How to search Lua table values

Question

I have a project that calls for a relational database like structure in an environment where an actual database isn't possible. The language is restricted to Lua, which is far from being my strongest language. I've got a table of tables with a structure like this:

table={
  m:r={
    x=1
    y=1
    displayName="Red"
  }
  m:y={
    x=1
    y=2
    displayName="Yellow"
  }
}

Building, storing and retrieving the table is straightforward enough. Where I'm running into issues is searching it. For the sake of clarity, if I could use SQL I'd do this:

SELECT * FROM table WHERE displayName="Red"

Is there a Lua function that will let me search this way?

Answer 1

The straightforward way is to iterate through all elements and find one that matches your criteria:

local t={
  r={
    x=1,
    y=1,
    displayName="Red",
  },
  y={
    x=1,
    y=2,
    displayName="Yellow",
  },
}
for key, value in pairs(t) do
  if value.displayName == 'Red' then
    print(key)
  end
end

This should print 'r'.

This may be quite slow on large tables. To speed up this process, you may keep track of the references in a hash that will provide much faster access. Something like this may work:

local cache = {}

local function findValue(key)
  if cache[key] == nil then
    local value
    -- do a linear search iterating through table elements searching for 'key'
    -- store the result if found
    cache[key] = value
  end
  return cache[key]
end

If the elements in the table change their values, you'll need to invalidate the cache when the values are updated or removed.