Using a conditional on a custom defined variable

Question

Last month, I asked this question, about how to personalize a flowchart based on data available in a text file. The idea was to be able to adjust the width of arrows depending on numbers in the text file. Now I have been wanting to go one step further, which is: not plotting the arrow in case the corresponding number in the data file is zero.

I managed to introduce a conditional to test whether my data is larger than zero, which works if I use an arbitrary number (e.g. \ifnum2>0), but not if I use my custom-defined command for fetching the data (e.g. \ifnum\flowwidth{A2B}>0, see below in the code chunk)

I have tried a few different options to try to make it work. Here is the base code, on which I started to make modifications (some parts come from the answer to this other question I asked last month):

\documentclass[tikz]{standalone}
\usepackage{readarray}
\usetikzlibrary{arrows,positioning}
\usepackage{filecontents}

\begin{filecontents*}{flow.txt}
A2B, 110$\pm$6, 1.5 mm
A2C, 0$\pm$0, 0 mm
A2D, 110$\pm$6, 0.4 mm
\end{filecontents*}

\begin{document}

% USE readarray  TO GET THE DATA INTO A \def
\readarraysepchar{;}
\readdef{flow.txt}\myflowdef

% USE listofitems TO DIGEST THE \def APPROPRIATELY
\setsepchar{;/,}
\ignoreemptyitems
\readlist*\myflowdata{\myflowdef}
\foreachitem\i\in\myflowdata[]{%
  \expandafter\edef\csname\myflowdata[\icnt,1]var\endcsname{\myflowdata[\icnt,2]}%
  \expandafter\edef\csname\myflowdata[\icnt,1]width\endcsname{\myflowdata[\icnt,3]}%
}

% For importing data
\newcommand{\flowvar}[1]{\csname#1var\endcsname}
\newcommand{\flowwidth}[1]{\csname#1width\endcsname}

\begin{tikzpicture}[BOX/.style   ={rectangle, draw, text centered},
                    ARROW/.style ={draw,-latex', line width = 0.5mm}]

\node [BOX] (a) {A};   
\node [BOX, right= of a] (c) {C};   
\node [BOX, above= of c] (b) {B};

\draw [draw,-stealth, line width = \flowwidth{A2B}] (a.north) |- (b.west);
\draw [draw,-stealth, line width = \flowwidth{A2C}] (a.east) -- (c.west);  

\end{tikzpicture}
\end{document}

I have tried to store the width in an intermediate variable instead of having the command after the if:

\def\myvar{\flowwidth{A2B}}
\ifnum\myvar>0%
    \draw [draw,-stealth, line width = \flowwidth{A2B}, color = red] (a.north) |- (b.west);
\fi

Or use \pgfmathsetmacro :

\pgfmathsetmacro\myvar{\csname\flowwidth{A2B}\endcsname}
\ifnum\myvar>0%
    \draw [draw,-stealth, line width = \flowwidth{A2B}, color = red] (a.north) |- (b.west);
\fi

Or use \expandafter :

\expandafter\ifnum\expandafter\flowwidth{A2B}>0%
    \draw [draw,-stealth, line width = \flowwidth{A2B}, color = red] (a.north) |- (b.west);
\fi

But those approaches yield nothing. I have also tried to play around with \csname and \endcsname. I have to admit that I made these changes rather randomly, with no true understanding of the programming principles behind. I would be very glad if someone would have suggestions on how to solve this and also if anyone knows a good resource/tutorial that explains that sort of LaTeX programming.

Answer 1

The final expansion of \flowwidth{A2B} is 1.5mm, so a test for it being positive should use \ifdim:

\ifdim\flowwidth{A2B}>0pt

(remember to leave a space, or end-of-line, after pt in order to avoid untimely expansion of what follows).