How to plot every 4th row using pgfplotstable?

Question

This seems very related to How to display every 4th row in a large data file using pgfplotstable?. I just try to plot the filtered rows but am missing something here. Here is the MWE (I copied the example from previous link).

\pgfplotstableread{
 obs      number
  1         2
  2         5
  3         3
  4         2
  5         4
  6         1
  7         2
  8         5
  9         3
  10        2
  11        4
  12        1
}\loadedtable

\pgfplotstabletypeset[
  row predicate/.code={%
    \pgfplotstablegetelem{#1}{obs}\of{\loadedtable}
    \pgfmathparse{int(Mod(\pgfplotsretval,4)}
    \ifnum \pgfmathresult = 0 
      \else \pgfplotstableuserowfalse
    \fi}
]{\loadedtable}

\begin{tikzpicture}
    \begin{axis}
   \addplot[only marks, 
    x filter/.code={\pgfplotstablegetelem{\coordindex}{obs}\of{\loadedtable}
                    \pgfmathparse{int(Mod(\pgfplotsretval, 4))}
                    \ifnum \pgfmathresult = 0 
                    \else
                    \def \pgfmathresult{}
                    \fi},
    ] table[x=obs,y=number] {\loadedtable};
   \end{axis}
\end{tikzpicture}

The tabletypeset works fine but the plot does not work. Is there an easy fix?

On a side note, is there anyway to save the filtered table as a new macro so that I can use it in a plot directly?

Any help is appreciated.

Answer 1

For plotting only each n-th coordinate, PGFPlots already provides the mechanism mark repeat=<integer> and mark offset=<integer>:

\documentclass[border=5mm]{standalone}

\usepackage{pgfplots, pgfplotstable}
\begin{document}


\pgfplotstableread{
 obs      number
  1         2
  2         5
  3         3
  4         2
  5         4
  6         1
  7         2
  8         5
  9         3
  10        2
  11        4
  12        1
}\loadedtable

\pgfplotstabletypeset[
  row predicate/.code={%
    \pgfplotstablegetelem{#1}{obs}\of{\loadedtable}%
    \pgfmathparse{int(Mod(\pgfplotsretval,4)}%
    \ifnum \pgfmathresult = 0 %
      \else \pgfplotstableuserowfalse%
    \fi}
]{\loadedtable}

\begin{tikzpicture}
    \begin{axis}
   \addplot[only marks,
   mark repeat=4, mark phase=4
    ] table[x=obs,y=number] {\loadedtable};
   \end{axis}
\end{tikzpicture}

\end{document}