Otdfbt

The function I'm trying to create is one that takes two numbers and prints the result with some math. The following is my code:

documentclass[12pt,a4paper]article

begindocument
 directlua
 function prod(a,b)
 tex.print(a "$times$" b "$=$" a*c)
 end
 

 The product of 2 and 3: directluaprod(2,3).
enddocument

I can't make it print the whole statement correctly. How to solve it?

documentclass[12pt,a4paper]article

directlua
 function prod(a,b)
 tex.print("$" .. a .. "string\times" .. b .. "=" .. a*b .. "$")
 end


begindocument 
 The product of 2 and 3: directluaprod(2,3).
enddocument

One tricky thing is getting the backslash escaping game right: LuaTeX: How to handle a Lua function that prints TeX macros. directlua expands macros before passing them on to Lua, so times gets messed up. But something like stringtimes, which should stop that expansion does not quite work as intended because t is a special escape for the tab in Lua. Hence we need to escape the backslash there. In Lua you would have to type \times, but in TeX we need to stop the \ from being expanded, so we need string\times. That is one of the reasons why it is often recommended to use the luacode package or externalise Lua functions into their own .lua files and then load them with dofile or require (see for example How to do a 'printline' in LuaTeX, a bit on dofile and require can be found at LuaLatex: Difference between `dofile` and `require` when loading lua files).

Another thing is that you need .. to concatenate strings.

Finally, you probably want the entire expression in math mode and not just certain bits.

Also moved the directlua function definition into the preamble. (Thanks to Mico for the suggestion.)

documentclass[12pt,a4paper]article

begindocument
 directlua
 function prod(a,b)
 tex.print(a.. "$string\times$".. b.. "$=$".. a*b)
 end
 

 The product of 2 and 3: directluaprod(2,3).
enddocument

Just for completeness, here's a solution that shows how to (a) write the Lua code to an external file, (b) load the Luacode via a directluadofile("...") directive, and (c) set up a LaTeX "wrapper" macro (called showprod in the example below) whose function (pun intended) is to invoke the Lua function.

Note that with this setup, one can write \ rather than string\ to denote a single backslash character. (This is also the case for the luacode and luacode* environments that are provided by the luacode package.)

RequirePackagefilecontents
beginfilecontents*show_prod.lua

function show_prod ( a , b )
 tex.sprint ( "$"..a.."\times"..b.."="..a*b.."$" )
end

endfilecontents*

documentclassarticle
%% Load Lua code from external file and define a LaTeX "wrapper" macro
directluadofile("show_prod.lua")
newcommandshowprod[2]directluashow_prod(#1,#2)

begindocument
The product of 2 and 3: showprod23.
enddocument