How to get nth occurrence in a string?

Question

I get a string from an external program, and I would like to get each part of that string:

\documentclass{article}
\usepackage{bashful}

\begin{document}

% Test.sh would do echo "4 8 15 16 23 42"
\let\data{\splice{./test.sh}}

\begin{itemize}
    \item \getOccurences{2}{\data} %15
    \item \getOccurences{5}{\data} %23
\end{itemize}

\end{document}

The information read from stdout is a series of numbers space separated.

How should I implement the \getOccurences macro?

Answer 1

This answer addresses how to parse the string, once you have inputted it. For this example, therefore, I merely define the string using \def. I use the listofitems package to create an array of the individual string elements.

\documentclass{article}
%\usepackage{bashful}
\usepackage{listofitems}
\setsepchar{ }
\ignoreemptyitems
\begin{document}

% Test.sh would do echo "4 8 15 16 23 42"
%\let\data{\splice{./test.sh}}
\def\data{4 8 15 16 23 42}

\readlist\mylist{\data}

\mylist[3] should be 15

\mylist[5] should be 23

\end{document}

Answer 2

The xstring package has a macro StrBetween[n,m]{full string}{string A}{string B} that may help here. It will select the substring between the nth occurrence of string A and the mth occurrence of string B. In this case you can set both string A and string B to be a space and extract the string between the 2nd and 3rd space to get 15 and the string between the 4th and 5th space to get 23.

The second part of the question is to get the external output into a format that xstring can handle. This is a bit tricky. \splice is doing a lot under the hood that causes trouble when expanding the macro for string processing. A better alternative is to use \bashStdout as defined by bashful that captures the output of the last \bash \END block as a plain string. The only problem here is that spaces have category code 12 which also confuses xstring. As a solution you can 'flatten' the string using \detokenize. This has the added advantage that you can use \edef which expands the argument, allowing for other \bash \END blocks (which will overwrite \bashStdout) in between the execution and the actual use of the values.

MWE:

\documentclass{article}
\usepackage{bashful}
\usepackage{xstring}

\begin{document}

\bash
echo "4 8 15 16 23 42"
\END
\edef\data{\detokenize\expandafter{\bashStdout}}
All data: \data

\begin{itemize}
    \item \StrBetween[2,3]{\data}{ }{ } %15
    \item \StrBetween[4,5]{\data}{ }{ } %23
\end{itemize}
\end{document}

Result: