[Tex/LaTex] Piecewise Function Using Pgfplots

pgfplots

Using the question here I have tried to plot a piecewise function. My code looks like

\begin{document}
\pgfmathdeclarefunction{func}{1}{%    
\pgfmathparse{%
(and(#1<=-2)  * (#1*#1 + #1*6 + 8)   +%     
(and(#1>=-2  ,#1<=1) * (2 - #1 - #1*#1)   +%    
(and(#1>=1  ,#1<=2) * (6 - #1*8 + #1*#1*2)   +%
(and(#1>=2) * (-10 + #1*6 - #1*#1)   %
}%
}


\begin{tikzpicture}[scale=0.8    
\begin{axis}[
axis x line=middle, 
axis y line=middle, 
ymin=-5, ymax=5, ytick={-1,-2,-3,-4,-5,1,2,3,4,5}, ylabel=$y$, 
xmin=-5, xmax=5, xtick={-5,-4,-3,-2,-1,1,2,3,4,5}, xlabel=$x$
]

\addplot[blue,domain=-5:5]{func(x)};
\end{axis}
\end{tikzpicture} 

\end{document}

The function will be x^2+6x+8 for x<=-2, 2-x-x^2 for -2<=x<=1, 2x^2-8x+6 for 1<=x<=2, and -x^2+6x-10 for x>=2. When I try to compile it gives me a PGF Math Error: internal routine of the floating point. How do I fix this?

Best Answer

The and function has at least two arguments, otherwise it can’t and anything. Simply remove the ands from the first and the last term.

You should also make sure that the pieces do not overlap which they do in your case as you always use <= and >= on the same border value.

Furthermore, I have used the top-level declare function key to declare a function.

Code

\documentclass[tikz]{standalone}
\usepackage{pgfplots}
%\pgfplotsset{compat=1.8}
\begin{document}
\begin{tikzpicture}[
  declare function={
    func(\x)= (\x<=-2) * (\x*\x + 6*\x + 8)   +
     and(\x>-2, \x<=1) * (2 - \x - \x*\x)     +
     and(\x>1,  \x<=2) * (6 - 8*\x + 2*\x*\x) +
                (\x>2) * (-10 + 6*\x - \x*\x);
  }
]
\begin{axis}[
  axis x line=middle, axis y line=middle,
  ymin=-5, ymax=5, ytick={-5,...,5}, ylabel=$y$,
  xmin=-5, xmax=5, xtick={-5,...,5}, xlabel=$x$,
]
\pgfplotsinvokeforeach{-2, 1, 2}{
  \draw[dashed] ({rel axis cs: 0,0} -| {axis cs: #1, 0}) -- ({rel axis cs: 0,1} -| {axis cs: #1, 0});}
\addplot[blue, domain=-5:5, smooth]{func(x)};
\end{axis}
\end{tikzpicture} 
\end{document}

Output

enter image description here

Related Solutions

[Tex/LaTex] Axis limits in user-defined style

Rather than defining your style in the first plot, you can define it in your pre-amble using \pgfplotsset; this allows you to put xmin, xmax, etc in your own style.

screenshot

\documentclass{article}
\usepackage{pgfplots}
\pgfplotsset{compat=1.5}

\pgfplotsset{mystyle/.style={%
        width=6cm,
        ylabel={mystyle (kg)},
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\begin{document}

\begin{tikzpicture}
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\end{axis}
\end{tikzpicture}
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\end{axis}
\end{tikzpicture}

\end{document}

And if you want to inherit some of the properties from mystyle and overwrite others, then you can use, for example,

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[Tex/LaTex] Apply shift to (current axis.left of origin)

I really don't know why but this works for me

\documentclass{article}
\usepackage{pgfplots}
\usetikzlibrary{calc}

\pgfplotsset{compat=1.10}

\begin{document}
\begin{tikzpicture}
\begin{axis}
    \addplot {x*x};

    \draw [ultra thick, dotted, draw=brown] 
        (current axis.left of origin) -- 
        (current axis.right of origin);

    \draw [ultra thick, draw=red] 
        ($(current axis.left of origin)-(axis cs:0,-12)$) -- 
        ($(current axis.right of origin)+(axis cs:0,12)$);
\end{axis}
\end{tikzpicture}
\end{document}