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\pgfmathresultThe 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}
