Strange extra \else in pgfplots slope field plot

Question

I'm trying to plot a slope field of the function 2x/y using pgfplots such that the length of each quiver is fixed. The code is below

\documentclass{minimal}
\usepackage{tikz}
\usepackage{pgfplots}
\begin{document}
    \begin{tikzpicture}
        \begin{axis}[
            declare function={
                f(\x,\y) = (\y != 0) * (2\x/\y);
                length(\x,\y) = sqrt(1+(2\x/\y)^2);
                result = (6 +0.5)/14;
            },
            domain=-0.5:6, 
            view={0}{90},
            axis lines=center
            ]
            \addplot3[blue, quiver={u={1/length(x,y)}, v={f(x,y)/length(x,y)}, scale arrows=result}, -stealth, samples=14] {0};
        \end{axis}
    \end{tikzpicture}% Extra \else. ...e arrows=result}, -stealth, samples=14] {0};
\end{document}

Here I define my functions and try to use them in the quiver plot. However, I get the following unhelpful error

! Extra \else.
\pgfmath@local@next ...@local@function@body \else
                                                  \if #1(\let \pgfmath@local...
l.32 ...e arrows=length}, -stealth, samples=14] {0};

I can't see what's wrong with the code. I have tried, without success

Changing the /= sign into other possible alternatives, such as !=,
Plugging in the function calls with their literals

Can I please have some help with this?

You're missing a ; at the end of the result definition. Aside from that, what is /= supposed to represent? The "not equal" comparison operator is !=. — Torbjørn T., Dec 09 '21 at 16:28
@TorbjørnT. that solved those errors, however, there's a new error now saying Package PGF Math Error: Could not parse input 'result' as a floating point number, sorry. The unreadable part was near 'result'.. ...e arrows=result}, -stealth, samples=14] {0}; — Kookie, Dec 09 '21 at 17:09
Please post a complete compilable example that also shows the packages and libraries you are using. — Jasper Habicht, Dec 09 '21 at 17:19
@JasperHabicht I did not use any package other than pgfplots and tikz. So I don't think it's necessary to put those in too. None of the things I used in my example seem to come from other packages either — Kookie, Dec 09 '21 at 17:29
@Kookie My point is, that with a complete example it is far easier to help you, because we can easily copy and paste the full code and compile it. Also, it is clear than, that you did not use any other packages, and I wouldn't need to ask =) — Jasper Habicht, Dec 09 '21 at 17:30
@JasperHabicht Understood, I'll edit the code in my question — Kookie, Dec 09 '21 at 17:32

score 3 · Accepted Answer · answered Dec 09 '21 at 17:46

In your original code you had forgotten a ; at the end of the definition of result. That you get a Could not parse input 'result' as a floating point number, ... error indicates that the argument given to scale arrows isn't parsed as a number, so the function is interpreted. I don't whether this is a bug or by design. One possible workaround is to use \pgfmathsetmacro to parse result and save to a macro, as in the example below.

\documentclass{article} % don't use minimal https://tex.stackexchange.com/questions/42114
\usepackage{pgfplots} % loads tikz
\begin{document}
    \begin{tikzpicture}
        \begin{axis}[
            declare function={
                f(\x,\y) = (\y != 0) * (2\x/\y);
                length(\x,\y) = sqrt(1+(2\x/\y)^2);
                result = (6 +0.5)/14;
            },
            domain=-0.5:6, 
            view={0}{90},
            axis lines=center
            ]
            % make an intermediate macro for "result"
            \pgfmathsetmacro{\tmpres}{result}
        % and use \tmpres for &quot;scale arrows&quot;
        \addplot3[blue, quiver={u={1/length(x,y)}, v={f(x,y)/length(x,y)}, scale arrows=\tmpres}, -stealth, samples=14] {0};
    \end{axis}
\end{tikzpicture}    

\end{document}

Strange extra \else in pgfplots slope field plot

1 Answers1