any idea on lookup table item both by index and item

alexzheng · May 2012

suppose you have a table
t = {"a", "b", "c", "d", "e"}
and you can pick a random value and remove it from table t by
local object = table.remove(t, math.random(#t))
but when you have a object,for example, "b", and want to get its index in table t and remove it from t,is there any effect way
to find the value "b" in table t(not iterate over the elements)?

Just like the these functions in Sprite
Sprite:removeChild(child) ---look up by item
Sprite:removeChildAt(index) ----lookup by index
Sprite:getChildAt(index)
Sprite:getChildIndex(child)

OZApps · May 2012

I guess that there will be some iteration, if it is supported behind the scenes in C, then you do not get to see it, otherwise you can do the same in Lua. In short, you will need to iterate through to find the value by the item.
Another point is that "b" is a string, where as the other values might not be a string or a number, but a table (object/userdata, etc).

Scouser · May 2012

@alexzheng:
I think you might be out of luck, reading the LUA wiki on tables gives this little snippet at the bottom of this page:
Note that table.remove only works with numeric indexes. For dictionaries you can just unset tables entries with tablevariable["index"] = nil

Doh!!!
Ninja'd by @OZApps

alexzheng · May 2012

yes, iterate through the table is the straight way to find the item,but it's not very effect for large table, so I'm Curious about if there are some magic code in gideros.

alexzheng · May 2012

@alexzheng:
I think you might be out of luck, reading the LUA wiki on tables gives this little snippet at the bottom of this page:
Note that table.remove only works with numeric indexes. For dictionaries you can just unset tables entries with tablevariable["index"] = nil

Doh!!!
Ninja'd by @OZApps

By use dictionaries,how to randomly pick an item from the table?

Scouser · May 2012

I may be wrong but I think their term dictionary is used to describe a table of Key : Value pairs.

ar2rsawseen · May 2012

Well in every aspect there is a balance between memory usage and performance.

For example, in this case, you can set up an index table, so if you have table t = {"a", "b", "c", "d", "e"}
You can create

tIndex = {}
tIndex["a"] = 1
tIndex["b"] = 2
tIndex["c"] = 3
tIndex["d"] = 4
tIndex["e"] = 5

Then you can easily remove random values:

local randomObject = math.random(#t)
table.remove(t, randomObject)
tIndex[randomObject] = nil

And you can easily check if object exists and retrieve it's value:

function checkObject(obj)
    if(tIndex[obj] ~= nil) then
         return t[tIndex[obj]]
    end
end

But as you've guessed, you'll probably use twice as much memory

alexzheng · May 2012

I have also considered use two tables exactly as you depicted, but after you perfrom table.remove, the remaining elements are reindexed, so the index stored in another table will become out of date.

Scouser · May 2012

@alexzheng: Surely then you could use something like

local randomObject = math.random(#t)
table.remove(t, randomObject)
table.remove(tIndex, randomObject)

which should keep the indices ticking over quite nicely..

alexzheng · May 2012

@alexzheng: Surely then you could use something like
local randomObject = math.random(#t)
table.remove(t, randomObject)
table.remove(tIndex, randomObject)
which should keep the indices ticking over quite nicely..

As you said above:
Note that table.remove only works with numeric indexes.

OZApps · May 2012

@alex,
I am not sure what you are trying to do, maybe you do not have to have two tables, you can access the entire table as an array, so all of table.xxx functions work. The data that you store can be structured a little better maybe with fields, or something that will help you ID the data. Dunno if you have heard of bitmap indexes, if that is going to be helpful (if you do go with non numeric indexes)

alexzheng · May 2012

Thanks all.
I just want to give up, I have to accept to get the value by iterating over the elements in a numeric indexed table. It performs just ok for a 9X9 table in my new game.

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