[AMPL 1826] Seemingly incorrect behavior: AMPL if block evaluation

[David Veerasingam]

Hello all,

It seems to me that AMPL is evaluating `if' blocks even when the
condition is false.

Example:
1) I have an `if' block that looks like this

    if (time >= 2) then {
        include process.run;
    }

2) However, I've found that even when time = 0, the block still
executes and AMPL quits on this error:

        Can't find  file "process.run"
context:  include  >>> process.run; <<<

Well, the process.run file isn't generated at the conditions time = 0
or time = 1. The `if' statement is supposed to prevent that block from
executing.

Therefore my question is: How can I force AMPL to avoid throwing an
error?

Thanks for your help!

David
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[Robert Fourer-2]

David,

"include" takes a copy of the specified file and makes it a part of the
current file, regardless of any surrounding conditional statements.  (It's
like a "macro" in programming languages.)

To get the effect you want, write

    if (time >= 2) then {
        commands process.run;
    }

"commands" is an ordinary command that causes all of the statements in the
specified file to be executed, and then returns control to the current file.

Bob Fourer
4er@...

> -----Original Message-----
> From: ampl@... [mailto:ampl@...] On Behalf Of
> David Veerasingam
> Sent: Thursday, June 12, 2008 2:40 PM
> To: AMPL Modeling Language
> Subject: [AMPL 1826] Seemingly incorrect behavior: AMPL if block
> evaluation
>
>
> Hello all,
>
> It seems to me that AMPL is evaluating `if' blocks even when the
> condition is false.
>
> Example:
> 1) I have an `if' block that looks like this
>
>     if (time >= 2) then {
>         include process.run;
>     }
>
> 2) However, I've found that even when time = 0, the block still
> executes and AMPL quits on this error:
>
>         Can't find  file "process.run"
> context:  include  >>> process.run; <<<
>
> Well, the process.run file isn't generated at the conditions time = 0
> or time = 1. The `if' statement is supposed to prevent that block from
> executing.
>
> Therefore my question is: How can I force AMPL to avoid throwing an
> error?
>
> Thanks for your help!
>
> David

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[David Veerasingam]

Thanks, Bob for being ever so helpful!

'commands' works great.

David

On Jun 12, 7:07 pm, "Robert Fourer" <4...@...> wrote:

> David,
>
> "include" takes a copy of the specified file and makes it a part of the
> current file, regardless of any surrounding conditional statements.  (It's
> like a "macro" in programming languages.)
>
> To get the effect you want, write
>
>     if (time >= 2) then {
>         commands process.run;
>     }
>
> "commands" is an ordinary command that causes all of the statements in the
> specified file to be executed, and then returns control to the current file.
>
> Bob Fourer
> 4...@...
>
> > -----Original Message-----
> > From: ampl@... [mailto:ampl@...] On Behalf Of
> > David Veerasingam
> > Sent: Thursday, June 12, 2008 2:40 PM
> > To: AMPL Modeling Language
> > Subject: [AMPL 1826] Seemingly incorrect behavior: AMPL if block
> > evaluation
>
> > Hello all,
>
> > It seems to me that AMPL is evaluating `if' blocks even when the
> > condition is false.
>
> > Example:
> > 1) I have an `if' block that looks like this
>
> >     if (time >= 2) then {
> >         include process.run;
> >     }
>
> > 2) However, I've found that even when time = 0, the block still
> > executes and AMPL quits on this error:
>
> >         Can't find  file "process.run"
> > context:  include  >>> process.run; <<<
>
> > Well, the process.run file isn't generated at the conditions time = 0
> > or time = 1. The `if' statement is supposed to prevent that block from
> > executing.
>
> > Therefore my question is: How can I force AMPL to avoid throwing an
> > error?
>
> > Thanks for your help!
>
> > David

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