Conversion pt to mm or cm

Question

Is there any way how to convert pt to mm or cm in plain TeX?

Does this help? http://tex.stackexchange.com/questions/8260/what-are-the-various-units-ex-in-pt-etc-expressed-in-mm/ — Torbjørn T., Dec 06 '11 at 17:08
Sorry, the above was was LaTeX, not sure if any of it works in plain. — Torbjørn T., Dec 06 '11 at 17:14
@TorbjørnT. Well, the accepted answer there only requires a few basic LaTeX macros that are easy enough to copy from latex.ltx. — Joseph Wright, Dec 06 '11 at 17:15
@TorbjørnT. It is not difficult to modify Philippe Goutet's solution to use eTeX only. — Leo Liu, Dec 06 '11 at 17:16
@Peter: Are we allowed to use the e-TeX extensions or does it have to be TeX82? — Joseph Wright, Dec 06 '11 at 17:16
@Peter: All calculations can be done under Knuth TeX without eTeX extensions. But it needs much longer code using several registers. — Leo Liu, Dec 06 '11 at 18:04
@JosephWright: Does anybody outside of Stanford still use TeX90 instead of eTeX et.al.? — Martin Schröder, Dec 06 '11 at 22:19
@MartinSchröder I have some contacts in chemistry publishing, for whom I wrote achemso. For that, e-TeX was not an option as it's not available on their server. — Joseph Wright, Dec 06 '11 at 22:20
@MartinSchröder I get the impression that the 'back end' publishers are happy with what works. It sounded as if an upgrade was possible but costly. — Joseph Wright, Dec 06 '11 at 22:28

score 11 · Answer 1 · answered Dec 06 '11 at 20:15

To convert from a dimension given in points, save the dimension in a counter first, then do the conversion to what you need. When TeX saves the dimension in a counter it converts to sp units.

\catcode`@=11
\@tempdimb25.4mm 
\@tempcnta=\@tempdimb
\the\@tempcnta 
\divide\@tempcnta by65536\par 
\the\@tempcnta pt
\bye

Conversion from any unit to a point is a bit more involved:

\catcode`@=11
\newdimen\@tempdimb
\newcount\@tempcnta
\def\topoint#1#2{%
\@tempdimb=#1
\@tempcnta=\@tempdimb
\multiply\@tempcnta by10
\divide\@tempcnta by18647 \advance\@tempcnta by1
\multiply\@tempcnta by72 \divide\@tempcnta by2540
\expandafter\def\expandafter#2\expandafter{\the\@tempcnta}}

\topoint{25.4mm}{\test}

\test pt
\bye

score 10 · Answer 2 · answered Dec 06 '11 at 21:26

Expandable solution without eTeX, using Heiko's bigintcalc package. Really, this is a mess, and you should use one of the solutions which make use of the eTeX extensions.

\input bigintcalc.sty
\catcode`@=11

% Step 1: expand the number.
\def\pttocm#1{\romannumeral\expandafter\pttocm@i\romannumeral-`0#1pt;;;;}
\def\pttocm@i#1{\pttocm@clean\ifcat\relax#1\the\fi#1}
\def\pttocm@clean{\expandafter\pttocm@clean@\romannumeral-`0}
\def\pttocm@clean@#1{%
  \ifcase\ifnum`#1<"2B 1\else\ifnum`#1>"39 1\else 2\fi\fi
  \or\expandafter\pttocm@clean@end
  \or\expandafter\pttocm@clean@do\fi #1}
\def\pttocm@clean@do#1#2;#3;{\pttocm@clean#2;#3#1;}

% Step 2: convert the decimal number to an integer/10^n
\def\pttocm@clean@end#1;#2;{\pttocm@clean@end@#2.;#2;.;!}
\def\pttocm@clean@end@#1.#2;#3.#4;#5!{\pttocm@decimal#4.;;#1#4}
\def\pttocm@decimal#1#2;{\ifx.#1\pttocm@decimal@\fi\pttocm@decimal#2;0}

% Step 3: convert from pt to nm (25400000nm=7227pt)
\def\pttocm@decimal@#1;0#2;#3;{\fi
  \expandafter\expandafter\expandafter\pttocm@result
  \bigintcalcDiv{\bigintcalcMul{25400000}{#3}}{7227#2}.;}

% Step 4: check the sign
\def\pttocm@result#1{%
  \ifx-#1\expandafter\pttocm@result@neg
  \else\expandafter\pttocm@result@pos\fi#1}
\def\pttocm@result@neg-{\expandafter\pttocm@rmstop
  \expandafter-\romannumeral\pttocm@result@pos}
\chardef\pttocm@rmstop=0

% Step 5: shift the period 5 steps to the left, by reversing twice.
\def\pttocm@result@pos#1;{%
  \pttocm@reverse 00000#1%
  {?\pttocm@result@shift;}?;%
  {?\pttocm@result@print;}?;}
\def\pttocm@reverse#1#2;{%
  \pttocm@gobble#1%
  \pttocm@reverse#2;#1}
\def\pttocm@gobble#1{}
\def\pttocm@result@shift#1;#2;#3;#4.#5#6#7#8#9%
  {\pttocm@reverse #4#5#6#7#8#9.}
\def\pttocm@result@print#1;#2;#3;#4;{\pttocm@trimzeros{#4}}

% Step 6: remove spurious zeros.
\def\pttocm@trimzeros#1{\pttocm@trim@#1;\pttocm@trim@ 0;\pttocm@trim@@}
\def\pttocm@trim@#10;#2{#2#1;}
\def\pttocm@trim@@#1;#2;{\pttocm@trim@@@#1}
\def\pttocm@trim@@@#1{%
  \ifx0#1\else\expandafter\pttocm@trim@end\expandafter#1\fi
  \pttocm@trim@@@}
\def\pttocm@trim@end#1#2{%
  \ifx.#1\expandafter\pttocm@rmstop\expandafter0%
  \else\expandafter\pttocm@rmstop\fi #1}
\catcode`@=12

\pttocm{-12.1232pt}

\pttocm{283000000000}

\def\foo{23}
\pttocm{12\foo.1\foo2pt}

\newdimen\testdimen
\testdimen=1cm
\pttocm{\testdimen}

\testdimen=-1mm
\pttocm{\testdimen}

\bye

score 6 · Answer 3 · edited Apr 13 '17 at 12:35

6

Here is a combination of Yiannis' question from Stripping the pt from a dimension and Philippe Goutet's answer from What are the various units (ex, em, in, pt, bp, dd, pc) expressed in mm?:

\catcode`@=11
\begingroup
  \catcode `P=12  % digits and punct. catcode
  \catcode `T=12  % digits and punct. catcode
  \lowercase{%
  \def\x{\def\rem@pt##1.##2PT{##1\ifnum##2>\z@.##2\fi}}}
     \expandafter\endgroup\x%
\def\strip@pt{\expandafter\rem@pt\the}
\def\convertto#1#2{\strip@pt\dimexpr #2*65536/\number\dimexpr 1#1\relax\relax}
\catcode`@=12
\convertto{mm}{10pt}
\bye

which requires e-TeX extensions.

edited Apr 13 '17 at 12:35

Community

1

answered Dec 06 '11 at 17:28

morbusg

25,490
4
81
162

1

And what about something without e-TeX extensions? – Peter Dec 06 '11 at 17:32
@Peter: Without e-TeX you need an extra assignment, which makes the macro non-expanable. – Martin Scharrer Dec 06 '11 at 17:39
And what about some solution to convert \the\hsize to mm? Any ideas? \catcode T=12 is not useful here. – Peter Dec 06 '11 at 17:50
3

You should terminate both \dimexpr with a \relax. Otherwise any dimen expression afterwards, like '* 10' will be taken as part of the inner \dimexpr. The \relax is removed by \dimexpr so you don't have to worry about expandable contexts. – Martin Scharrer Dec 06 '11 at 17:52
2

@Peter: \convertto{mm}{\the\hsize} will work just fine. The catcode changes are only valid during the definition of the macro, not during the execution. – Martin Scharrer Dec 06 '11 at 17:53
@Peter: What is your intention? Why do you need plain tex? – Marco Daniel Dec 06 '11 at 18:10
1

@MarcoDaniel: Why do you need Windows? Why do you need Linux? Why do you need Peugeot? Why do you need... >:-/ – morbusg Dec 06 '11 at 18:16
I agree with @MarcoDaniel. :) Thank you guys. – Peter Dec 06 '11 at 18:31
@morbusg: Maybe he want to create a command for a package and doesn't know the e-TeX macros or ... ;-). I think sometimes it easier to know the result. More users will find more ways ;-) – Marco Daniel Dec 06 '11 at 18:31
Unfortunately not everybody has "some-TeX" including the e-TeX extensions (yet). – Stephen Dec 06 '11 at 18:39
1

@Stephen: I only know one distribution which doesn't come with e-TeX, that's the commercial Scientific Workbench. Are there more? The majority of users should have e-TeX. – Martin Scharrer Dec 06 '11 at 20:52
@MartinScharrer: Scientific WorkPlace, Scientific Word, Scientific Notebook of mackichan.com - and then there are all those users who depend on some administrator who just refuses to update some system originating from about 1492... ("We do not do updates. The users know and want the system as it is." Not even installing security patches for Windows...) – Stephen Dec 08 '11 at 19:11

Leo Liu · Answer 4 · 2011-12-06T17:57:54.680

6

With help of eTeX's \scantokens, \strip@pt can be also:

\def\strip#1.#2pt{#1\ifnum#2>0.#2\fi}
\def\convertto#1#2{%
  \expandafter\expandafter\expandafter\strip
  \expandafter\expandafter\scantokens
  \expandafter{\the\dimexpr#2*65536/\number\dimexpr1#1\relax\relax\endinput}}

\convertto{pt}{1pc}

\bye

edited Dec 06 '11 at 17:57

answered Dec 06 '11 at 17:52

Leo Liu

77,365

Conversion pt to mm or cm

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