copying unordered keys from one table to ordered values in another
I have a table mapping strings to numbers like 开发者_运维百科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?
-- 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"
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
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.
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