How to search Lua table values - search

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.

Answer 2

There are no built-in functions for searching tables. There are many ways to go about it which vary in complexity and efficiency.
local t = {
r={displayname="Red", name="Ruby", age=15, x=4, y=10},
y={displayname="Blue", name="Trey", age=22, x=3, y=2},
t={displayname="Red", name="Jack", age=20, x=2, y=3},
h={displayname="Red", name="Tim", age=25, x=2, y=33},
v={displayname="Blue", name="Bonny", age=10, x=2, y=0}
}
In Programming in Lua they recommend building a reverse table for efficient look ups.
revDisplayName = {}
for k,v in pairs(t) do
if revDisplayName[v.displayname] then
table.insert(revDisplayName[v.displayname], k)
else
revDisplayName[v] = {k}
end
end
Then you can match display names easily
for _, rowname in pairs(revDisplayName["Red"]) do
print(t[rowname].x, t[rowname].y)
end
There is code for creating SQL-like queries in Lua, on Lua tables, in Beginning Lua Programming if you want to build complex queries.
If you just want to search through a few records for matches, you can abstract the searching using an iterator in Lua
function allmatching(tbl, kvs)
return function(t, key)
repeat
key, row = next(t, key)
if key == nil then
return
end
for k, v in pairs(kvs) do
if row[k] ~= v then
row = nil
break
end
end
until row ~= nil
return key, row
end, tbl, nil
end
which you can use like so:
for k, row in allmatching(t, {displayname="Red", x=2}) do
print(k, row.name, row.x, row.y)
end
which prints
h Tim 2 33
t Jack 2 3