PGFplots: Logarithmic scale showing 1020 insteead of 1024?

Question

I am trying to make a plot with a base-2 logarithmic axis and everything is turning out great, except that the labels are 32, 64, 128, 256, 512, and 1020. The last one should be 1024. I assume that this has something to do with significant digits, but I'm not sure how to change it.

Here is the code that I am using:

\documentclass{article}
\usepackage{pgfplots}
\usepackage[active, tightpage]{preview}
\PreviewEnvironment{tikzpicture}

\begin{document}
\begin{tikzpicture}
    \begin{semilogxaxis}[
        every axis legend/.append style={nodes={right}},
        xlabel=$n$,
        log basis x=2,
        ymode=log,
        ylabel=Time (sec),
        log ticks with fixed point,
        legend style={at={(0.05,0.95)},
        title={My title},
        anchor=north west}]
        \addplot coordinates {
            (32,0.04)
            (64,0.09)
            (128,0.14)
            (256,0.27)
            (512,0.56)
            (1024,1.66)
        };
        \addplot coordinates {
            (32,0.04)
            (64,0.09)
            (128,0.26)
            (256,0.92)
            (512,5.71)
            (1024,42.14)
        };
        \legend{With minimization,Without minimization}
    \end{semilogxaxis}
\end{tikzpicture}
\end{document}

Thanks so much.

Answer 1

This is indeed a precision issue.

Two possible options:

• Use

  xticklabel={
      \pgfkeys{/pgf/fpu=true}
      \pgfmathparse{int(2^\tick)}
      \pgfmathprintnumber[fixed]{\pgfmathresult}
  }
  

  This gives the correct results up to 2^21 = 2097152, after that, you also run into precision problems

• Load the xint package (by our own jfbu!) and then use

  xticklabel={\xinttheiexpr2^\tick\relax}
  

  This works up to a few hundred digits. I tried it up to 2^42 = 4398046511104, which works correctly. At some point, you probably don't want fixed format output anymore, though.

---

\documentclass{article}
\usepackage{pgfplots}
\usepackage{xintexpr}
\usepackage[active, tightpage]{preview}
\PreviewEnvironment{tikzpicture}

\begin{document}
\begin{tikzpicture}
    \begin{semilogxaxis}[
        every axis legend/.append style={nodes={right}},
        xlabel=$n$,
        log basis x=2,
        ymode=log,
        ylabel=Time (sec),
        /pgf/number format/1000 sep={\,},
        xticklabel={\xinttheiexpr[0]2^\tick\relax},
        legend style={at={(0.05,0.95)},
        title={My title},
        anchor=north west}]
        \addplot coordinates {
            (32,0.04)
            (64,0.09)
            (128,0.14)
            (256,0.27)
            (512,0.56)
            (1024,1.66)
        };
        \addplot coordinates {
            (32,0.04)
            (64,0.09)
            (128,0.26)
            (256,0.92)
            (512,5.71)
            (1024,42.14)
        };
        \legend{With minimization,Without minimization}
    \end{semilogxaxis}
\end{tikzpicture}
\end{document}