Generating random strings

Question

I've created the following string manipulation function for randomizing my passed string in Lua:

require "string"
require "math"
math.randomseed( os.time() )
function string.random( self )
 local tTemporary, tNew = {}, {}
 if not self or self:len() < 5 then
 return nil
 end
 self:gsub( "%a", function( cChar )
 table.insert( tTemporary, cChar )
 end
 )
 for i = 1, #tTemporary, 1 do
 local iRandom = math.random(1, #tTemporary)
 tNew[i] = tTemporary[iRandom]
 table.remove( tTemporary, iRandom )
 end
 return table.concat(tNew, " ")
end

Can this be optimized/more-randomised?

Answer 1

A couple of points to consider:

• Calling your function shuffle instead of random would be a better name since that's really what it's doing.
• The extra intermediate tables aren't really necessary to perform the shuffle. A single table is enough and you can perform the shuffle in-place right on that table.
• Consider a better separation between the process of shuffling and how the input is stored. For example, if you have your shuffle function take a table instead then that function can be reused on any kind of data and not just strings.
• I would do away with the Hungarian notation and just drop the type prefix on the variable names. Remember Lua variables themselves doesn't have a type. If you later change what that variable refers to then the Hungarian notation becomes misleading.

So I would refactor your original code into two functions. Here's one possibility:

function shuffle(self)
 if type(self) ~= 'table' then return nil end
 for i = #self, 2, -1 do
 local randi = math.random(i)
 self[i], self[randi] = self[randi], self[i]
 end
 return self
end
function shuffle_words(str)
 local strtable = {}
 for each in str:gmatch("%a+") do
 table.insert(strtable, each)
 end
 return table.concat(shuffle(strtable), ' ')
end

• hjpotter92: "%a+" should be "%a"
• greatwolf: This is just an example of course, it assumes the input string is a bunch of words delimited by whitespace. If you have a different set of constraints then yes you should change the regex pattern to fit your needs.