map varargs function lua - lua

Question

I want to be able to map functions that take multiple arguments like
function(a, b) return a+b end
onto a table so that I can write stuff like
answer = varmap(function(a, b) return a+b end, {1, 7, 3}, {5, 4, 8}
but I am not comfortable with lua varargs and the code samples on wikibooks use table.getn, and when you replace them with # it doesn't work and returns "attempt to preform arithmatic on local 'a' (a nil value)"

Answer 1

One more possibility:
local unpack = table.unpack or unpack
--------------------------------------------------------------------------------
-- 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 ans
end
end
--------------------------------------------------------------------------------
function map(f,...)
assert(type(f) == 'function','Function expected for 1st arg')
local t = {...}
return coroutine.wrap(
function()
for t in zip(unpack(t)) do
coroutine.yield(f(unpack(t)))
end
end)
end
--------------------------------------------------------------------------------
-- Example use
for item in map(function(a, b) return a+b end, {1, 7, 3}, {5, 4, 8}) do
print(item)
end
print()
for item in map(function(a) return a*2 end, {1, 7, 3}) do
print(item)
end

Answer 2

local function imap(func, ...) -- imap(func, src_table_1, src_table_2, ...)
local result = {}
local src_tables_arr = {...}
if #src_tables_arr == 1 then
for k, v in ipairs(src_tables_arr[1]) do
result[k] = func(v)
end
else
for k = 1, #src_tables_arr[1] do
result[k] = func(
(table.unpack or unpack)
(
imap(
function(src_t) return src_t[k] end,
src_tables_arr
)
)
)
end
end
return result
end
table.imap = imap
Usage:
local arr = table.imap(function (a, b) return a+b end, {1, 7, 3}, {5, 4, 8})

Answer 3

Perhaps you are looking for something like this:
function varmapn(func, ...)
local args, result = { ... }, {}
for arg_i = 1, #(args[1]) do
local call_args = {}
for arg_list = 1, #args do
table.insert(call_args, args[arg_list][arg_i])
end
table.insert(result, func(table.unpack(call_args)))
end
return result
end
Sample interaction:
> answer = varmapn(function (a, b) return a+b end, {1, 7, 3}, {5, 4, 8})
> print(answer)
table: 0x970eb0
> for i = 1, 3 do print(answer[i]) end
6
11
11
Or, here is a slightly more involved function that is more general. For argument lists it takes arrays, or it takes tables with arbitrary keys:
function mapn(func, ...)
local args, call_args = { ... }, {}
local result = {}
for k in pairs(args[1]) do
call_args[k] = {}
end
for arg_list, v in pairs(args) do
for k in pairs(args[1]) do
table.insert(call_args[k], v[k])
end
end
for k, v in pairs(call_args) do
result[k] = func(table.unpack(v))
end
return result
end
Sample interaction:
> answer = mapn(function (a, b) return a+b end, {x=1, y=7, z=3}, {x=5, y=4, z=8})
> for k,v in pairs(answer) do print(k .. " = " .. v) end
z = 11
y = 11
x = 6
> answer = mapn(function (a, b) return a+b end, {1, 7, 3}, {5, 4, 8})
> for i = 1, 3 do print(answer[i]) end
6
11
11