CODEWITHSUNDEEP
lualua-table
Yuri Astrakhan
Yuri Astrakhan

Reputation: 10035

How do I get first table value in Lua

Is there an easier way to do this? I need to get the very first value in a table, whose indexes are integers but might not start at [1]. Thx!

local tbl = {[0]='a',[1]='b',[2]='c'}  -- arbitrary keys
local result = nil
for k,v in pairs(tbl) do -- might need to use ipairs() instead?
    result = v
    break
end

Upvotes: 13

Views: 43794

Answers (4)

Andrew Eades
Andrew Eades

Reputation: 19

ipairs() will provide an iterator that starts at 1 and iterates in steps of 1 until there is no key there so it will miss out the 0th element in the example code.

pairs() will iterate over all the keys but in an undefined order so will not give what the question asked for either except by accident.

To get the first element with arbitrary integer keys, you would have to sort the keys.

local function first(t)
    local keys = {}
    for k,_ in pairs(t) do
        table.insert(keys, k)
    end
    
    table.sort(keys)
    
    return t[keys[1]]
end

Having said that, if you are needing to do this a lot you might want to rethink the data structure a bit.

Upvotes: 1

Martin Kunc
Martin Kunc

Reputation: 471

It is possible to call the first iterator without the current state, which returns initial value, but the order is still not guarantueed.

a = {[1]="I", [2]="II", [3]="III"}
-- create iterator
iter = pairs(a)

print("Calling iterator first time ")
currentKey, currentValue = iter(a)
print(currentKey, currentValue)

print("Calling iterator second time")
currentKey, currentValue = iter(a, currentKey)
print(currentKey, currentValue)

print("Calling iterator third time")
currentKey, currentValue = iter(a, currentKey)
print(currentKey, currentValue)

print("Calling iterator fourth time")
currentKey, currentValue = iter(a, currentKey)
print(currentKey, currentValue)

Upvotes: 0

Colonel Thirty Two
Colonel Thirty Two

Reputation: 26609

If the table may start at either zero or one, but nothing else:

if tbl[0] ~= nil then
    return tbl[0]
else
    return tbl[1]
end

-- or if the table will never store false
return tbl[0] or tbl[1]

Otherwise, you have no choice but to iterate through the whole table with pairs, as the keys may no longer be stored in an array but rather in an unordered hash set:

local minKey = math.huge
for k in pairs(tbl) do
    minKey = math.min(k, minKey)
end

Upvotes: 11

MBlanc
MBlanc

Reputation: 1783

pairs() returns the next() function to iterate the table. The Lua 5.2 manual says this about next:

The order in which the indices are enumerated is not specified, even for numeric indices. (To traverse a table in numeric order, use a numerical for.)

You'll have to iterate the table until you find the key. Something like:

local i = 0
while tbl[i] == nil do i = i + 1 end

This code snippet makes the assumption that the table has at least 1 integer index.

Upvotes: 0

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