CODEWITHSUNDEEP
luaiterationlua-tablelistiterator
mike bayko
mike bayko

Reputation: 385

Is it possible to make an iterator in Lua that can iterate over a dictionary?

Lets say that I have a dictionary in Lua (i.e. a Lua table that contains indexes of type string) like so:

local my_dictionary = {a = 123; b = 321; c = 456; d = 654};

What I am trying to do is create an iterator function that can iterate over a table even if its indexes are of type string; kind of like pairs, however whenever I try to call next() to get the next index,value it will only return the index,value if the index is of type int. An idea I had was maybe to call (index):byte(1, -1) and add up the tuple of ints, and use that as a sort of pretend index, just to keep track of the indexes, but I do not think that would work with next. Here is basically what I have so far:

local function each(list)
    if #list > 0 then
        local function my_itr(lst, ind)
            return next(lst, ind);
        end
        return my_itr, List, 0;
    end
    return function() end, nil, nil;
end

this only works for a table with int indexes (an array table), so I was wondering if anyone could help me out. Thanks. Edit: To make this less vague here is an example piece of code of what I am trying to accomplish:

local mytable = {a = 123; b = 321; 3, 2, 1, c = "bca"};
for i,v in each(mytable) do
    print(i,v);
end

what it should output:

>a 123
>b 321
>1 3
>2 2
>3 1
>c bca

The output would not have to be in exact order.

Upvotes: 1

Views: 1865

Answers (3)

William Wilkins
William Wilkins

Reputation: 37

I took a swing at this problem for kicks, and came up with a solution that I hope can save someone a lot of time one day.

I found using a custom generator function did the trick:

--- A generator function for traversing a table in lexicographical key order.
---
--- Returns an iterator function that generates the `key`, `value`, and `index` of each row in the table. 
--- 
--- @generic K,V
--- @param t table<K,V> Table to iterate over
--- @return fun(): key:K, value:V, index:integer
local function apairs(t)
    local index  = 0

    -- Get Keys
    local keys = {}
    for key, _ in pairs(t) do table.insert(keys, key) end

    -- Sort keys lexicographically (converting to string to handle mixed types)
    table.sort(keys, function(a, b) return tostring(a) < tostring(b) end)

    --- Iterator function.
    --- @generic K,V
    --- @return K,V,integer
    return function()
        index = index + 1
        local key = keys[index]
        return key, t[key], index
    end
end

Example Output

What it might look like in the wild:

-- Setting the test data
local actionResources = {
    WarlockSpellSlot              = { Hidden = "False", UUID = "e9127b70-22b7-42a1-b172-d02f828f260a", ReplenishType = "ShortRest" },
    Interupt_Shield_MindFlayer    = { Hidden = "True" , UUID = "7bd0ceb4-9191-4208-b924-909897948fb6", ReplenishType = "Rest"      },
    MagicStones                   = { Hidden = "True" , UUID = "c12836e6-0f4e-44a2-b84d-da7b1bcc9986", ReplenishType = "Never"     },
    Interrupt_EntropicWard_Charge = { Hidden = "True" , UUID = "89c063f2-dadf-49e4-830b-ceb9f50f3538", ReplenishType = "ShortRest" },
    Interrupt_Portent_7           = { Hidden = "True" , UUID = "504b4d64-5832-45f7-b09a-ba994b0939ce", ReplenishType = "NA"        },
    ReactionActionPoint           = { Hidden = "False", UUID = "45ff0f48-b210-4024-972f-b64a44dc6985", ReplenishType = "NA"        },
    BreathoftheDragonUse          = { Hidden = "False", UUID = "ddadea14-74b6-4da9-b9da-b5e5b00c0547", ReplenishType = "Rest"      },
    Interrupt_Portent_18          = { Hidden = "True" , UUID = "1d766f02-04e4-4086-9b75-cb0621130e06", ReplenishType = "NA"        },
    FungalInfestationCharge       = { Hidden = "False", UUID = "281f289d-3c0b-433a-a7ed-0505be88ec9e", ReplenishType = "Rest"      },
    SorceryPoint                  = { Hidden = "False", UUID = "46886ba5-6505-4875-a747-ac14118e1e08", ReplenishType = "Rest"      },
    BardicInspiration             = { Hidden = "False", UUID = "46bbeb43-9973-40fb-a11f-e386bc425a8e", ReplenishType = "Rest"      },
    Interrupt_Portent_19          = { Hidden = "True" , UUID = "32dc0c71-b26e-482c-b439-4b261823373f", ReplenishType = "NA"        },
    WarPriestActionPoint          = { Hidden = "False", UUID = "30cca4c5-a808-4c96-bd1c-f57bbb92dc1d", ReplenishType = "Rest"      },
    SteadyAimUse                  = { Hidden = "False", UUID = "f187bd93-f245-4d8d-b2c1-7e97c5ca831e", ReplenishType = "Rest"      },
    AncestralBoon                 = { Hidden = "False", UUID = "f3881182-64bc-415f-ad18-b0ac8b09d17b", ReplenishType = "Rest"      },
    ArcaneRecoveryPoint           = { Hidden = "False", UUID = "74737a08-7a77-457b-9740-ae363be2b80f", ReplenishType = "Rest"      },
    InfectiousInspiration         = { Hidden = "False", UUID = "a7795e7c-66bc-4cc5-a58e-2630434ab238", ReplenishType = "Rest"      },
    Interrupt_Portent_2           = { Hidden = "True" , UUID = "12cd6971-2efd-4780-b7a0-0cdba87045b9", ReplenishType = "NA"        },
    TidesOfChaos                  = { Hidden = "False", UUID = "733e3365-082c-4a0a-8df9-98273c23186e", ReplenishType = "ShortRest" },
    VigilantDefenderReaction      = { Hidden = "False", UUID = "7301fe0b-148f-4be9-8ab6-1e3b37a38ae1", ReplenishType = "NA"        },
    ActionPoint                   = { Hidden = "False", UUID = "734cbcfb-8922-4b6d-8330-b2a7e4c14b6a", ReplenishType = "NA"        }
}

-- The obligatory aesthetics step, which aligns the columns so I can sleep at night
local w    = 0; for x in pairs(actionResources) do w = math.max(w, #x) end
local fmt = "\t%-"..w.."s\t%s\t%2d"

-- And for the grand finally:
for key, value, index in apairs(actionResources) do
    print(string.format(fmt, key, value, index))
end

The output then looks like this:

    ActionPoint                     table: 0x55e64cc2a950    1
    AncestralBoon                   table: 0x55e64cc2a530    2
    ArcaneRecoveryPoint             table: 0x55e64cc2a5e0    3
    BardicInspiration               table: 0x55e64cc2a270    4
    BreathoftheDragonUse            table: 0x55e64cc29fb0    5
    FungalInfestationCharge         table: 0x55e64cc2a110    6
    InfectiousInspiration           table: 0x55e64cc2a690    7
    Interrupt_EntropicWard_Charge   table: 0x55e64cc29da0    8
    Interrupt_Portent_18            table: 0x55e64cc2a060    9
    Interrupt_Portent_19            table: 0x55e64cc2a320   10
    Interrupt_Portent_2             table: 0x55e64cc2a740   11
    Interrupt_Portent_7             table: 0x55e64cc29e50   12
    Interupt_Shield_MindFlayer      table: 0x55e64cc2c740   13
    MagicStones                     table: 0x55e64cc2c780   14
    ReactionActionPoint             table: 0x55e64cc29f00   15
    SorceryPoint                    table: 0x55e64cc2a1c0   16
    SteadyAimUse                    table: 0x55e64cc2a480   17
    TidesOfChaos                    table: 0x55e64cc2a7f0   18
    VigilantDefenderReaction        table: 0x55e64cc2a8a0   19
    WarPriestActionPoint            table: 0x55e64cc2a3d0   20
    WarlockSpellSlot                table: 0x55e64cc2c700   21

As a bonus, here's a really short version for any heathens who don't document their code or hate type completion:

--- A generator function that returns an iterator that traverses a table in alphabetical key order.
local function apairs(t)
    local keys, index = {}, 0
    for k in pairs(t) do keys[#keys+1] = k end
    table.sort(keys, function(a, b) return tostring(a) < tostring(b) end)

    return function() index = index + 1; return keys[index], t[keys[index]], index end
end

And here's a one-liner version in-case you hate your coworkers are allergic to readability: (please don't use this)

local function apairs(t) local k,i={},0 for x in pairs(t) do k[#k+1]=x end table.sort(k,function(a,b) return tostring(a)<tostring(b) end) return function() i=i+1 return k[i],t[k[i]],i end end

Upvotes: 0

Paul Kulchenko
Paul Kulchenko

Reputation: 26794

It should work exactly as you want it to with a couple of tweaks: fix typo in List and pass nil instead of 0:

local function each(list)
  local function my_itr(lst, ind)
    return next(lst, ind)
  end
  return my_itr, list, nil
end

local mytable = {a = 123; b = 321; 3, 2, 1, c = "bca"}
for i,v in each(mytable) do
  print(i,v)
end

This prints the following for me, which is what you'd need:

1   3
2   2
3   1
a   123
b   321
c   bca

Upvotes: 3

iehrlich
iehrlich

Reputation: 3592

You can achieve this very behavior by using pairs. Don't confuse it with ipairs though - these are two different table traversal functions!

While ipairs only traverses integer keys of the table (usually it also stops at the first non-existent integer key), pairs traverses all key-value pairs in the table.

So, by writing

local mytable = {a = 123; b = 321; 3, 2, 1, c = "bca"};
for i, v in pairs(mytable) do
    print(i, v);
end

You'll get all key-value pairs printed, in some random order. Here's the demo.

As a sidenote, there's no such thing as 'dictionary' in Lua - all associative arrays are referred to as 'tables'.

Upvotes: 2

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