How to test if a macro has the shape of a coordinate from TikZ?

Question

How can I know if a macro contains a coordinate, that is, in the sense of having the shape of a coordinate num,num.

What I want to do is to know if a given macro, \pos, has the shape (in sense of parsing) of a coordinate or not, and then do something in case it has it and do something else if the shape is not a coordinate. For example, I'm looking for a way of achieving:

\pos{0,0}
\ifcoordinate\pos
It is a coordinate
\else
It is not a coordinate
\fi

In that case, what \ifcoordinate should be? In other words, how can I know if the contents of the given macro are in the sequence: number, coma, number?.

Note that I want to use this verification, later on, in some TikZ related code, so a TikZ related solution is appreciated.

Answer 1

Here is a version using the xstring package based on @egreg's examples:

This also yields

YES
NO
YES

Known Issues:

• \IfDecimal allows the decimal separator to be a dot or a comma. Since we uses a comma as a separator for the two numbers, the first coordinate can not use a comma as the decimal separator, but the second one can. This inconsistency is not good so what is needed is a way to change the decimal separator for xtring to just be the period.

Code:

\documentclass{article}
\usepackage{xstring}
\usepackage{etoolbox}

\newtoggle{wasCoordinate}
\newcommand*{\ifcoordinate}[3]{%
    \toggletrue{wasCoordinate}%
    \StrBefore{#1}{,}[\StringBeforeComma]
    \StrBehind{#1}{,}[\StringBehindComma]
    \IfDecimal{\StringBeforeComma}{}{\togglefalse{wasCoordinate}}
    \IfDecimal{\StringBehindComma}{}{\togglefalse{wasCoordinate}}
    \iftoggle{wasCoordinate}{#2}{#3}
}%

\begin{document}

\def\pos{1.23,1}
\ifcoordinate{\pos}{YES}{NO}

\def\pos{1.23}
\ifcoordinate{\pos}{YES}{NO}

\def\pos{1.23,-1}
\ifcoordinate{\pos}{YES}{NO}

\end{document}
\end{document}

Answer 2

\documentclass{article}
\usepackage{xparse,l3regex}
\ExplSyntaxOn
\NewDocumentCommand\ifcoordinate{ m m m }
  {
   \bool_set_true:N \l_iscoord_bool
   \seq_set_split:Nnx \l_tmpa_seq {,} {#1}
   \int_compare:nTF { \seq_length:N \l_tmpa_seq = 2 }
     {
      \seq_map_inline:Nn \l_tmpa_seq
        { \bool_if:NT \l_iscoord_bool { \iscoord_checkfornumber:n { ##1 } } }
     }
     { \bool_set_false:N \l_iscoord_bool }
   \bool_if:NTF \l_iscoord_bool {#2} {#3}
  }
\cs_generate_variant:Nn \seq_set_split:Nnn { Nnx }
\cs_new:Npn \iscoord_checkfornumber:n #1
  {
   \regex_match:nnTF {\A \-? \d+ (\.\d+)? \Z}{#1}
     {\bool_set_true:N \l_iscoord_bool}
     {\bool_set_false:N \l_iscoord_bool}
  }
\bool_new:N \l_iscoord_bool
\ExplSyntaxOff
\begin{document}

\def\pos{1.23,1}
\ifcoordinate{\pos}{YES}{NO}

\def\pos{1.23}
\ifcoordinate{\pos}{YES}{NO}

\def\pos{1.23,-1}
\ifcoordinate{\pos}{YES}{NO}

\end{document}

This will print

YES
NO
YES

Important change

Due to the changes to expl3 made in Summer 2012, in the code above \seq_length:N should be replaced by \seq_count:N. Here's a new complete implementation.

\documentclass{article}
\usepackage{xparse,l3regex}
\ExplSyntaxOn
\NewDocumentCommand\ifcoordinate{ m m m }
 {
  \iscoord_main:nnn { #1 } { #2 } { #3 }
 }
\bool_new:N \l__iscoord_iscoord_bool
\seq_new:N \l__iscoord_arg_seq
\cs_new_protected:Npn \iscoord_main:nnn #1 #2 #3
  {
   \bool_set_true:N \l__iscoord_iscoord_bool
   \seq_set_split:Nnx \l__iscoord_arg_seq {,} {#1}
   \int_compare:nTF { \seq_count:N \l__iscoord_arg_seq = 2 }
     {
      \seq_map_inline:Nn \l__iscoord_arg_seq 
        { \bool_if:NT \l__iscoord_iscoord_bool { \__iscoord_checkfornumber:n { ##1 } } }
     }
     { \bool_set_false:N \l__iscoord_iscoord_bool }
   \bool_if:NTF \l__iscoord_iscoord_bool {#2} {#3}
  }
\cs_generate_variant:Nn \seq_set_split:Nnn { Nnx }
\cs_new_protected:Npn \__iscoord_checkfornumber:n #1
  {
   \regex_match:nnTF {\A \-? \d+ (\.\d+)? \Z}{#1}
     {\bool_set_true:N \l__iscoord_iscoord_bool}
     {\bool_set_false:N \l__iscoord_iscoord_bool}
  }
\ExplSyntaxOff
\begin{document}

\def\pos{1.23,1}
\ifcoordinate{\pos}{YES}{NO}

\def\pos{1.23}
\ifcoordinate{\pos}{YES}{NO}

\def\pos{1.23,-1}
\ifcoordinate{\pos}{YES}{NO}

\end{document}