Mfcttrf

This question is inspired by Jake's answer to this question. The problem is illustrated by this MWE:

documentclassstandalone
usepackagetikz
begindocument

defri1.0cm % define inner diameter
defro2.0cm % define outer diameter

begintikzpicture
 pgfmathsetmacrodrro-ri % compute width
 pgfmathsetmacrorm(ri+ro)/2 % compute mean diameter
 draw[red,line width=dr] (0:ro) arc (0:360:ro);
 draw[black] (0,0) circle (ro) circle (ri);
endtikzpicture

enddocument

This produces the following output:

But this is not what I want, which is that the red strip should be between the two black circles. I expected that I could achieve this by replacing the first draw command by

draw[red,line width=dr] (0:rm) arc (0:360:rm);

but then I get a non-sensical result in which the red strip is far too large:

This is not surprising given the remarks in Section 94.1.2 "Considerations concerning units" in the pgf manual (v3.1.2). What does surprise me is that simply adding cm like this

draw[red,line width=dr] (0:rm cm) arc (0:360:rm cm);

does not work - in the sense that there is no change in the output. However, in Jake's answer, he adds pt which seems to work (as witnessed by his comment Somehow, the [...] units got lost, so we add 'pt' at the end. Not nice...).

So my questions are:

1. Why does adding pt work in Jake's answer but not when I add cm?


2. I cannot make sense of Section 94.1.2. There seem to be commands for checking whether units are "declared" but there do not seem to be any commands for "adding them back" to a result. What good is it to be able to check for units but not "adding them back"?

pgfmathsetmacro calculates a length and gives back the result in pt but without the unit pt attached - the macro contains simply a number. So if you want to use this number in a sensible way you should reattach the pt:

documentclassstandalone
usepackagetikz
begindocument

defri1.0cm % define inner diameter
defro2.0cm % define outer diameter

begintikzpicture
 pgfmathsetmacrodrro-ri % compute width
 pgfmathsetmacrorm(ri+ro)/2 % compute mean diameter
 showrm %<--- shows > rm=macro: ->42.67912.
 draw[red,line width=dr] (0:rm pt) arc (0:360:rm pt);
 draw[black] (0,0) circle (ro) circle (ri);
endtikzpicture

enddocument

Instead of pdfmathsetmacro you can use pgfmathsetlengthmacro

documentclassstandalone
usepackagetikz
begindocument

defri1.0cm % define inner diameter
defro2.0cm % define outer diameter

begintikzpicture
 pgfmathsetlengthmacrodrro-ri % compute width
 pgfmathsetlengthmacrorm(ri+ro)/2 % compute mean diameter
 showrm % > rm=macro: ->42.67912pt.
 draw[red,line width=dr] (0:rm) arc (0:360:rm);
 draw[black] (0,0) circle (ro) circle (ri);
endtikzpicture

enddocument

Very simple: divide or multiply by 1cm. That way you can work in any units you like, such as cm. (I do not recommend using rm for a macro name, though.)

documentclassstandalone
usepackagetikz
begindocument

defri1.0cm % define inner diameter
defro2.0cm % define outer diameter

begintikzpicture
 pgfmathsetmacrodrro-ri % compute width
 pgfmathsetmacrorm(ri+ro)/2cm % compute mean diameter
 typeoutrm
 draw[red,line width=dr] (0:rm*1cm) arc (0:360:rm*1cm);
 draw[black] (0,0) circle (ro) circle (ri);
endtikzpicture

enddocument