Lua

In one of my projects, I had to choose which language I would use to write some small module - C, C++ or Lua. I didn’t want to use C, because the module required a lot of string handling. Another option was C++, but some other parts of the system were written in Lua, so I thought it would be much easier to integrate everything without switching languages, and I had heard some good things about Lua, so in the end, I decided to give it a shot. And this was a right decision.

Lua is a minimal language. In fact, there’s only one primary data structure - an associative array. It’s something similar to Python’s dictionary, but you can build nearly everything based on it, e.g., check the https://en.wikipedia.org/wiki/Lua_(programming_language) to see how to implement namespaces or objects using Lua’s arrays.

I found only one confusing thing in that language. When you want to iterate over an array using for loop, you need to use an iterator. So far so good. ipairs works only if the keys are numeric starting from 1, and are continuous (there’re no spaces between them). See the example below:

arr = {}
for i=1,5 do 
   arr[i] = i*i
end
for _, val in ipairs(arr) do print(val) end
-- 1
-- 4
-- 9
-- 16
-- 25

print("Arr size", #arr)
-- Arr size 5

arr[7] = 49
arr['error'] = 1000
print("Arr size", #arr)
-- Arr size 5  <--- note that the array size is the same, despite we just added two elements 

print(arr['error']) -- 'error' key exists
-- 1000

for _, val in ipairs(arr) do print(val) end
-- 1
-- 4
-- 9
-- 16
-- 25   <-- the last element is still 25, arr[7], and arr['error'] are omited

arr[6] = 36 -- fill the space between 5 and 7
print("Arr size", #arr)
-- Arr size 7   <-- the size is now 7 because the numeric indexes are now continuous

for _, val in ipairs(arr) do print(val) end
-- 1
-- 4
-- 9
-- 16
-- 25
-- 36
-- 49   <-- arr[7] is present!

-- print with keys
for i,val in ipairs(arr) do print(i, val) end
-- 1    1
-- 2    4
-- 3    9
-- 4    16
-- 5    25
-- 6    36
-- 7    49

If you need to iterate over all elements in the table you need to use pairs iterator. However, the order of elements is not guaranteed, which makes sense because the array is not a vector but something more like a map. See the example:

-- create an array with string keys
arrString = {}
for i=1,5 do 
    arrString["tmp" .. i] = i*i
end

for i,val in ipairs(arrString) do print(i, val) end -- nothing happens

for i,val in pairs(arrString) do print(i, val) end
-- tmp1 1
-- tmp3 9
-- tmp4 16  <---
-- tmp2 4   <---
-- tmp5 25
-- note that the result is not in the order