Problem with x filter when trying to filter points according to x value

Question

I am trying to filter points according to its x values. The data may be from a table-formatted text file, or directly typed in coordinates{...}. I tried to use x filter/.code, but for data from text file it seems fine, while the same code yield compiling error if the data are directly typed through coordinates{...}.

\addplot[scatter, only marks] table[x=xx, y=yy, col sep=comma]{tmp.txt}; 
% Seems working

vs

\addplot[scatter, only marks] coordinates{(0,0) (1,1) (1,1.5) (2,2)}; 
%|16 error| Missing = inserted for \ifnum. Y ...s] coordinates{(0,0) (1,1) (1,1.5) (2,2)};

I suspect it might be my problematic usage of \pgfmathresult in x filter, but not sure how to fix. The following are the MWE along with the compilation error. Assuming file tmp.txt is:

xx, yy
0,0 
1,1 
1,1.5 
2,2 

And the code is:

\documentclass{article}
\usepackage{pgfplots}   

\begin{document}

\begin{tikzpicture}[scale=0.8]
\begin{axis}[
  x filter/.code= {
    \ifnum\pgfmathresult=1
      \def\pgfmathresult{}
    \fi
 }]
  % This seems working fine
  \addplot[scatter, only marks] table[x=xx, y=yy, col sep=comma]{tmp.txt}; 

  % This cause compilation error, which is
  % |16 error| Missing = inserted for \ifnum. Y ...s] coordinates{(0,0) (1,1) (1,1.5) (2,2)};

  %\addplot[scatter, only marks] coordinates{(0,0) (1,1) (1,1.5) (2,2)};
\end{axis} 
\end{tikzpicture}

\end{document}

Answer 1

It's because coordinates are handled via internal floating point representation which is Klingon for me such 1.1Y0.e1 (or something like that) so you see that Y in the error message. The solution is to convert it to regular decimal.

Also \ifnum works only with integers but pgfmath tends to spit out 1.0 even for integer 1 so better make a dimension comparison to get rid of the complications.

\documentclass{article}
\usepackage{pgfplots}   

\pgfplotstableread{
xx yy
0 0
1 1
1 1.5
2 2
}\mytable
\begin{document}

\begin{tikzpicture}[scale=0.8]
\begin{axis}
\addplot[scatter, only marks,  x filter/.code= {
    \ifnum\pgfmathresult=1
      \def\pgfmathresult{}
    \fi
 }] table[x=xx, y=yy]{\mytable}; 

  \addplot+[no marks,  x filter/.code={
\pgfkeys{/pgf/fpu,/pgf/number format/.cd,fixed,verbatim}
\pgfmathprintnumberto{\pgfmathresult}{\mytempvar}
    \ifdim\mytempvar pt=1pt%
      \def\pgfmathresult{}
    \fi
}] coordinates{(0,0) (1,1) (1,1.5) (2,2)};
\end{axis} 
\end{tikzpicture}
\end{document}