Re: Units of xpart and ypart of a pair in MetaFun

[Taco Hoekwater <taco@elvenkind.com>]

> On 26 Nov 2020, at 16:09, Keith McKay <mckaymeister@gmail.com> wrote:
> 
> Hi,
> 
> I have been using xpart and ypart to extract these values from pairs of points in a path but I wasn't getting the correct result. I was expecting:
> 
> pair (2cm, 11cm) to give xpart 2 and ypart 11

As explained by various people, Metapost does not have dimensions. All lengths are implicit Postscript big points
(72/inch). Variables like “cm” just add a multiplication factor to your expressions, at no point is a dimension stored inside Metapost. 

Your equation:

  a0 = (2cm,11cm)

first expands into 
  
  a0 = (2*28.34645,11*28.34645)

because “cm” is a variable with a value (Its definition in plain.mp is "cm = 28.34645”)

Then, the two expressions in the a0 equation are resolved before the assignment, so what you actually wrote at the statement level is

  a0 = (56.6929, 311.8096);

At no point is there a “cm”-sized dimension.

===

Perhaps another option would be for you to do all your work without units, e.g. 

  a0 = (2,11) % assume scaling will happen later

and then in the end do the drawing on a scaled path:

  draw (pp scaled cm) …

as that may be less confusing? If the precision is an issue, you could counter the loss of precision you could use implied millimeters instead of centimeters.

Best wishes,
Taco





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