lua - how to compare different array

Question

im trying to compare 2 arrays but i dont know how

for example:

local array1 = { 'friend', 'work', 'privat' }
local array2 = { 'apple', 'juice', 'privat' }

if both arrays have the same value it should do a print.

i know i need to work with something like this

for x in ipairs(array1) do
    if x == array2 then
        print ("Hi")
    end
end

but ofcourse it didnt work. so how can i check if the array1 value contains a values from array2?

score 3 · Answer 1 · answered Mar 09 '21 at 18:52

Think of it this way: You have to check each element in the first array to its counterpart in the second. If any element is not equal, you know right away that the arrays aren't equal. If every element checks out as equal, the arrays are equal.

local function arrayEqual(a1, a2)
  -- Check length, or else the loop isn't valid.
  if #a1 ~= #a2 then
    return false
  end

  -- Check each element.
  for i, v in ipairs(a1) do
    if v ~= a2[i] then
      return false
    end
  end
  
  -- We've checked everything.
  return true
end

This is the way! Just in case, if the order of values in the two tables is random, but they should still be equivalent, then both tables should be alphabetically sorted after comparing lengths. Here's a quick function to pass to table.sort: — Optimum, Mar 09 '21 at 21:40

Toast · Answer 2 · 2021-03-10T18:33:10.707

how can i check if the array1 value contains a values from array2?

@luther's answer will not always work for your question..

If the arrays are different sizes, it completely fails.

If you have an array where similar element are not in the exact same index, it can return a false negative.

for example a = {'one', 'two'}; b = {'two', 'one'} will return false

Using table.sort to solve this would be a band-aid solution without fixing the real problem.

The function below will work with arrays of different sizes containing elements in any order

function array_compare(a, b)
  for ia, va in ipairs(a) do 
    for ib, vb in ipairs(b) do 
      if va == vb then 
        print("matching:",va)
      end
    end
  end
end

In array_compare we go through all the combinations of elements in table a and table b, compare them, and print if they are equal.

ipairs(table) uses index, value (instead of just value)

For example

local array1 = { 'friend', 'work', 'privat' }
local array2 = { 'apple', 'juice', 'privat' }
array_compare(array1, array2)

will print

matching: privat

The simplest interpretation of the question is that they want to know if two arrays are exactly equal ("if both arrays have the same value..."), in which case I'm sure my answer is correct. Your interpretation might be valid, but the OP would have to add some clarification before we can ever know. — luther, Mar 10 '21 at 20:15

score 0 · Answer 3 · answered Mar 10 '21 at 20:46

(I'm writing a second answer to account for another possible interpretation of the question.)

If you want to see if array1 contains any value that's also in array2, you can do the following:

Convert array1 to a set. A set is a new table where the array's values become keys whose values are true.
Iterate through array2 to see if any of its values are a key in the set.

local set = {}
for _, v in ipairs(array1) do
  set[v] = true
end

for _, v in ipairs(array2) do
  if set[v] then
    print'Hi'
    -- Use a break statement if you only want to say hi once.
  end
end

If the arrays are large, this algorithm should be faster than a nested loop that compares every value in array1 to every value in array2.

lua - how to compare different array

3 Answers3