[metapost] test for empty text?

Daniel H. Luecking luecking at uark.edu
Tue May 28 15:54:32 CEST 2013

[X]Colin Fraizer [metapost at cfraizer.com] wrote:

> -          Is there a way to test if the replacement text of a macro
>is empty?

>I have a macro which expands to a color or to empty (0 tokens),
>in which case I want to use “background”.

I might make use of the fact that a for-loop ignores empty
items in a list. For example:
 def getmycolor (text clr) =
   for c = clr,background:
     mycolor := c;
     exitif true;

When the parameter clr is empty, one should get
  mycolor := background
on the first (only) iteration of the loop, but when clr
is an actual color one should get
on the first iteration, and then immediately exit the

I did not test this, but I have done similar things before.

Daniel H. Luecking
Department of Mathematical Sciences
University of Arkansas
Fayetteville, Arkansas
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