How to loop through Lua table using key, value, and index

Question

I want to loop through a table that looks like this

function add(t, k, v, ...)
    if k ~= nil then
        t[k] = v
        t[#t+1] = k
        return add(t, ...)
    end
    return t
end

my_table = add({ }, "a", 5, "b", 4, "c", 3)

for i,k in ipairs(my_table) do
    local v = my_table[k]
    print(k, v)
end

Result:

a - 5

b - 4

c - 3

But I want to be able to loop through the table using the index, the key, and the value, so it looks like this:

1 - a - 5

2 - b - 4

3 - c - 3

Is this possible in Lua?

Egor Skriptunoff · Accepted Answer · 2019-03-24 19:32:16Z

3

Iterator:

function triples(t)   
  local function next_triple(tbl, idx)
    idx = idx + 1
    local k = tbl[idx]
    if k ~= nil then 
      return idx, k, tbl[k]
    end
  end
  return next_triple, t, 0
end

Usage:

local a = {"q", "w", "e", q = 11, w = 22, e = 33}
for i, k, v in triples(a) do
  print(i, k, v)
end

Output:

1   q   11
2   w   22
3   e   33

answered Mar 24, 2019 at 19:32

Egor Skriptunoff

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2 Comments

Piglet Over a year ago

you could also use ipairs and print(i,v,a[v])

Egor Skriptunoff Over a year ago

@Piglet - Yes. I don't know why OP don't like his own code.

glenn jackman · Accepted Answer · 2019-03-25 14:26:49Z

2

An alternate implementation of Egor's triples function using coroutines:

function triples(t)
  return coroutine.wrap(function()
    for i, k in ipairs(t) do
      coroutine.yield(i, k, t[k])
    end
  end)
end

answered Mar 25, 2019 at 14:26

community wiki

glenn jackman

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