row index of first non-zero element in pgfplotstable

Question

I am using the "functions" \findmin and findmax from here quite much to dynamical format and set plots from CSV data I read in.

In the same vein, I want a function \findFirstNonZeroIndex, which returns the integer index of the first element which is non-zero in a specified column. Since all my data are non-negative integers, non-zero is equal to being greater than 0.

Since the indices in pgfplotstable start with 0, how do I add 1 to this number?

From the pgfplotstable manuel, the iflessthan command seems capable of doing what I want:

\pgfplotstablesort[sort key={#2},sort cmp={
  iflessthan/.code args={#1#2#3#4}{%
    % here comes your code
  }%
}]{\result}{\datatable}

Some example data:

\begin{filecontents}{table.dat}
  x y
  1 0
  2 0
  3 30
  4 0
  5 50
\end{filecontents}

If possible, I do only want to use the column =y=index[1] and get something like \findFirstNonZeroIndex{\inputtable}{\outputvalue}. In this example, \outputvalue should be 3.

If it is easier, up to now I only use integer values in column y sorted ascending. So there would not be a 0 in row x=4.

I just do not know enough on how to get started. Any help would be appreciated.

Edit

Steven B. Segletes just answered my question with the help of package readarray.

Out of curiosity, is there a solution using the pgf framework?

Answer 1

Here's how you can do this using PGFPlotstable. I defined a macro that takes three arguments: The macro containing the table that was loaded using \pgfplotstableread, the name of the column, and the macro that contains the output. Note that PGFPlotstable uses zero based indexing, so if the first non-zero element is in the third row, the output will be 2. If no non-zero values are found, the output will be -1.

\documentclass{article}
\usepackage{filecontents}
\usepackage{pgfplotstable}

\begin{filecontents}{table.dat}
  x y
  1 0
  2 0
  3 30
  4 0
  5 50
\end{filecontents}

\newcommand\findFirstNonZeroIndex[3]{
    \def#3{-1}
    \pgfplotstableforeachcolumnelement{#2}\of{#1}\as\cell{
        \ifnum#3=-1
            \ifnum\cell>0
                \xdef#3{\pgfplotstablerow}
            \fi
        \fi
    }
}

\begin{document}

\pgfplotstableread{table.dat}\datatable
\findFirstNonZeroIndex{\datatable}{y}{\outputvalue}
\outputvalue

\end{document}

Answer 2

The macro \firstdata{column} will find the first positive entry in the specified column (beginning at row 2, since row 1 is alphabetic). Once found, it will output the value of column 1 from that row, which, in the data structure, is the datapoint number.

Thus, this MWE outputs the result 3.

The readarray package stores the file in one large \def, in this case \mydata. It then creates an array structure, here named thedata, by reading \mydata as a 2 column array of space-separated values. Individual cells of the array may be recalled with \arrayij{thedata}{row}{column} and compared against 0 to detect positiveness.

\documentclass{article}
\usepackage{readarray,filecontents,ifthen}
\begin{filecontents*}{table.dat}
  x y
  1 0
  2 0
  3 30
  4 0
  5 50
\end{filecontents*}
\readdef{table.dat}{\mydata}
\readArrayij{\mydata}{thedata}{2}
\newcounter{currentrow}
\def\firstdata#1{%
  \setcounter{currentrow}{1}%
  \whiledo{\value{currentrow}<\thedataROWS}{%
    \stepcounter{currentrow}%
    \ifnum\arrayij{thedata}{\thecurrentrow}{#1}>0
      \arrayij{thedata}{\thecurrentrow}{1}%
      \setcounter{currentrow}{\thedataROWS}%
    \fi%
  }%
}
\begin{document}
\firstdata{2}
\end{document}