Parallel iteration in lua

Question

I would like to loop through multiple tables in parallel in Lua. I could just do:

for i in range(#table1)
  pprint(table1[i])
  pprint(table2[i])
end

But I would prefer something like python zip:

for elem1, elem2 in zip(table1, table2):
  pprint(elem1)
  pprint(elem2)
end

Is there such a thing in standard Lua (or at least that comes with a torch?).

Answer 1

- Lua, - Python, Penlight. seq.zip. , Penlight Torch, LuaRocks ( Torch).

seq.zip Penlight . , Python zip, .. ( ) :

local zip
do
  local unpack = table.unpack or unpack
  local function zip_select( i, var1, ... )
    if var1 then
      return var1, select( i, var1, ... )
    end
  end

  function zip( ... )
    local iterators = { n=select( '#', ... ), ... }
    for i = 1, iterators.n do
      assert( type( iterators[i] ) == "table",
              "you have to wrap the iterators in a table" )
      if type( iterators[i][1] ) ~= "number" then
        table.insert( iterators[i], 1, -1 )
      end
    end
    return function()
      local results = {}
      for i = 1, iterators.n do
        local it = iterators[i]
        it[4], results[i] = zip_select( it[1], it[2]( it[3], it[4] ) )
        if it[4] == nil then return nil end
      end
      return unpack( results, 1, iterators.n )
    end
  end
end

-- example code (assumes that this file is called "zip.lua"):
local t1 = { 2, 4, 6, 8, 10, 12, 14 }
local t2 = { "a", "b", "c", "d", "e", "f" }
for a, b, c in zip( {ipairs( t1 )}, {ipairs( t2 )}, {io.lines"zip.lua"} ) do
  print( a, b, c )
end

Answer 2

--------------------------------------------------------------------------------
-- Python-like zip() iterator
--------------------------------------------------------------------------------

function zip(...)
  local arrays, ans = {...}, {}
  local index = 0
  return
    function()
      index = index + 1
      for i,t in ipairs(arrays) do
        if type(t) == 'function' then ans[i] = t() else ans[i] = t[index] end
        if ans[i] == nil then return end
      end
      return unpack(ans)
    end
end

--------------------------------------------------------------------------------
-- Example use:
--------------------------------------------------------------------------------

a = {'a','b','c','d'}
b = {3,2,1}
c = {7,8,9,10,11}

for a,b,c,line in zip(a,b,c,io.lines(arg[0])) do
  print(a,b,c,line)
end

print '\n--- Done! ---'