Unpack several lua fields from a table by name

Question

Can I unpack several named fields from a table in lua by name? I know that I can use table.unpack to unpack the numbered fields from a table into separate variables and I also can extract just one named field from the table.

local a, b = table.unpack({1,2,3})
print(a, b) -- will print "1    2"
local t = {some=1, stuff=2}
local field = t.some
print(field) -- will print "1"

But I want to know if there is an equivalent to the following snipped in php

$x = ["a"=>1, "b"=>2, "c"=>3];
list("a"=>$a, "c"=>$c) = $x;
echo "$a $c";  // will print "1 3"

My use case is a require which returns a table with many named fields and I am only interested in some. So currently I am doing

local a = require("file/where/I/just/need/one/field").the_field
local tmp = require("file/that/returns/table/with/many/fields")
local b, c = tmp.x, tmp.y

But I was wondering if I could do that second one on one line.

Alexander Mashin Alexander Mashin · Accepted Answer · 2020-10-12T09:01:15

If you are going to do that thing often, you could define a function:

local function destruct (tbl, ...)
    local insert = table.insert
    local values = {}
    for _, name in ipairs {...} do
        insert (values, tbl[name])
    end
    return unpack(values)
end

-- Test:
local a, b = destruct ({a = 'A', b = 'B', c = 'C'}, 'a', 'b')
print ('a = ' .. tostring (a) .. ', b = ' .. tostring (b))

So, in your example, it will be: local b, c = destruct (require 'file/that/returns/table/with/many/fields', 'x', 'y').

But you shouldn't.

Unpack several lua fields from a table by name

2 Answers