Table Iteration

[Clamsoda]

Good Morning,

I was curious about how I may go about ensuring a table of mine is iterated over in a certain order (simply, top to bottom.)

I tried to use the ipairs() function for my loop, and it didn't work; and using pairs(), it reads my table in a rather erratic fashion.

My table utilizes the key and value as information, so I can't use the key as an index. The next() function seems to produce the same results as pairs().

Thanks!

[Phanx]

The order in which key/value pairs are returned by pairs is undefined, so, basically random. next works the same way. The only way to directly walk through a table in a specific order is to use ipairs.

There are some workarounds, but they all involve creating an extra table with only the keys from the original table as indexed values, sorting it somehow, and then using ipairs on the extra table and value lookups on the original table, eg:

local t = {
    ["cow"] = "moo",
    ["pig"] = "oink",
    ["other"] = "?",
}

local order = { "cow", "pig", "other" }

for i, k in ipairs(order) do
    local v = t[k]
    -- do something with v
end

The other option would be to switch your table to an indexed table with subtables:

local t = {
    { "cow", "moo" },
    { "pig", "oink" },
    { "other", "?" },
}

Then you can sort it however you like, and use ipairs on it. However, you can't do direct value lookups anymore.

Without knowing what you're actually doing, it's hard to say what the best method will be.

[Clamsoda]

Hey Phanx,

Thanks for the reply.

What I am trying to accomplish is to have two tables of relevant buffs and debuffs, and their respective modification to healing. An example would be:

-- ["Spell ID"] = percent
local generalHealingIncreases = {
    [47788] = 60,   -- Guardian Spirit
    [55233] = 25    -- Vampiric Blood; needs to be adjusted for glyph.
}

The way healing modifications are handled is multiplicative, from highest to lowest value. What I aim to do is sort the table from highest to lowest by hand, and by having my loop iterate over it in descending order, I can ensure the appropriate buff is calculated first.

That being said, I need to be able to modify the values through a look-up of the keys (to reflect glyphs); and the table would then need to be re-sorted to reflect the new values.

I am not sure that I like the idea of two tables. Is this a situation where metatables would be useful? I was poking around some Lua websites and there was a method utilizing metatables to emulate the next() function, but I wasn't too sure it was what I was looking for.

Thanks for your time and help (as usual ).

[Sharparam]

If your code is not performance critical, it may be easier to have a table of tables instead, something like:

local buffs = { -- Unsorted buffs table
    {1234, 25},
    {4321, 60}
}
 
for i, v in ipairs(buffs) do
    print(i, v[1], v[2])
end
 
--[[ Output:
1       1234    25
2       4321    60
]]
 
local function compare(a, b)
    return a[2] > b[2]
end
 
table.sort(buffs, compare)
 
for i, v in ipairs(buffs) do
    print(i, v[1], v[2])
end
 
--[[ Output:
1       4321    60
2       1234    25
]]
 
-- Now to find a specific buff ID and get the healing value, you could use something like:
 
local function getHealing(id)
    for _, v in ipairs(buffs) do
        if v[1] == id then return v[2] end
    end
    return nil -- Functions return nil by default, but whatever
end
 
local healing = getHealing(1234) -- Returns 25
healing = getHealing("this buff does not exist") -- Returns nil

[semlar]

Originally Posted by Clamsoda I am not sure that I like the idea of two tables. Is this a situation where metatables would be useful?

Using a metatable would still be using two tables.

Also why does the order matter if it's multiplicative?

[Clamsoda]

Also why does the order matter if it's multiplicative?

Because the order in which you multiply, when dealing with percents, affects the out-come? Increasing an amount by 60%, then 40% yields a different answer than increasing an amount by 40%, then 60%.

I suppose you are correct about the two table approach, but the metatable could be universal for two seperate tables, yielding only 3 tables, instead of four? (Bare in mind I know nothing about metatables, I just saw it used as a solution to a problem similar to mine).

If your code is not performance critical...

I guess you could say that code is performance critical.

My approach was, and this is pretty theoretical, to sort a table of healing increases (and decreases in a separate table) by hand, and as a for-loop iterates over it (assuming it is in descending order), if a player has a buff in the table, that percentage will be applied to an amount, and the process continues. In the example posted above, if a player had BOTH Guardian Spirit and Vampiric Blood, a potential heal would be increased by 60%, then 25(or 40)%.

My approach could be horrid, but it seemed like the most simple approach, until I ran into the issue with my table being impossible to index, and impossible to iterate over with ipairs().

Thanks for the replies!

Edit:

Here was the script I was using (granted, it doesn't work in that it reads the table in any order, but it still works)

for k, v in pairs(generalHealingIncrease) do
    if UnitAura("player", k) then
        totalDamageAmount = addPercent(totalDamageAmount, v)
    end
end

[Sharparam]

Originally Posted by Clamsoda Because the order in which you multiply, when dealing with percents, affects the out-come? Increasing an amount by 60%, then 40% yields a different answer than increasing an amount by 40%, then 60%.

local value = 100
value = value * 1.6
value = value * 1.4
print(value) -- 224
value = 100
value = value * 1.4
value = value * 1.6
print(value) -- 224
 
-- Since (a * b) * c == a * b * c == a * (b * c) == a * k where k = (b * c)

Originally Posted by Clamsoda I guess you could say that code is performance critical.

Does it run very often (like in an OnUpdate script or CLEU)? If not, I think it would be pretty safe to say a few milliseconds extra won't matter too much. Or possibly you could cache the results somewhere(?) and use that in OnUpdate/CLEU.