Order list of names with Lua

Question

I want to define a command using \directlua that looks something like:

\sort{c, b, a}

and outputs a, b, c to the document.

Answer 1

In ConTeXt you do not have to reinvent the wheel.

\def\sort#1{%
  \ctxlua{
    context(
      table.concat(
        table.sorted(
          utilities.parsers.settings_to_array([[#1]]) ) , ", " ) )
}}

\starttext

\sort{c, b, a}

\stoptext

---

You can also use it with LaTeX by including the appropriate Lua headers from ConTeXt.

\documentclass{article}

\directlua{
dofile(kpse.find_file("l-lpeg.lua"))
dofile(kpse.find_file("util-sto.lua"))
dofile(kpse.find_file("util-prs.lua"))
}

\def\sort#1{%
  \directlua{\unexpanded{
    tex.sprint(
      table.concat(
        table.sorted(
          utilities.parsers.settings_to_array([[#1]]) ) , ", " ) )
}}}

\begin{document}

\sort{c, b, a}

\end{document}

The output is similar.

---

To omit empty entries (empty string) one has to write one additional helper function, which I could not find in ConTeXt. However, multiple empty entries side by side seem to confuse at least one of the functions, i.e. \sort{,,,,a} will produce spurious commas.

\documentclass{article}

\directlua{
dofile(kpse.find_file("l-lpeg.lua"))
dofile(kpse.find_file("l-string.lua"))
dofile(kpse.find_file("util-sto.lua"))
dofile(kpse.find_file("util-prs.lua"))

function table.strip_empty(tab)
    for k, v in pairs(tab) do
        if ( string.strip(v) == "" ) then
            table.remove(tab, k)
        end
    end
    return tab
end
}

\def\sort#1{%
  \directlua{\unexpanded{
    tex.sprint(
      table.concat(
        table.sorted(
          table.strip_empty(
            utilities.parsers.settings_to_array([[#1]]) ) ) , ", " ) )
}}}

\begin{document}

\sort{, d, , f, c, b, a, }

\end{document}

Answer 2

Here's a possible solution. The main Lua function, called dojob, converts the input string into a Lua table (via a call to an auxiliary Lua function called string_to_table), performs a simple sort, and outputs the table elements as a string, with the elements separated by "," ("comma&space"). Any leading or trailing whitespace characters and commas are discarded automtically.

The Lua function dojob may be accessed from within the body of a LaTeX document via the LaTeX macro called \sort.

Aside: Much of the Lua code in the string_to_table function was obtained from this site. (Look for the header "Method: Using only string.gsub" for more details on how the code works.) For good measure, I added a routine that removes leading and trailing whitespace from the table entries.

The sorting routine is simple. I trust your comma-separated list isn't so long as to require a more efficient sorting algorithm (e.g., QuickSort) in order to avoid getting bogged down.

% !TeX program = lualatex
\documentclass{article}
%% Lua-side code
\usepackage{luacode}
\begin{luacode}

function string_to_table (str)
   sep = ","
   fields = {}
   str:gsub( "([^"..sep.."]*)" .. sep , 
               function(c) 
                 c = string.gsub ( c, "^%s*(.-)%s*$", "%1" )
                 table.insert (fields, c)   
               end)
   return fields
end
function dojob ( s )
   -- append "," (if needed) to end of string
   if (string.sub(s,-1) == ",") then else s = s.."," end 
   -- convert string to table
   t = string_to_table ( s , "," )
   -- sort the table entries
   table.sort ( t , function(a,b) return a<b end)
   -- convert table to string, with items separated by commas
   s = table.concat ( t, ", " )
   -- remove any leading whitespace and comma characters, output the string
   tex.sprint ( (string.gsub( s, "^[,%s]*(.*)$", "%1" ) ) )
end

\end{luacode}

%% TeX-side code
\newcommand\sort[1]{\directlua{dojob(\luastring{#1})}}

\begin{document}
\sort{c, b, a}

\sort{Carla , Eric , Anna Michelle, Brenda  , Daniel}

\sort{Eric,Carla,Brenda,Anna Michelle,Daniel,,}

\sort{aaa,15}
\end{document}