In lua, array sub element

Question

In Lua, I want to select the parts of the array. The below example selects from the second elements

a = { 1, 2, 3}
print(a)

b = {}
for i = 2, table.getn(a) do
  table.insert(b, a[i])
end
print(b)

In python a[1:] works. Does Lua have the similar syntax?

Answer 1

Lua does not have similar syntax. However, you can define your own function to easily wrap this logic up.

local function slice (tbl, s, e)
    local pos, new = 1, {}
    
    for i = s, e do
        new[pos] = tbl[i]
        pos = pos + 1
    end
    
    return new
end

local foo = { 1, 2, 3, 4, 5 }
local bar = slice(foo, 2, 4)

for index, value in ipairs(bar) do
    print (index, value)
end

Note that this is a shallow copy of the elements in foo to bar.

---

Alternatively, in Lua 5.2 you could use table.pack and table.unpack.

local foo = { 1, 2, 3, 4, 5 }
local bar = table.pack(table.unpack(foo, 2, 4))

Although the manual has this to say:

table.pack (···)

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

---

While Lua 5.3 has table.move:

local foo = { 1, 2, 3, 4, 5 }
local bar = table.move(foo, 2, 4, 1, {})

---

And finally, most would probably opt to define some kind of OOP abstraction over this.

local list = {}
list.__index = list

function list.new (o)
    return setmetatable(o or {}, list)
end

function list:append (v)
    self[#self + 1] = v
end

function list:slice (i, j)
    local ls = list.new()
    
    for i = i or 1, j or #self do
        ls:append(self[i])
    end
    
    return ls
end

local foo = list.new { 1, 2, 3, 4, 5 }
local bar = foo:slice(2, 4)