Is there a reason why the code below leads to the error:
! Illegal unit of measure (pt inserted).
<to be read again>
e
l.16 ...t [domain=-.1:.1,samples = 100] (\x,{G(\x)})
\documentclass[tikz]{standalone}
\directlua{
function G (x)
return (-(x^2)/3)
end
}
\pgfmathdeclarefunction{G}{1}{%
\edef\pgfmathresult{\directlua{tex.print("" .. G(#1))}}%
}
\begin{document}
\begin{tikzpicture}
\draw [blue] plot [domain=-.1:.1,samples = 100] (\x,{G(\x)});
\end{tikzpicture}
\end{document}
If you modify the function to
\pgfmathdeclarefunction{G}{1}{%
\edef\pgfmathresult{\directlua{tex.print("" .. G(#1))}}%
\show\pgfmathresult
\edef\pgfmathresult{1}%
}
then the plot plots the (wrong:-) function without error but you get to see what you were plotting. The first few values are OK but then:
> \pgfmathresult=macro:
->-9.5052e-06.
Presumably pgf isn't expecting the e notation. Presumably lua has some number formatting functions to prevent it using that notation?
edit ah yes:
\makeatletter
\pgfmathdeclarefunction{G}{1}{%
\edef\pgfmathresult{\directlua{tex.print(string.format("\@percentchar f",G(#1)))}}%
}
\makeatother
G function definition could be \pgfmathparse{\directlua{tex.print("" .. G(#1))}}% as the math parser can convert the e notation into something pgf can use.
I would suggest to use Lua for all the calculations and also to create the relevant LaTeX code. Of course as Dave suggested use string.format('% .2f','number') to format the number. Two decimal places would be more than adequate for plotting.
\documentclass[tikz]{standalone}
\usepackage{luacode}
\begin{document}
\begin{luacode}
local function G (x)
return string.format('% .2f',-(x^2)/3)
end
tex.print('\\begin{tikzpicture}[scale=.3]')
tex.print('\\draw[color=red] (0,0)')
local z=-.1
for i=1,50 do
tex.sprint('-- (' .. i .. ',' .. G(z)..') ')
z=z+.2
end
tex.sprint(';')
tex.print('\\end{tikzpicture}')
\end{luacode}
\end{document}
This normally produces cleaner code, is easier to debug and faster. Add formatting code for axes and co-ordinates to suit your requirements. For example using the above it is trivial to also export the values as a table and also do any transformations you may want.
lua and tex codes?