Evaluating local variables

[David Jondreau]

I'd like to pass a parameter to a custom function and return the local value for that parameter. I know that's a clunky way of saying what I mean. I want to code some values in a custom function return those values based on the parameter passed.

I want Eval ( "dog" ) to return "bark".

I tried this custom function:

Eval ( parameter )

Let( [

dog = "bark" ;

cat = "meow" ;

mouse = "squeak"

];

Evaluate ( parameter )

)

But that didn't work. I could use a Case () function but I expecting a lot of values in my list.

Thanks for any tips.

[comment]

I could use a Case () function but I expecting a lot of values in my list.

Is using Let() any shorter? IIUC, you still need to define each input/output pair.

[David Jondreau]

Yes, you're right, it wouldn't be shorter. In fact, it the whole calc would evaluate faster, as not all the local variables would have to be evaluated. Once a true test is met, the Case() stops.

But now I realize there's something about Evaluate() I don't understand and I'd like to to learn why it doesn't work the way I think it should.

[comment]

But now I realize there's something about Evaluate() I don't understand

I believe Evaluate() does not "look back" at the parameters defined by Let() or by a CF. So for example in:

Let ([

code = "message" 

] ;

Evaluate ( "code" )

)

the formula to be evaluated is code - and this produces Error#102 (Field is missing), because code is perceived as meaningless. OTOH, 

Let ([

$code = "message" 

] ;

Evaluate ( "$code" )

)

will return "message" as expected.

[Fitch]

Or you could evaluate the whole thing:

Evaluate ( "Let ( [" & 

"dog = \"bark\" // etc.

& "] ;" & parameter & " )" )

[comment]

Or you could evaluate the whole thing:

True, but then why would you?

[Fitch]

I use a CF that is a modified version of that to parse name/value pairs when I pass multiple script parameters. What's your preferred method to pass multiple script parameters?

[comment]

Yeah, OK - if they cannot be return-separated (my preferred method). Otherwise:

http://fmforums.com/forum/index.php?showtopic=39249&view=findpost&p=183000

However, with Set Variable[] I find the need for multi-valued parameter very rare.

Edited March 15, 2011 by comment

[Fitch]

I've been burned too many times by return-separated variables:

1. you have to be careful about the order

2. unreliable for passing lists

I find the need for multi-value parameters very common. I often find myself coming back to a button that originally passed a single param and redefining it to pass name/value pairs.

What I like about the custom function method is that it takes so little thought when defining the parameter, e.g.:

"name=" & nameField &

"; id=" & IDfield &

"; ice cream=good" // etc

And then in my script:

Set Variable( $param ; Get( ScriptParameter ))

GetNamed( $param ; "id" ) // GetNamed being the name of the CF

[David Jondreau]

I recently started using SetProperty( name ; value ) and GetScriptProperty ( name ) for passing parameters between scripts and I'm really like it: http://www.goya.com.au/blog/using-name-value-pairs-parameters-and-storing-data

I don't think that relates to my question though.

The Evaluate() won't take a Let-defined variable as a valid expression is what tripped me up. It seems the answer is simply, "The computer says no."

[Fitch]

Sorry if I derailed your thread, DJ. Michael asked why I would use Evaluate that way. (I still don't know why you're trying to use it the way you are.)

For any interested lurkers, I've posted the CF I use here:

http://fmforums.com/forum/topic/77148-passing-multiple-script-parameters/