Passing variables into context

Passing variables into context

[Matthijs Kooijman]

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Hi all,

I've been trying to make a metapost graphic, based on the \textwidth. However,
it seems I can't just pass in \textwidth as an option. For example, the
following breaks:

  \startuniqueMPgraphic{HLine}
  draw origin -- (\MPvar{width}, 0mm); 
  \stopuniqueMPgraphic

  \starttext

  \setupMPvariables[HLine][width=\textwidth]
  \uniqueMPgraphic{HLine}

  \stoptext

with the following error:

  mplib | mp terminal: >> textwidth
    ! Undefined x coordinate has been replaced by 0.
    <to be read again>
                   ;
    <*> ...d:=3441; draw origin -- (\textwidth , 0mm);

It seems that the \textwidth option is not expanded before being passed to MP.
Is there any way to force this expansion? I've tried \expandafter (without
really understanding what that does), but that didn't help.

I've tried putting this graphic as an overlay under an \framed, which allows
me to use the \overlaywidth macro in metapost (which I assume is expanded
specially by context?). This worked, but is not very elegant. Is there
something more straightforward possible?

Gr.

Matthijs

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Re: Passing variables into context

[Taco Hoekwater]

Matthijs Kooijman wrote:
> 
> It seems that the \textwidth option is not expanded before being passed to MP.
> Is there any way to force this expansion? I've tried \expandafter (without
> really understanding what that does), but that didn't help.

You should try \the\textwidth

Best wishes,
Taco
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___________________________________________________________________________________

Re: Passing variables into context

[Matthijs Kooijman]

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Hi Taco,

> You should try \the\textwidth
Thanks, that works. Could you enlighten me as to what \the does and why this
helps? Does Context treat \the specially within metapost code (which seems so,
since there are more \the examples in the metafun manual), or is it expanded
before getting inserted into metapost?

Using this in arithmetic gets slightly more complicated (0.5\the\textwidth
doesn't work, but since this gets put into metapost, 0.5 * \the\textwidth does
work).

Gr.

Matthijs

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If your question is of interest to others as well, please add an entry to the Wiki!

maillist : ntg-context@ntg.nl / http://www.ntg.nl/mailman/listinfo/ntg-context
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___________________________________________________________________________________

Re: Passing variables into context

[Taco Hoekwater]

Matthijs Kooijman wrote:
> Hi Taco,
> 
>> You should try \the\textwidth
> Thanks, that works. Could you enlighten me as to what \the does and why this
> helps? Does Context treat \the specially within metapost code (which seems so,
> since there are more \the examples in the metafun manual), or is it expanded
> before getting inserted into metapost?

\the is an expandable command that 'converts' internal tex parameters
into their value. The expansion takes places during \edef (like inside
\expanded) or during write to a buffer.

> Using this in arithmetic gets slightly more complicated (0.5\the\textwidth
> doesn't work, but since this gets put into metapost, 0.5 * \the\textwidth does
> work).

If you want a single value inside mpost, you could use \dimexpr to do
the calculation in tex.

Best wishes,
Taco
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maillist : ntg-context@ntg.nl / http://www.ntg.nl/mailman/listinfo/ntg-context
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archive  : https://foundry.supelec.fr/projects/contextrev/
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___________________________________________________________________________________