[gradsusr] If Statements and Drawing Variables

[Jeff Duda]

Unfortunately you can't define plottable fields like that. You need to use
a combination of maskout() and const() to get the desired field. Nesting of
const((maskout)) usually gets you what you want. maskout() works like an if
statement, filtering out values you don't want to plot based on an
inequality. The way it works is maskout(field,mask), where field is the raw
field you want to plot, and mask is an expression which may include other
fields and/or constants. For example, if you wanted to just plot EHI where
it was > 1, you could do 'd maskout(ehi,ehi-1)'. The result of maskout is a
field plotted only where the mask expression is > 0 (might be >= 0, I
forget, but if you're dealing with a continuous field, the end result is
the same). Anywhere the mask field does not satisfy the equation, the raw
field is treated like missing data. This is where const() comes in. You can
nest maskout() within const() to tell grads what to do with the missing
data (like the ELSE part of the if-else block). If you wanted to fill the
rest of the EHI-masked field with 0s, for example, you could do 'd
const(maskout(ehi,ehi-1),0,-u)', where the -u flag specifies const() to
only apply the else value to undefined values (created by the maskout).

While maskout works best for one-sided (i.e., semi-infinite) inequalities
(e.g., x > 5 or x <= -10), you can use it for bounded inequalities as well
using the absolute value function. You'll have to get a little clever with
your masking expression, but some simple algebra will do it for you. You
can also sum values and take an average (where the bounded inequality is
valid, you'll sum the same field twice, so taking the average will restore
the original values to the field. For example, for your first expression:
'd 0.5*(maskout(ehi,ehi-0.25)+maskout(ehi,0.5-ehi))' would plot a band of
EHI values at grid points where 0.25 < EHI < 0.5. You could wrap this in a
const() function to change those values to 1: 'd
const(0.5*(maskout(ehi,ehi-0.25)+maskout(ehi,0.5-ehi)),1)'. You can then do
that for all your bands. There will not be any overlap.

Of course, that's a longhand way of doing it. You can also just use
repeated multiplied instances of nested const(maskout()) functions to get
the same result in one grand command, although it will be quite involved.
I'll give you a start and leave you to figure the rest out:

'd const(const(maskout(ehi,0.125 - abs(ehi-0.375)),1),0,-u)...
This first chunk gives your first condition where index would be set to 1.
Use multiplication for the other terms.

Good luck!

Jeff Duda


On Mon, Apr 17, 2017 at 4:59 PM, Carter Humphreys - Mods for GRX <
carter.humphreys at mods-for-grx.com> wrote:

> Hello,
>
> I'm fairly new to GrADS and am trying use a script to search through a
> model product using if statements and assign a value to a certain range of
> that product. (I.e. if temp is between 45 and 55, x = 2) I then try to have
> GrADS draw that variable but all I end up with is a blank map.
>
> Any help would be appreciated, it's probably simple but I've been pretty
> much lost after trying this for a few days now.
>
> My Script:
>
> 'clear'
> 'set mpdset hires'
> 'sdfopen http://nomads.ncep.noaa.gov/dods/nam/nam20170417/nam_12z'
>
> 'define ehi = (capesfc*hlcy1000_0m)/160000'
> 'define index = 0'
>
> if (ehi>=0.25 & ehi<0.50)
> index = 1
> endif
>
> if (ehi>=0.50 & ehi<0.75)
> index = 2
> endif
>
> if (ehi>=0.75 & ehi<1.00)
> index = 3
> endif
>
> if (ehi>=1.00)
> index = 4
> endif
>
> 'set gxout shaded'
>
> 'd index'
>
> 'cbar'
> 'draw title GrADS Script Test'
> 'printim C:/GrADS/GrADS_script_test.png png white'
>
> --
>
> Sincerely,
>
> *Carter J. Humphreys*
>
> *Developer*
>
>
> Mods for GRX
>
> carter.humphreys at mods-for-grx.com | Support Email: support at mods-for-grx.
> com | Website: www.mods-for-grx.com
>
> _______________________________________________
> gradsusr mailing list
> gradsusr at gradsusr.org
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>
>


-- 
Jeff Duda
Post-doctoral research fellow
University of Oklahoma School of Meteorology
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