`Missing = inserted for \ifnum` when using addplot

Question

I'm trying to plot a normalized sinc function with PGFPlots/TikZ. As this is somewhat tricky because of the limiting value at 0, I used a suggestion provided by percusse.

Here's a minimal example (note that I've renamed the function to sincf due to conflicts with the rest of the document and inserted an approximation of Pi to normalize):

\documentclass{scrreprt}

\usepackage{pgfplots}
\pgfplotsset{compat=1.12}


\begin{document}
\begin{tikzpicture}

% https://tex.stackexchange.com/a/235009
\pgfmathdeclarefunction{sincf}{1}{%
        \pgfmathparse{abs(#1)<0.01 ? int(1) : int(0)}%
        \ifnum\pgfmathresult>0 %
            \pgfmathparse{1}%
        \else%
            \pgfmathparse{sin(3.14159*#1 r)/(3.14159*#1)}%
        \fi%
}

    \begin{axis}
        \addplot {sincf(\x)};
    \end{axis}

\end{tikzpicture}
\end{document}

However, I'm getting two errors which I can't figure out:

! Missing = inserted for \ifnum.
<to be read again>
Y
l.23 \addplot plot (\x,{sincf(\x)});
I was expecting to see `<', `=', or `>'. Didn't.

! Missing number, treated as zero.
<to be read again>
Y
l.23 \addplot plot (\x,{sincf(\x)});
A number should have been here; I inserted `0'.
(If you can't figure out why I needed to see a number,
look up `weird error' in the index to The TeXbook.)

Using MikTeX 2.9

Answer 1

pgfplots uses library fpu for better precision by default, which has a different internal number format. Therefore \pgfresult is not 1 or 0 anymore and \ifnum cannot be used.

\tikzset{fpu}
\pgfmathparse{int(0)}\show\pgfmathresult
\pgfmathparse{int(1)}\show\pgfmathresult

The lines show 0Y0 for zero and 1Y1 for one.

In this case, the whole math can be put in one \pgfmathparse expression:

\documentclass{scrreprt}

\usepackage{pgfplots}
\pgfplotsset{compat=1.12}

\begin{document}
\begin{tikzpicture}

    % http://tex.stackexchange.com/a/235009
    \pgfmathdeclarefunction{sincf}{1}{%
        \pgfmathparse{(abs(#1)<0.01) ? 1 : sin(pi*#1 r)/(pi*#1)}%
    }

    \begin{axis}
        \addplot {sincf(\x)};
    \end{axis}

\end{tikzpicture}
\end{document}

Answer 2

Using a related answer about fpu formatting, I was able to use \ifnum in the case that my result was within the range of an int() [−16383.9998, +16383.9998]. The trick is to use \pgfmathfloattofixed on the \pgfmathresult.

\pgfkeys{/pgf/fpu=true}
\pgfmathparse{...do floating point math here (with an int() result)...}
\pgfmathfloattofixed{\pgfmathresult} % this allows the conversion with int()
\edef\tmp{\pgfmathresult}
\pgfkeys{/pgf/fpu=false}
\pgfmathsetmacro\iTmp{int(\tmp)} % convert with int()

\ifnum \iTmp>0
  ... do stuff ...
\fi