Thanks Taco!
I understand much better now. I think it's best to leave the units out
until the drawing stage as you suggest.
Thanks again
Keith McKay

On Fri, 27 Nov 2020, 08:30 Taco Hoekwater, <taco@elvenkind.com> wrote:

>
>
> > 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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