Can I only extract the same value from two tables? [closed] - arrays

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What is the best way, in terms of code cleanness or efficiency, to find an "intersection" of two Lua numbered tables, i.e. a table that contains only values that are present in both tables?
For example, I have
a={1,2,3,4,5}
b={3,4,5,6,7}
Since 3, 4 and 5 are found in both a and b, I want {3,4,5}.

Answer 1

I suggest a slightly different approach, based on sets. The performance will not necessarily improve, bit the code will be clean, understandable and expandable.
In Lua a set can be easily implemented as a table, where the keys are set items and the values are true, therefore, to check whether an item is present in the set, you just need if set[item] then.
I will give three variants of the code. The first cleanly implements an intersect operator but then has to transform the resulting set back to an indexed Lua table. The second immediately returns the intersections of two sets as an array. The third is a compromise between the first two.
Note that neither contains nested loops over a and b: it's O(a + b) rather than O(ab).
First:
local insert, sort = table.insert, table.sort -- localise for performance.
local a = {1,2,3,4,5}
local b = {3,4,5,6,7}
local function toset (tbl)
local set = {}
for _, value in ipairs (tbl) do
set [value] = true
end
return set
end
local function intersect (set1, set2)
local intersection = {}
for item, _ in pairs (set1) do
if set2 [item] then
intersection [item] = true
end
end
return intersection
end
local function toarray (set)
local array = {}
for item, _ in pairs (set) do
insert (array, item)
end
sort (array) -- since you want {3,4,5} not {4,5,3}
return array
end
-- Fortunately, table.concat does not require previous tostring:
print ('{' .. table.concat (toarray (intersect (toset (a), toset (b))), ', ') .. '}')
Second:
local insert, sort = table.insert, table.sort -- localise for performance.
local a = {1,2,3,4,5}
local b = {3,4,5,6,7}
local function toset (tbl)
local set = {}
for _, value in ipairs (tbl) do
set [value] = true
end
return set
end
local function intersect_and_to_array (set1, set2)
local array = {}
for item, _ in pairs (set1) do
if set2 [item] then
insert (array, item)
end
end
sort (array) -- since you want {3,4,5} not {4,5,3}
return array
end
-- Fortunately, table.concat does not require previous tostring:
print ('{' .. table.concat (intersect_and_to_array (toset (a), toset (b)), ', ') .. '}')
Third:
local insert = table.insert -- localise for performance.
local a = {1,2,3,4,5}
local b = {3,4,5,6,7}
local function toset (tbl)
local set = {}
for _, value in ipairs (tbl) do
set [value] = true
end
return set
end
local function filter_array_through_set (array, set)
local filtered = {}
for _, value in ipairs (array) do
if set [value] then
insert (filtered, value)
end
end
return filtered -- the order of array is preserved.
end
-- Fortunately, table.concat does not require previous tostring:
print ('{' .. table.concat (filter_array_through_set (a, toset (b)), ', ') .. '}')

Answer 2

You need to iterate on one of the table and output the value if you find it in the other table. On, my computer the following code returns {3, 4, 5}, it basically implement the intersection of 2 sets.
a={1,2,3,4,5}
b={3,4,5,6,7}
function Contains (Array, Item)
local Found = false
local Index = 1
while ((not Found) and (Index <= #Array)) do
if Array[Index] == Item then
Found = true
else
Index = Index + 1
end
end
return Found
end
function Intersects (Ta, Tb)
local Result = { }
local Index
for Index = 1, #Tb do
if Contains(Ta, Tb[Index]) then
Result[#Result + 1] = Tb[Index]
end
end
return Result
end
Result = Intersects(a, b)
-- Print the results
local Index, Value
for Index, Value in ipairs(Result) do
print(Value)
end