+1)-height(z-1))"
>> >> >>>
>> >> >>> if (rh=100) then
>> >> >>> "mai=elr-malr"
>> >> >>> else
>> >> >>> "mai=0"
>> >> >>> endif
>> >> >>
>> >> >> GrADS expression parser and the scripting language do not share the
>> >> >> same
>> >> >> name space. Therefore you cannot have an if statement as above where
>> >> >> you
>> >> >> reference a gridded variable. If you are using opengrads, the if()
>> >> >> function
>> >> >> could accomplish what you want:
>> >> >> 'mai = if(rh,=,0,elr-malr,0)'
>> >> >> For more information on if() see:
>> >> >> if(EXPR1,OP,EXPR2,TRUE_EXPR,FALSE_EXPR)
>> >> >> In regular grads, a combination of maskout()/const() functions can
>> >> >> accomplish the same as the if() function above.
>> >> >> Arlindo
>> >> >>
>> >> >> --
>> >> >> Arlindo da Silva
>> >> >> dasilva at alum.mit.edu
>> >> >>
>> >> >> _______________________________________________
>> >> >> gradsusr mailing list
>> >> >> gradsusr at gradsusr.org
>> >> >> http://gradsusr.org/mailman/listinfo/gradsusr
>> >> >>
>> >> >
>> >> >
>> >> > _______________________________________________
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>> >> >
>> >>
>> >>
>> >>
>> >> --
>> >> *************************************************
>> >> Dr. Yaqiang Wang
>> >> Chinese Academy of Meteorological Sciences (CAMS)
>> >> 46, Zhong-Guan-Cun South Avenue
>> >> Beijing, 100081
>> >> China
>> >>
>> >> yaqiang.wang at gmail.com
>> >>
>> >> http://www.meteothinker.com
>> >> **************************************************
>> >>
>> >> _______________________________________________
>> >> gradsusr mailing list
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>> >> http://gradsusr.org/mailman/listinfo/gradsusr
>> >
>> >
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>>
>>
>>
>> --
>> *************************************************
>> Dr. Yaqiang Wang
>> Chinese Academy of Meteorological Sciences (CAMS)
>> 46, Zhong-Guan-Cun South Avenue
>> Beijing, 100081
>> China
>>
>> yaqiang.wang at gmail.com
>>
>> http://www.meteothinker.com
>> **************************************************
>>
>> _______________________________________________
>> gradsusr mailing list
>> gradsusr at gradsusr.org
>> http://gradsusr.org/mailman/listinfo/gradsusr
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>
>
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