One loop for iterating through multiple Lua tables - lua

Question

Is it possible to iterate through multiple Lua tables with the same loop?
For looping through indexed tables I can do something like this:
local t1 = {"a", "b", "c"}
local t2 = {"d", "e", "f"}
local num = #t1+#t2
for i=1, num, do
local j
local val
if i <= #t1 then
j = i
val = t1[j]
else
j = i-#t1
val = t2[j]
end
-- Do stuff
end
but how about key-value tables?
E.g. something like this:
local t1 = {a="a", b="b", c="c"}
local t2 = {d="d", e="e", f="f"}
for key, val in pairs(t1) or pairs(t2) do
print(key..": '"..val.."'")
end
should result in this:
a: 'a'
b: 'b'
c: 'c'
d: 'd'
e: 'e'
f: 'f'

Answer 1

function pairs(t, ...)
local i, a, k, v = 1, {...}
return
function()
repeat
k, v = next(t, k)
if k == nil then
i, t = i + 1, a[i]
end
until k ~= nil or not t
return k, v
end
end
local t1 = {a="a", b="b", c="c"}
local t2 = {d="d", e="e", f="f"}
for key, val in pairs(t1, t2) do
print(key..": '"..val.."'")
end
Note: this implementation does not respect __pairs metamethod.

Answer 2

For the given example, I think it is much more concise and clear to just wrap the loop in an outer loop that iterates the tables.
I am assuming the primary reason OP was looking for a solution other than two loops was to avoid having to write the inner logic twice. This is a good solution to that problem and only adds two lines of code:
local t1 = {a="a", b="b", c="c"}
local t2 = {d="d", e="e", f="f"}
for _, tbl in ipairs({ t1, t2 }) do
for key, val in pairs(tbl) do
print(key..": '"..val.."'")
end
end

Answer 3

While it's always nice to have an iterator like Egor's, a more efficient solution would simply be
local t1 = {a="a", b="b", c="c"}
local t2 = {d="d", e="e", f="f"}
for key, val in pairs(t1) do
print(key..": "..val)
end
for key, val in pairs(t2) do
print(key..": '"..val)
end
It's simple, concise, and easily understandable.