Re: [ferret_users] If then elif end if

[Jaison Kurian <jaison@xxxxxxxxxxxxxxxxxx>]

Hi Nageswara,
              The IF-THEN-ELSE-ENDIF block construct is only meant for 
scalars in Ferret according to user manual, Ref Sec14.1. IF-THEN-ELSE 
conditional execution :

"If the expression is a numeric expression, then it may be a digit (e.g. 1 or 
0), a variable, or a symbol.  If it is a variable, it must evaluate to a 
scalar, and be enclosed in grave accents to force its evaluation in the IF 
expression.  If it is a symbol, it must be evaluated with ($symbolname)."  

Here is an example :

 let x1 = 1
 let x2 = 2

 IF `x1 EQ x2` THEN
    say " x1 = x2"
 ELIF `x1 GT x2` THEN
    say " x1 > x2"
 ELIF `x1 LT x2` THEN
    say " x1 < x2"
 ENDIF

Note : Ferret versions previous to 6.00 got a bug : Ferret attempted to parse 
       commands even when they were not executed. For details see
       http://ferret.wrc.noaa.gov/Ferret/Documentation/Release_Notes/
                        v600.html#if_immediate

User manual continues as :
If a variable is not a scalar, it cannot be used with this particular syntax 
for conditional execution, however the use of masking allows you to operate on 
a whole field based on a condition by defining a new variable (See Ch3 Sec2.5.
IF-THEN logic ("masking"))

So the answer to your question is to use successive "masking" steps instead
of using a block/construct as above. Here is an example :

Suppose we want to compare two arrays of similar dimensions (this is very 
important), say var1 and var2. Our aim is to assign -1 for all points where
var1 < var2, 0 where var1 = var2 and 1 where var1 > var2. Remember that the 
negative values should be enclosed in brackets "()". The most simpler 
procedure will be as follows

   let var1 = XSEQUENCE({1,2,3,4,5,6,7,8,9,10})
   let var2 = XSEQUENCE({1,4,3,6,5,2,9,8,7,10}) 

   let var_diff = var1 - var2
   let var_m1   = IF var_diff LT 0 THEN (-1) ELSE var_diff ! var1 < var2
   let var_p1   = IF var_m1   GT 0 THEN   1  ELSE var_m1   ! var1 > var2
 
 ! please note that var1 = var2 case will be automatically taken care off
 !   by var_diff and we used "var_m1" in the second IF construct for "var_p1".
 !   this is the trick --> carry over variables from each step (or even you
 !   can merge all masked variables at a later stage to get the final one).

   list var1, var2, var_p1

Please note that there are several examples on "Masking" in user manual and
on Ferret FAQ page (http://ferret.wrc.noaa.gov/Ferret/FAQ/ferret_FAQ.html).

Hope this helps,

Jaison

On Fri, 24 Nov 2006, golla nageswararao wrote:

> Hi all,
>   Can anyone say me how to use if then elif end if in
> ferret. I saw the ferret users guide but i didnt
> understand it and i cannot apply it to my application.
> If possible send me any jnl file which contains if
> then elif endif statement so that i can understand. i
> want multiple options for if.
> Thanks in advance. 
> 
> 
> From,
>     G.Nageswara Rao,
>   Research Scholar,
>   INCOIS,
>   Jeeidmetla,
>   Hyderabad.
>   
> 
> 
> 
> 		
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