Using `\pgfgetlastxy`

Question

I'm trying to use this answer to extract the coordinates of a point without the pt. But why is the following not working? The first \JustNumber works as exected, but the second not, although \show\X gives > \X=macro:->0.0pt..

\documentclass{article}
\usepackage{tikz}
\def\JustNumber#1pt{#1}
\begin{document}
    \JustNumber0.0pt.
    \begin{tikzpicture}
        \path (0,0);
        \pgfgetlastxy{\X}{\Y}
        \show\X
        \xdef\X{\expandafter\JustNumber\X}
    \end{tikzpicture}
\end{document}

Why do I want this? I want to fill the rectangle between two points with a dotted grid. Because patterns seem to cause various problems when it comes to precise positioning, I thought of extracting the coordinates (without the pt) and feeding them into a \foreach.

Answer 1

The reason why it does not work is:

With your definition of \JustNumber the argument delimiter pt is tokenized while reading from the .tex-input-file.
Hereby you get two explicit character tokens of category 11(letter), i.e., p₁₁t₁₁.
But character tokens delivered via \pgfgetlastxy seem to come from \the which in turn delivers explicit character tokens of category 12(other), i.e., p₁₂t₁₂. (Exception: spaces delivered by \the are of category 10(space).)
Thus TeX does not find a matching delimiter.

With your definition ensure that the explicit character tokens forming the delimiter of the argument are of category 12(other):

\documentclass{article}
\usepackage{tikz}
% \string delivers explicit character tokens with the same catcode-régime
% as \the, so let's stringify the delimiter:
\csname @ifdefinable\endcsname\JustNumber{%
  \edef\JustNumber{##1\string p\string t}%
  \expandafter\def\expandafter\JustNumber\JustNumber{#1}%
}%
\begin{document}
    \begin{tikzpicture}
        \path (0,0);
        \pgfgetlastxy{\X}{\Y}
        \show\X
        \xdef\X{\expandafter\JustNumber\X}
        \show\X
    \end{tikzpicture}
\end{document}