Re: defining variables using IF-THEN-ELSE syntax

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

Hi Lia,
The grave-accent syntax is only valid for scalar variables, but without 
the grave accents the IF test applies to all the values in the 
variable.  The syntax you want is this. 
  LET nav_lon_pos = IF nav_lon LT 0 THEN  nav_lon+360 ELSE nav_lon


Here's a quick example, using a short list of values for the starting 
variable

yes? let a = {118,158,180,-178,-100,-62}

yes? let b = IF a LT  0 THEN  (a+360) ELSE a

yes? list a, b
            X: 0.5 to 6.5
Column  1: A is {118,158,180,-178,-100,-62}
Column  2: B is IF A LT 0 THEN A + 360 ELSE A
            A      B
1   / 1:  118.0  118.0
2   / 2:  158.0  158.0
3   / 3:  180.0  180.0
4   / 4: -178.0  182.0
5   / 5: -100.0  260.0
6   / 6:  -62.0  298.0

Ansley

Lia wrote:

> Hello Ferreters,
>
> I'm hoping there is an elegant solution to this- I'm trying to redefine
> my variable nav_lon so it will be monotonic (so I can use nav_lon to
> define a new x axis), by adding 360 to nav_lon when nav_lon is negative.
>
> However, I'm having problems with my IF statement because nav_lon is
> both positive AND negative (118:180, -178:-62), so I don't get a scalar
> output.  I've tried two primary tacks, with minor variations (i.e.
> including and not including the LET command):
>
> yes? IF `nav_lon LT 0` THEN LET nav_lon2= nav_lon + 360 ELSE LET
> nav_lon2 = nav_lon
> ** ERROR: invalid command: grave accent doesnt evaluate to scalar
> nav_lon LT 0
>
> yes? IF `nav_lon LT 0 THEN nav_lon2 = nav_lon + 360 ELSE nav_lon2 =
> nav_lon`
> ** ERROR: command syntax: options: "P=prec", "B=bad","W=width",
> "ZW=zero-width" or "R=return-item"
>
> I read a little in the users guide about P,B,W, and R syntax, but didn't
> find it to really apply to my problem.  Any ideas?
>
> Thanks,
> Lia
>
>