What is a simple example of something you can do with LuaTeX?

Question

I've heard that with LuaTeX, you can embed Lua code in your document that work directly with TeX internals, making it possible to do several things that are hard to do with macros. What is a good specific example of stuff that you can accomplish easily with LuaTeX?

(I see here that LuaLaTeX is already available on Debian testing, and you can write documents like:

\documentclass{article}
\usepackage{luacode}

\begin{document}
A random number:
\begin{luacode}
tex.print(math.random())
\end{luacode}
\end{document}

resulting in something like

A random number: 0.135494265234

so I expect that other useful nontrivial things are possible.)

Answer 1

Here's an example of something I'm doing.

I'm writing a document which has a lot of sections. At the end of each section is a score, made of 3 numbers. The first number is on a scale of 0-10, and the second two are 0-3. Also the scale of the first is inverted, meaning that 1 is the biggest and 10 is the smallest, but 0 should still be 0. This code outputs each score and the average score it calculates.

\def\score#1#2#3{
    \directlua{
        s1 = #1;
        s2 = #2;
        s3 = #3;

        if s1 > 0 then
            s1 = 11 - s1
        end

        avgscore = (s1 + (10 * s2 / 3) + (10 * s3 / 3)) / 3;

        tex.print("Score 1: ", s1, ", ")
        tex.print("Score 2: ", s2, ", ")
        tex.print("Score 3: ", s3, ", ")
        tex.print("Combined Score: ", string.format("\%0.2f", avgscore))
    }
}

And then I use it multiple places in my document like this:

\section{stuff}
....
\score{8}{3}{0}
....
\section{even more stuff}
....
\score{0}{1}{1}

I'm also dumping these scores to a file (using lua), reading them in later (again with lua), and using pgfplots to create a chart of some things. There may be a much better way to do this, but I'm not willing to spend the rest of my life learning how to do a few calculations.

Answer 2

Programming in TeX is quite complicated as it was never meant to be a general programming language (despite being Turing complete). Adding Lua makes a lot of programming jobs much easier. At the same time, the LuaTeX people are exposing the internals of TeX so they can be altered by Lua. This makes lots of clever stuff possible. Some of the best examples at the moment can be seen on the ConTeXt wiki, as ConTeXt and LuaTeX are very closely linked.

Answer 3

Triggered by the Epub question: here is something useful you can easily do with luatex:

{\catcode`\#=12
\directlua {
local words = io.open('hyphens-' .. tex.jobname .. '.txt', 'w');
local outchar = unicode.utf8.char
local function dumphyphens (head)
   local data = {}
   for v in node.traverse(head) do
       if v.id == node.id('glyph') then
         data[#data+1] = outchar(v.char);
       elseif v.id == node.id('disc') then
          data[#data+1] = '-'
       elseif v.id == node.id('glue') then
         data[#data+1] = outchar(32)
       elseif v.id == node.id('hlist') then
         data[#data+1] = dumphyphens(v.list)
       end
   end
   return table.concat(data)
end
callback.register ('hyphenate', function (head,tail)
   lang.hyphenate(head, tail) 
   words:write (dumphyphens(head) .. outchar(10))
   end)
}}

\input knuth
\bye

This will write to hyphens-\jobname.txt a dump of all the characters that luatex has been asked to add hyphenation points to from this point on in the source, with the output of each callback call on a single line.

Although you cannot use this to generate Epub (mostly because macros like \TeX create two lines of output, once 'E' only for the lowered hbox and once 'TEX' for the actual macro use) the output is still useful because you can check to make sure there are no potential bad breaks allowed by the hyphenation patterns you are using.

Answer 4

LuaLaTeX and circuitikz example: A one-phase transformer approximately equivalent circuit (T-cicuit). Give the equivalent circuit parameters manually in the code.

\documentclass{article}
\usepackage[utf8x]{inputenc}
\usepackage{luacode}
\usepackage{amsmath,graphicx}
\usepackage{circuitikz}

\begin{document}
\luaexec{
tp=tex.print
local R1 = 0.1
local X1 = 0.3
local R2 = 0.13
local X2 = 0.35
local RC = 100
local XM = 40
tp("\\begin{figure}[h]")
tp("\\begin{circuitikz}")
tp("\\draw (0,0) to[short,*-*] (10.5,0);")
tp("\\draw (0,4) to[short,*-,i=$I_1$] (1,4);")
tp("\\draw (1,4) to[R,l=$"..R1.."\\Omega$] (3,4);")
tp("\\draw (3,4) to[L,l=j$"..X1.."\\Omega$] (5,4);")
tp("\\draw (5,4) to[short] (5.5,4);")
tp("\\draw (5.5,4) to[L,l=j$"..X2.."\\Omega$] (7.5,4);")
tp("\\draw (7.5,4) to[R,l=$"..R2.."\\Omega$] (9.5,4);")
tp("\\draw (9.5,4) to[short,-*,i=$I_2'$] (10.5,4);")
tp("\\draw (5.25,4) to[short,*-*,,i=$I_0$] (5.25,3);")
tp("\\draw (5.25,3) to[short] (4.5,3)to [short] (4.5,2.75);")
tp("\\draw (5.25,3) to[short] (6,3) to [short] (6,2.75);")
tp("\\draw (4.5,2.75) to[R,l=$"..RC.."\\Omega$] (4.5,1.65);")
tp("\\draw (6,2.75) to[L,l=j$"..XM.."\\Omega$] (6,1.65);")
tp("\\draw (4.5,1.65) to[short] (4.5,1.4);")
tp("\\draw (6,1.65) to[short] (6,1.4) to [short] (4.5,1.4);")
tp("\\draw (5.25,1.4) to[short,*-*] (5.25,0);")
tp("\\draw (0,4) to [open,v=$V_1$] (0,0);")
tp("\\draw (10.5,4) to [open,v^=$V_2'$] (10.5,0);")
tp("\\end{circuitikz}")
tp("\\caption{One-phase transformer equivalent circuit (T-circuit)}")
tp("\\end{figure}")
}

\end{document}

Answer 5

Check out those examples. Even if "normal" users might see no need for a general purpose language in LaTeX they will benefit from packages using Lua scripts internally.