copying unordered keys from one table to ordered values in another

Question

I have a table mapping strings to numbers like this:

t['a']=10
t['b']=2
t['c']=4
t['d']=11

From this I want to create an array-like table whose values are the keys from the first table, ordered by their (descending) values in the first table, like this:

T[1] = 'd' -- 11
T[2] = 'a' -- 10
T[3] = 'c' -- 4
T[4] = 'b' -- 2

How can this be done in Lua?

Answer 1

-- Your table
local t = { }
t["a"] = 10
t["b"] = 2
t["c"] = 4
t["d"] = 11

local T = { } -- Result goes here

-- Store both key and value as pairs
for k, v in pairs(t) do
  T[#T + 1] = { k = k, v = v }
end

-- Sort by value
table.sort(T, function(lhs, rhs) return lhs.v > rhs.v end)

-- Leave only keys, drop values
for i = 1, #T do
  T[i] = T[i].k
end

-- Print the result
for i = 1, #T do
  print("T["..i.."] = " .. ("%q"):format(T[i]))
end

It prints

T[1] = "d"
T[2] = "a"
T[3] = "c"
T[4] = "b"

Answer 2

Alexander Gladysh's answer can be simplified slightly:

-- Your table
local t = { }
t["a"] = 10
t["b"] = 2
t["c"] = 4
t["d"] = 11

local T = { } -- Result goes here

-- Store keys (in arbitrary order) in the output table
for k, _ in pairs(t) do
  T[#T + 1] = k
end

-- Sort by value
table.sort(T, function(lhs, rhs) return t[lhs] > t[rhs] end)

-- Print the result
for i = 1, #T do
  print("T["..i.."] = " .. ("%q"):format(T[i]))
end

Answer 3

Tables in Lua do not have an order associated with them.

When using a table as an array with sequential integer keys from 1 to N, the table can be iterated in order using a loop or ipairs().

When using keys that are not sequential integers from 1 to N, the order can not be controlled. To get around this limitation a second table can be used as an array to store the order of the keys in the first table.