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Say, I have an array
a = { 1, 2, 10, 15 }
I would like to divide each element by 3 and store the result in a new array. Is there a more efficient / elegant way of doing that than this:
b = { }
for i,x in pairs(a) do
b[i] = x / 3
end
In R, I would simply do b <- a/3. Is there anything like that in lua, or maybe a way of applying a function to each element of a table?
Lua is lightweight, so there is no ready-made functions, but you can create a similar function with metatable.
local mt_vectorization = {
__div = function (dividend, divisor)
local b = {}
for i,x in pairs(dividend) do
b[i] = x / divisor
end
return b
end
}
a = setmetatable({ 1, 2, 10, 15 }, mt_vectorization)
b = a / 3
In addition to the answer by shingo, I found in the meanwhile that writing an R-style mapping function is very easy in lua:
function map(x, f)
local ret = { }
for k, v in pairs(x) do
ret[k] = f(v)
end
return ret
end
This makes some operations easy, for example
a = { 1, 2, 3, 5, 10 }
map(a, function(x) return x * 2 end)
I see similar answers to this question given in different programming languages like Haskell and Python but all of them use build-in functionality that Lua doesn't have, so please don't mark this question as duplicate.
Let's say i have two tables like bellow:
table1 = {A,B,C}
table2 = {D,E,F}
I would like to find all unique ways of matching the items from two tables, the answer should be (in informal notation):
AD,BE,CF
AD,BF,CE
AE,BD,CF
AE,BF,CD
AF,BD,CE
AF,BE,CD
so the answer will store in a table that table[1] would be {{A, D}, {B, E}, {C, F}} and so on.
tables length can be anything but both will be the same size.
we can get all shuffles via induction (not the fastest way, but pretty easy to write/understand)
local function deepcopy(orig)
local copy
if type(orig) == 'table' then
copy = {}
for orig_key, orig_value in next, orig, nil do
copy[deepcopy(orig_key)] = deepcopy(orig_value)
end
setmetatable(copy, deepcopy(getmetatable(orig)))
else
copy = orig
end
return copy
end
local function get_shuffles(N)
if N == 1 then
return {{1}}
end
local shuffles = get_shuffles(N-1)
local result = {}
for index = 1, #shuffles do
local shuffle = shuffles[index]
for position = 1, #shuffle do
local new_shuffle = deepcopy(shuffle)
table.insert(new_shuffle, position, N)
table.insert(result, new_shuffle)
end
local new_shuffle = deepcopy(shuffle)
table.insert(new_shuffle, N)
table.insert(result, new_shuffle)
end
return result
end
table1 = {"A", "B", "C"}
table2 = {"D","E", "F"}
assert(#table1 == #table2)
local result = {}
local shuffles = get_shuffles(#table1)
for index = 1, #shuffles do
local shuffle = shuffles[index]
local part = {}
for i = 1, 3 do
table.insert(part, {})
table.insert(part[i], table1[i])
table.insert(part[i], table2[shuffle[i]])
end
table.insert(result, part)
end
for index = 1, #result do
print(result[index][1][1], result[index][1][2], result[index][2][1], result[index][2][2], result[index][3][1], result[index][3][2])
end
function get_all_combinations(arr1, arr2)
local n, e, all_comb = #arr1, {}, {}
for j = 1, n do
e[j] = arr2[j]
end
local function generate(m)
if m <= 1 then
local comb = {}
all_comb[#all_comb + 1] = comb
for j = 1, n do
comb[j] = arr1[j]..e[j] -- it should be {arr1[j], e[j]} to fulfill your requirements
end
else
for j = 1, m do
generate(m - 1)
local k = j < m and m % 2 == 1 and 1 or j
e[k], e[m] = e[m], e[k]
end
end
end
generate(n)
return all_comb
end
for i, v in ipairs(get_all_combinations({"A", "B", "C"}, {"D", "E", "F"})) do
print(i, table.concat(v, ";"))
end
An alternate way of doing it is with the following code. This was written to help with a game (Typeshift) to discover all possible combinations of variable groups of letters. I've modified it to fit your example, though.
-- table array: { {1, 2}, {3, 4}, {5, 6} }
-- Should return { 135, 136, 145, 146, 235, 236, 245, 246 }
--
-- This uses tail recursion so hopefully lua is smart enough not to blow the stack
function arrayCombine(tableArray)
-- Define the base cases
if (tableArray == nil) then
return nil
elseif (#tableArray == 0) then
return {}
elseif (#tableArray == 1) then
return tableArray[1]
elseif (#tableArray == 2) then
return arrayCombine2(tableArray[1], tableArray[2])
end -- if
-- We have more than 2 tables in the input parameter. We want to pick off the *last*
-- two arrays, merge them, and then recursively call this function again so that we
-- can work our way up to the front.
local lastArray = table.remove(tableArray, #tableArray)
local nextToLastArray = table.remove(tableArray, #tableArray)
local mergedArray = arrayCombine2(nextToLastArray, lastArray)
table.insert(tableArray, mergedArray)
return arrayCombine(tableArray)
end -- arrayCombine
function arrayCombine2(array1, array2)
local mergedArray = {}
for _, elementA in ipairs(array1) do
for _, elementB in ipairs(array2) do
table.insert(mergedArray, elementA .. elementB)
end -- for
end -- for
return mergedArray
end -- arrayCombine2
-- You can set it up this way:
combinedArray = {}
table.insert(combinedArray, {"A", "B", "C"})
table.insert(combinedArray, {"D", "E", "F"})
for i,v in ipairs(arrayCombine(combinedArray)) do
print(i,v)
end
-- Or go this way, which may be somewhat cleaner:
for i,v in ipairs(arrayCombine({{"A", "B", "C"}, {"D", "E", "F"}})) do
print(i,v)
end
Either way, it produces the results you're looking for.
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)"
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
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})
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
I'm trying to find a way to do element-by-element comparison in Lua using the standard < operator. For example, here's what I'd like to do:
a = {5, 7, 10}
b = {6, 4, 15}
c = a < b -- should return {true, false, true}
I already have code working for addition (and subtraction, multiplication, etc). My issue is that Lua forces the result of a comparison to a boolean. I don't want a boolean, I want a table as the result of the comparison.
Here is my code so far, with addition working, but less-than comparison not working:
m = {}
m['__add'] = function (a, b)
-- Add two tables together
-- Works fine
c = {}
for i = 1, #a do
c[i] = a[i] + b[i]
end
return c
end
m['__lt'] = function (a, b)
-- Should do a less-than operator on each element
-- Doesn't work, Lua forces result to boolean
c = {}
for i = 1, #a do
c[i] = a[i] < b[i]
end
return c
end
a = {5, 7, 10}
b = {6, 4, 15}
setmetatable(a, m)
c = a + b -- Expecting {11, 11, 25}
print(c[1], c[2], c[3]) -- Works great!
c = a < b -- Expecting {true, false, true}
print(c[1], c[2], c[3]) -- Error, lua makes c into boolean
The Lua programming manual says that the result of the __lt metamethod call is always converted to a boolean. My question is, how can I work around that? I heard that Lua is good for DSL, and I really need the syntax to work here. I think it should be possible using MetaLua, but I'm not really sure where to start.
A coworker suggested that I just use << instead with the __shl metamethod. I tried it and it works, but I really want to use < for less than, rather than a hack using the wrong symbol.
Thanks!
You only have two choices to make this work with your syntax:
Option 1: Patch the Lua core.
This is probably going to be very difficult, and it'll be a maintenance nightmare in the future. The biggest issue is that Lua assumes on a very low level that the comparison operators <, >, ==, ~= return a bool value.
The byte-code that Lua generates actually does a jump on any comparison. For example, something like c = 4 < 5 gets compiled to byte-code that looks much more like if (4 < 5) then c = true else c = false end.
You can see what the byte-code looks like with luac -l file.lua. If you compare the byte-code of c=4<5 with c=4+5 you'll see what I mean. The addition code is shorter and simpler. Lua assumes you'll do branching with comparisons, not assignment.
Option 2: Parse your code, change it, and run that
This is what I think you should do. It would be very hard, expect most of the work is already done for you (using something like LuaMinify).
First of all, write a function you can use for comparisons of anything. The idea here is to do your special comparison if it's a table, but fall back on using < for everything else.
my_less = function(a, b)
if (type(a) == 'table') then
c = {}
for i = 1, #a do
c[i] = a[i] < b[i]
end
return c
else
return a < b
end
end
Now all we need to do is replace every less than operator a<b with my_less(a,b).
Let's use the parser from LuaMinify. We'll call it with the following code:
local parse = require('ParseLua').ParseLua
local ident = require('FormatIdentity')
local code = "c=a*b<c+d"
local ret, ast = parse(code)
local _, f = ident(ast)
print(f)
All this will do is parse the code into a syntax tree, and then spit it back out again. We'll change FormatIdentity.lua to make it do the substitution. Replace the section near line 138 with the following code:
elseif expr.AstType == 'BinopExpr' then --line 138
if (expr.Op == '<') then
tok_it = tok_it + 1
out:appendStr('my_less(')
formatExpr(expr.Lhs)
out:appendStr(',')
formatExpr(expr.Rhs)
out:appendStr(')')
else
formatExpr(expr.Lhs)
appendStr( expr.Op )
formatExpr(expr.Rhs)
end
That's all there is to it. It will replace something like c=a*b<c+d with my_less(a*b,c+d). Just shove all your code through at runtime.
Comparisons in Lua return a boolean value.
There is nothing you can do about it short of changing the core of Lua.
Can you put up with a bit verbose v()-notation:
v(a < b) instead of a < b ?
local vec_mt = {}
local operations = {
copy = function (a, b) return a end,
lt = function (a, b) return a < b end,
add = function (a, b) return a + b end,
tostring = tostring,
}
local function create_vector_instance(operand1, operation, operand2)
local func, vec = operations[operation], {}
for k, elem1 in ipairs(operand1) do
local elem2 = operand2 and operand2[k]
vec[k] = func(elem1, elem2)
end
return setmetatable(vec, vec_mt)
end
local saved_result
function v(...) -- constructor for class "vector"
local result = ...
local tp = type(result)
if tp == 'boolean' and saved_result then
result, saved_result = saved_result
elseif tp ~= 'table' then
result = create_vector_instance({...}, 'copy')
end
return result
end
function vec_mt.__add(v1, v2)
return create_vector_instance(v1, 'add', v2)
end
function vec_mt.__lt(v1, v2)
saved_result = create_vector_instance(v1, 'lt', v2)
end
function vec_mt.__tostring(vec)
return
'Vector ('
..table.concat(create_vector_instance(vec, 'tostring'), ', ')
..')'
end
Usage:
a = v(5, 7, 10); print(a)
b = v(6, 4, 15); print(b)
c = a + b ; print(c) -- result is v(11, 11, 25)
c = v(a + b); print(c) -- result is v(11, 11, 25)
c = v(a < b); print(c) -- result is v(true, false, true)
As others have already mentioned, there is no straight-forward solution to this. However, with the use of a generic Python-like zip() function, such as the one shown below, you can simplify the problem, like so:
--------------------------------------------------------------------------------
-- 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 table.unpack(ans)
end
end
--------------------------------------------------------------------------------
a = {5, 7, 10}
b = {6, 4, 15}
c = {}
for a,b in zip(a,b) do
c[#c+1] = a < b -- should return {true, false, true}
end
-- display answer
for _,v in ipairs(c) do print(v) end
I am having a table
test = {a= {1,2}, b= {1},c= {2,3}}
I want output like.
test_out = {ab={1,2}, bc = {1,2,3}, ac={1,2,3}}
Apparently what you want are set operations. Here is a way you can do it without any library:
local test = {a = {1, 2}, b = {1}, c = {2, 3}}
local keys = {}
for k,_ in pairs(test) do keys[#keys+1] = k end
table.sort(keys)
local result = {}
local t1, t2, r, found
for i=1,#keys-1 do
for j=i+1,#keys do
t1 = test[keys[i]]
t2 = test[keys[j]]
r, found = {}, {}
for k=1,#t1 do
found[t1[k]] = true
r[k] = t1[k]
end
for k=1,#t2 do
if not found[t2[k]] then
r[#r+1] = t2[k]
end
end
result[keys[i] .. keys[j]] = r
end
end
The result is in the result table. If you can use a set library like pl.Set you can do it with less code:
local test = {a = {1, 2}, b = {1}, c = {2, 3}}
local keys = {}
for k,_ in pairs(test) do keys[#keys+1] = k end
table.sort(keys)
local result = {}
local Set = require "pl.Set"
for i=1,#keys-1 do
for j=i+1,#keys do
result[keys[i] .. keys[j]] = Set.values(
Set(test[keys[i]]) + Set(test[keys[j]])
)
end
end