CODEWITHSUNDEEP
arrayslua
VeLKerr
VeLKerr

Reputation: 3157

Lua doesn't go over 'nil' in table

I have such an anonymous Lua function:

an_func = function(x) return x == nil end

I need to test it on a table from 4 values: {'None', nil, 1, '1'}. So, I wrote the following code:

for num, element in pairs({'None', nil, 1, '1'}) do
    print(num .. ': ' .. tostring(an_func(element)))
end

When I ran it, I got only three lines:

1: false
3: false
4: false

The line with the true value (corresponding to nil table element) didn't print.

Can you explain, why did Lua print all results, but true? How can I fix the snipped make it output all 4 lines?

P.S. I'm newbee in Lua.

Upvotes: 3

Views: 5697

Answers (4)

Tom Blodget
Tom Blodget

Reputation: 20772

Other answers address your second question well but the simple explanatory facts are Any key with value nil is not considered part of the table. Conversely, any key that is not part of a table has an associated value nil. Assigning a value of nil removes any existing key-value pair with the same key.

Upvotes: 3

warspyking
warspyking

Reputation: 3113

In Lua, everything (a function, string, thread, ...) is a value, with nil (and technically none/void) being the only exception(s). To put simply, nil represents the absence of value. So when iterating over a table, why would Lua say there is nothing there. If it did it would have to iterate over every virtually possible index you can create.

However it appears you are using an array. You technically can iterate over it assuming you know the length. I'll make the assumption that the last item in the table is NOT nil, and that there wasn't any holes assigned after the initial length of the table with holes, however this may not necessarily be true in your case, if it isn't you will have to keep track of the length yourself and use it rather than the # operator:

for i = 1,#t do
    local v = t[i]
    print (v == nil)
end

Upvotes: 4

Rochet2
Rochet2

Reputation: 1156

You would need to loop through all keys you want to print the values for. Possible ways:

  1. Know the length beforehand. (warspyking noted about this)
  2. Using an end marker.
  3. Calculate the length somehow (for example with maxn).
  4. Use something to represent nil values. (EinsteinK showed an example of this)

Each has their good and bad sides, here are some:

For the first you would need to know the length beforehand in all cases which may be difficult. The end marker is convenient, but requires you to shift around the end marker. Calculating length would not work if the last element is nil.

Example of calculating the length - assuming last element is not nil. Note that lua 5.1 has table.maxn that you can use instead of this custom implementation of maxn.

an_func = function(x) return x == nil end

function maxn(t)
    local n = 0
    for k,v in pairs(t) do
        if k%1 == 0 then
            n = k
        end
    end
    return n
end

local t = {'None', nil, 1, '1'}
for num = 1, maxn(t) do
    print(num .. ': ' .. tostring(an_func(t[num])))
end

Example of using an end marker:

an_func = function(x) return x == nil end

local END = {}
local t = {'None', nil, 1, '1', END}

local num = 1
while t[num] ~= END do
    print(num .. ': ' .. tostring(an_func(t[num])))
    num = num+1
end

Upvotes: 1

EinsteinK
EinsteinK

Reputation: 755

When using pairs(), it goes over all elements of a table. nil means nothing, so it won't go over it. Why should it tell you there's nothing in the table? If you have a totally new table, it'll return nil if you index it with anything. print(({}).test) --> nil

If you really want to get the nil, you could use a dummy value:

local NIL = setmetatable({},{__tostring=function() return "nil" end})

an_func = function(x) return x == NIL end

for num, element in pairs({'None', NIL, 1, '1'}) do
    print(num .. ': ' .. tostring(an_func(element)))
end

Output:

1: false
2: true
3: false
4: false

Upvotes: 9

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