Create programmatically a macro whose name contains the value of a counter

Question

I often need to define macros whose name contains the value of a counter, notably to attribute a value to multiple objects: in python I would do:

myArray[0] = "mystyle"

Unfortunately, I can't use \expandafter\xdef\csname myArray\the\mycounter\endcsname because in LaTeX, it is not possible to have numbers in macros. For now I'm using \roman{mycounter} instead, but it feels very hacky as it can take a long time to computer the roman version of a number... Is there any better solution? I tried notably to check if LaTeX3 provided something better for this kind of associative arrays containing numbers, but I can only find integers arrays (I guess integers that can only contain arrays). The other option I imagine is maybe a sequence of tupples, but then the access time is linear which is not very efficient.

MWE

\documentclass[options]{article}
\begin{document}
\newcounter{mycounter}
\expandafter\edef\csname myArray\roman{mycounter}\endcsname{Hello}
Bonjour'' is\csname myArray\roman{mycounter}\endcsname''.
\end{document}

since you are using csname you can use digits anyway replace \roman{mycounter} with \themycounter (or \arabic{mycounter}) — David Carlisle, Feb 02 '22 at 23:56
you could use \expandafter\xdef\csname myArray\the\mycounter\endcsname if \mycounter was defined (eg by \newcount\mycounter) your code shown doesn't define it — David Carlisle, Feb 03 '22 at 00:01
@DavidCarlisle Oh, I was not aware of this rule, thanks a lot! — tobiasBora, Feb 03 '22 at 00:23
See these questions: Creating a command with numbers on it, Why does TeX not allow numbers in command names?, Defining commands/abbreviations that contain numbers, Command/macro name cannot include numbers and symbols. — Ulrich Diez, Feb 03 '22 at 02:16
\csname foo17$_B\endcsname 's replacement text is the control-word-token \foo17$_B. Expandable tokens, e.g. \the\mycounter, between \csname and \endcsname are expanded. In expl3-syntax you can use \use:c instead of \csname..\endcsname. You might be interested in What exactly do \csname and \endcsname do? — Ulrich Diez, Feb 03 '22 at 02:30
Nice, I would not expect csname to work for any strange macro name ^^ Thanks! — tobiasBora, Feb 03 '22 at 02:34
the macro name is not strange and it's not really a fearure of csname for example after \catcode\1=11you could use\def\zz11{abc}and use\zz11but if you restore the catcode of 1 you would need csname again as\zz11would parse as\zz 1 1` the restriction about digits is not a restriction on allowed names, it is just a feature of how input characters are parsed into tokens — David Carlisle, Feb 03 '22 at 07:31
@tobiasBora There is nothing strange about these macro names. :-) About category codes of characters: Category 11(letter) just denotes the set of characters whose elements at the time of tokenizing .tex-input might be taken for components of the name of a multiletter-control-sequence/of a control-word-token. But tokenization of .tex-input is not the only way in which tokens come into being. E.g. there is nothing against having TeX create control-word-tokens from a set of (explicit) character tokens via \csname..\endcsname when tokenization of .tex-input is already done. — Ulrich Diez, Feb 03 '22 at 12:42

score 3 · Accepted Answer · answered Feb 10 '22 at 15:17

3

If you define

\def\sdef#1{\expandafter\def\csname#1\endcsname}
\def\cs#1{\csname#1\endcsname}

then an equivalent of python's

myArray[0] = "mystyle"

is

\sdef{myArray[0]}{mystyle}

and the value is accessible by

\cs{myArray[0]}

You can declare a counter by \newcount\mycount and then you can extend the example above by replacing the 0 character by \the\mycount:

\newcount\mycount
\sdef{myArray[\the\mycount]}{mystyle}
\cs{myArray[\the\mycount]}

answered Feb 10 '22 at 15:17

wipet

74,238

It's a cool idea to define a function in order to limit the heavy usage of \csname. Thanks! – tobiasBora Feb 10 '22 at 15:27

score 1 · Answer 2 · answered Feb 10 '22 at 17:29

If you're afraid of the access time to a sequence, you can use \csname:

\documentclass{article}
\ExplSyntaxOn
\NewDocumentCommand{\newarray}{m}
  {
    % keep the next free index
    \int_new:c { g__tobias_array_#1_int }
  }
\NewDocumentCommand{\addtoarray}{omm}
  {
    \tobias_array_add:enn
      {% the index
        \IfNoValueTF { #1 }
          { \int_eval:n { \int_use:c { g__tobias_array_#2_int } } }
          { \int_eval:n { #1 } }
      }
      { #2 } % the array
      { #3 }
  }
\NewExpandableDocumentCommand{\getarray}{mm}
  {
   \tobias_array_get:ne { #1 } { \int_eval:n { #2 } }
  }
\cs_new_protected:Nn \tobias_array_add:nnn
 {
  \tl_gclear_new:c { g__tobias_array_#2_#1_tl }
  \tl_gset:cn { g__tobias_array_#2_#1_tl } { #3 }
  \int_gset:cn { g__tobias_array_#2_int } { #1 + 1 }
 }
\cs_generate_variant:Nn \tobias_array_add:nnn { e }
\cs_new:Nn \tobias_array_get:nn
  {
    \tl_if_exist:cT { g__tobias_array_#1_#2_tl }
      {
        \tl_use:c { g__tobias_array_#1_#2_tl }
      }
  }
\cs_generate_variant:Nn \tobias_array_get:nn { ne }
\ExplSyntaxOff
\begin{document}
\newarray{myarray}
\addtoarray{myarray}{An item}
\addtoarray{myarray}{Another item}
\addtoarray[1+3]{myarray}{Item #4}
0: \getarray{myarray}{0}
1: \getarray{myarray}{1}
3: \getarray{myarray}{3}
4: \getarray{myarray}{2+2}
\end{document}

You can add to an array sequentially or even jumping over some indices. And do arithmetic in order to find or set an index.

In the example, the next \addtoarray would use index 5.

Thanks for this LaTeX3 alternative! It's cool to be able to do arithmetic directly in the function. — tobiasBora, Feb 10 '22 at 17:43

Create programmatically a macro whose name contains the value of a counter

2 Answers2