[LUA] Calling the game database

[ThorHammerz]

Hey all, probably a really basic question, but how would I go about calling the game database from a base LUA script (such as AssignStartingPlots.lua)?

For example:

Since Firaxis hard-coded the values to "take into account the various terrain yields in determining fertility....." crap in order to "determine how much value to assign each starting region", I would like to access the table "Terrain_Yields" (as well as others) to retrieve the yields for say, plains, grass, tundra, etc. to feed into the existing algorithm.

What would be the general syntax for doing so (calling a specific element from any game database table - i.e. not using a loop to iterate through everything)?

 

[LeeS]

I think what you are look for is probably here, but I've not any actual experience in using the "DB.Query" methods.

 

[whoward69]

No need to use DB, you can use GameInfo (and several other examples of this in the core Lua files)

local sTerrain = "TERRAIN_PLAINS"
for yields in GameInfo.Terrain_Yields{TerrainType = sTerrain} do
  print(yields.TerrainType, yields.YieldType, yields.Yield)
end

will print

TERRAIN_PLAINS YIELD_FOOD 1
TERRAIN_PLAINS YIELD_PRODUCTION 1

 

[ThorHammerz]

For the line :

for yields in GameInfo.Terrain_Yields{TerrainType = sTerrain} do

Is that roughly translated to "if the element in GameInfo.[table name] has column TerrainType equal to sTerrain"?

Also, am I correct in assuming that to extract anything out of a game database table, we must use a loop to iterate (and use a check to filter results), as opposed to say, calling out a specific row?

 

[whoward69]

for yields in GameInfo.Terrain_Yields{TerrainType = sTerrain} do

Is that roughly translated to "for each element in GameInfo.[table name] has column TerrainType equal to sTerrain"?

Yes (but "for each" rather than "if" - as there may be 0, 1 or more entries)

Also, am I correct in assuming that to extract anything out of a game database table, we must use a loop to iterate (and use a check to filter results), as opposed to say, calling out a specific row?

No.

The result of GameInfo.table_name{something=value} is an iterator, which is why it can be used where the for statement expects an iterator. Each time an iterator is called it returns the next value, so we can write

local row = GameInfo.Terrain_Yields{TerrainType = sTerrain}()

Note the use of () after the closing curly bracket, which causes the iterator function to be called once and return the first row from the Terrain_Yields table.

row will either be nil or a Lua table holding a single row from the database table

If, and only if, we know that there will always be an entry in the database table, we could write

local iPlainsProd = GameInfo.Terrain_Yields{TerrainType="TERRAIN_PLAINS", YieldType="YIELD_PRODUCTION"}().Yield

which would directly retrieve the value

BUT, beware, if we wrote

local iTundraProd = GameInfo.Terrain_Yields{TerrainType="TERRAIN_TUNDRA", YieldType="YIELD_PRODUCTION"}().Yield

(unless a mod had added production on tundra) we would get a nil pointer error, so the safe code would be

local row = GameInfo.Terrain_Yields{TerrainType="TERRAIN_TUNDRA", YieldType="YIELD_PRODUCTION"}()
local iTundraProd = row and row.Yield or 0

(That last bit is a more condensed way or writing "local iTundraProd = 0; if (row) then iTundraProd = row.Yield end", it's (almost) equivalent to the C++/Java "condition ? true value : false value;" construct.)

[[For more on Lua iterators see here]]

W