CODEWITHSUNDEEP
luaiterable-unpackingargument-unpacking
Mateo Vial
Mateo Vial

Reputation: 710

How to "pack" a variable amount of outputs from a function in Lua?

I have a function in Lua that gives a variable number of outputs. How can I get all its outputs, regardless of how many outputs it has?

function my_function()
    -- whatever
end

outputs_list = my_function()

The above example doesn't work because outputs_list gets only the first output of the function, and all the others are discarded. I want outputs_list to contain all the outputs of the function so I can retrieve them later.

I think this is equivalent in python to doing *outputs_list = my_function()

Upvotes: 4

Views: 537

Answers (1)

Oka
Oka

Reputation: 26375

In general, you can capture the values into a table

local t = { foo() }

but it may not be a sequence if the function returns nil values that create holes in the list.

select may be used to choose where to begin capturing return values.

-- capture the second return value and onwards
local t = { select(2, foo()) }

In Lua 5.2+, you also have access to table.pack(...), which

Returns a new table with all arguments stored into keys 1, 2, etc. and with a field "n" with the total number of arguments. Note that the resulting table may not be a sequence, if some arguments are nil.

The caveat about the resulting table not being a sequence means that looping over all the returned values may require a numeric for, instead of a generic one using ipairs.

local function foo()
    return 'a', 'b', 'c', 'd', 'e'
end

local results = table.pack(foo())

for i = 1, results.n do
    print(results[i])
end

a
b
c
d
e

The opposite function is table.unpack.

In Lua 5.1, you can polyfill table.pack as

table.pack = function (...)
    local t = { ... }
    t.n = select('#', ...)
    return t
end

Upvotes: 5

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