Correct way of test blank element in array

Question

I'm trying to test whether an element in a defined list (array) is "empty." I tried using the \empty macro, or putting white spaces inside the quote, but nothing seems to work. I'm testing the emptiness of each element using the etoolbox \ifboolexpr construct with test (also, I tested \notblank with the same results), although I'm not sure if the fault lies in the conditional evaluation or in the \pgfmathparse that retrieves the element from the array.

Thus, what is the correct way of checking if the element is empty and branching into the else part of the conditional.

\documentclass{article}

\usepackage{tikz}
\usetikzlibrary{calc}
\usepackage{etoolbox}

\pagestyle{empty}

\begin{document}
\begin{tikzpicture}
\def\n{5}
\def\weights{{"1","2","3","4","","n"}}
\foreach \i [count=\c] in {0,...,\n}{
  \pgfmathparse{\weights[\i]} \let\label\pgfmathresult
  \ifboolexpr{ not test{\ifstrempty{\label}} }{%
    \node at (0,\c) {$l_{\label}$};
  }{ %
    \node at (0,\c) {empty};
  }
}
\end{tikzpicture}
\end{document}

Answer 1

Below, I have defined the macro \GetNthMember to get the nth member of a list. If node does not exist or is "", then it is displayed in red:

Notes:

• Your list members are not empty when you define them as "". They have a value which is two double quotes.

• If you eliminate the double quotes around your list members, one way you can test for an empty member is:

  \IfEq{<string to test>}{}{<true code>}{<false code>}
  

  or if you prefer to still use the double quotes you can compare as:

  \IfEq{<string to test>}{""}{<true code>}{<false code>}
  

  Am sure there are other ways to test fro empty strings but I find that the xstring package works well for me.

• The code below treats the first element of the list as number one. If you want that to start from zero the code will require adjustment.

Code:

\documentclass{article}
\usepackage{xstring}
\usepackage{tikz}
\usetikzlibrary{calc}
\usepackage{etoolbox}


\newcommand*{\GetNthMember}[3]{%
    % #1 = macro to assign value to
    % #2 = list
    % #3 = n
    \edef\List{#2}%
    \edef#1{}%
    \foreach [count=\c] \Member in \List {%
        \IfEq{\c}{#3}{%
            \xdef#1{\Member}%
            \breakforeach%
            %\typeout{****Found Member #3 = #1}%
        }{}%
    }%
}%

\newcommand*{\NodeColor}{}%

\begin{document}
\begin{tikzpicture}[ultra thick]
\def\n{5}
\def\weights{"1","2","3","4","","n"}
\foreach \i  in {0,...,\n}{
    \GetNthMember{\MyLabel}{\weights}{\i}%
    \def\NodeColor{blue}%
    \IfStrEq{\MyLabel}{}{\def\NodeColor{red}}{}%
    \IfStrEq{\MyLabel}{""}{\def\NodeColor{red}}{}%
    \node [shape=rectangle, draw=\NodeColor] at (\i,0) {\strut\MyLabel};
}
\end{tikzpicture}
\end{document}

Answer 2

I think this is a simpler solution:

\documentclass{article}

\usepackage{tikz}
\usepackage{xifthen}

\pagestyle{empty}
\begin{document}
    \begin{tikzpicture}
    \def\wt{1,2,3,4,,6}
    \newcounter{cnt}
    \foreach \x in \wt{
        \addtocounter{cnt}{1}
        \node at (0,\thecnt) {\ifthenelse{\equal{\x}{}}{empty}{\x}};
    }
    \end{tikzpicture}
\end{document}

Answer 3

(Just for completeness, using Peter's answer and the pgfmathparse array retrieval method.)

The xstring package works in my case. Using the pgf way of retrieving the elements (don't know if is the best or correct, though), this is the code that does the trick:

\documentclass{article}

\usepackage{tikz}
\usetikzlibrary{calc}
\usepackage{etoolbox}
\usepackage{xstring}

\pagestyle{empty}

\begin{document}
\begin{tikzpicture}
\def\n{5}
\def\weights{{"1","2","3","4","","n"}}

\foreach \i [count=\c] in {0,...,\n}{
  \pgfmathparse{\weights[\i]} \let\lbl\pgfmathresult
  \IfStrEq{\lbl}{}{%
    \node at (0,\c) {empty};%
  }{%
    \node at (0,\c) {$l_{\lbl}$};%
  }%
}

\end{tikzpicture}
\end{document}