Assuming I have a piece of code such as the following
aTable = {aValue=1}
aTable_mt = {}
print(aTable)
What must I do to make Lua print something like aTable current aValue = 1 as opposed to table: 0x01ab1d2.
So far I've tried setting the __tostring metamethod but that doesn't seem to be invoked by print. Is there some metamethod I've been missing or does the answer have nothing to do with metamethods?
__tostring works:
aTable = {aValue=1}
local mt = {__tostring = function(t)
local result = ''
for k, v in pairs(t) do
result = result .. tostring(k) .. ' ' .. tostring(v) .. ''
end
return result
end}
setmetatable(aTable, mt)
print(aTable)
This prints aValue 1 (with one extra whitespace, remove it in real code). The aTable part is not available, because aTable is a variable that references the table, not the content of the table itself.
I'm not sure how you set the metamethod, but the following code prints "stringified" for me:
local aTable = {a = 1, b = 2}
setmetatable(aTable, {__tostring = function() return "stringified" end})
print(aTable)
If you want lua to generally print all tables human readable, you could
hook up/overwrite the print function:
local orig_print = print
print = function(...)
local args = {...}
for i,arg in ipairs(args) do
if type(arg) == 'table' then
args[i] = serialize(arg)
end
end
orig_print(table.unpack(args))
end
serialize could be serpent or some other lib from here
Note that this must be done before any other module/script is loaded.
Related
I am trying to write a string join function that will work with both tables and variable arguments as input. Here is what I have so far:
function join(separator, ...)
local result = ""
local vargs = {...}
local n = #vargs
for i = 1, n do
local varg = vargs[i]
if type(varg) == "table" then
result = join(separator, result, table.unpack(varg))
elseif varg ~= nil then
result = result..tostring(varg)
if i < n then
result = result..separator
end
end
end
return result
end
However, when I try to use it with the following input:
print(join(",", "1", "2", "3"))
print(join(",", {"a", "b", "c"}))
The output is this:
1,2,3
,a,b,c
I did not expect the , at the beginning of a,b,c.
From what I understand, it seems somehow the separator is getting added to the variable arguments when calling the function inside the function (recursiveness). But why is that? And how can I fix it? Thank you!
Try
...
if type(varg) == "table" then
varg = join(separator, table.unpack(varg))
end
if varg ~= nil then
...
Well uh sorry if it's not really useful but maybe it's because you put the: , into "" that it print the ,
I would like to display all the variables of Lua code using some debugger functions, but i can't get the exacte size of the tables, and also i can't retrieve the element of some tables (value, name, etc).
The following code illustrate what i did :
local name, value;
local NIL = {} -- to represent nil variables
local locals = {}
local a = 9
local i = 1
local tab ={1,2,3,4,4,4}
local tab2 = {22,22,22,22}
local tab4 = {'X', 'Y','Z'}
while( true ) do
name, value = debug.getlocal( 1, i )
if ( name == nil ) then break end
locals[ name ] = value == nil and NIL or value
i = i + 1
end
for k, v in pairs( locals ) do
if(type(v) ~= "table") then -- if it's a simple variable
print( k, v )
else
print("tabel : "..k) -- if it's a table, display it's element
print("size : "..#k)
-- I want to diplay all the elements of the table !!
for x, y in pairs(v) do
print(y)
end
end
end
you can use inspect library (useful to print nested tables) or print_r function
I'm trying to get the name of all the file saved in two folders, the name are saved as :
1.lua 2.lua 3.lua 4.lua and so on
the folders name are :
first folder : "/const/"
second folder: "/virt/"
what I'm trying to do is only get the number of the files and this works but not in the right order, when I get the 17 file for example I get the 17th delivered from the function before the 15 and this causes for me a problem here the code of the function that I'm using :
local virt_path = "/virt/"
local const_path = "/const"
local fs = require "lfs"
local const = {}
for num = 1, (numberoffile)do -- numberoffile is predfined and can't be change
const[num] = assert(
dofile (const_path .. mkfilename(num)),
"Failed to load constant ".. num ..".")
end
local function file_number() --this is the function that causes me a headach
local ci, co, num = ipairs(const)
local vi, vo, _ = fs.dir(virt_path)
local function vix(o)
local file = vi(o)
if file == nil then return nil end
local number = file:match("^(%d+).lua$")
if number == nil then return vix(o) end
return tonumber(number)
end
local function iter(o, num)
return ci(o.co, num) or vix(o.vo, num)
end
return iter, {co=co, vo=vo}, num
end
As I said the function delive the need return values but not the right Arithmetic order.
any idea what I'm doing wrong here ?
I use my path[1] library.
1 We fill table with filenames
local t = {}
for f in path.each("./*.lua", "n") do
t[#t + 1] = tonumber((path.splitext(f)))
end
table.sort(t)
for _, i in ipairs(t) do
-- do work
end
2 We check if files exists
for i = 1, math.huge do
local p = "./" .. i .. ".lua"
if not path.exists(p) then break end
-- do work
end
[1] https://github.com/moteus/lua-path
I am trying to build an elegant transpose function using functions mapn and zip in Lua.
The mapn and zip are as follows (From the lua book):
function map(func, array)
local new_array = {}
for i,v in ipairs(array) do
new_array[i] = func(v)
end
return new_array
end
function mapn(func, ...)
local new_array = {}
local i=1
local arg_length = table.getn(arg)
while true do
local arg_list = map(function(arr) return arr[i] end, arg)
if table.getn(arg_list) < arg_length then return new_array end
new_array[i] = func(unpack(arg_list))
i = i+1
end
end
These work as expected.
I then define zip and transpose as:
function zip(...)
return mapn(function(...) return {...} end,...)
end
function transpose(...)
return zip(unpack(...))
end
Now transpose({{1,2},{3,4},{5,6}}) produces {{1,3,5},{2,4,6}} as expected.
But transpose({{1,2},{3,4},{5}}) does not produce {{1,3,5},{2,4}}. It only produces one row.
How can I get it to produce the result I wish for?
I just decided to write an "inelegant" function instead. It seems there's no smooth way to use mapn and friends.
function transp(L)
local n=#L
local m,M=1e42,0
--Get the beginning and end of resultant transpose list.
for i=1,n do
for k,v in pairs(L[i]) do
if M<k then M=k end
if m>k then m=k end
end
end
local nt={}
for i=m,M do
local rt={}
for j=1,n do
rt[j]=L[j][i]
end
table.insert(nt,rt)
end
return nt
end
Please critique and improve this candidate solution.
I fixed a few things in your code and I think it works now as intended, I've added comments inline.
function map(func, array)
local new_array = {}
for i, v in ipairs(array) do
new_array[#new_array + 1] = func(v)
end
return new_array
end
function mapn(func, ...)
-- Variadic arguments bound to an array.
local arrays = {...}
local new_array = {}
-- Simple for-loop.
local i = 1
while true do
local arg_list = map(function(arr) return arr[i] end, arrays)
if #arg_list == 0 then
break
end
new_array[i] = func(unpack(arg_list))
i = i + 1
end
return new_array
end
-- Using 'mapn' instead of 'map' (probably how you intended).
function zip(...)
return mapn(function(...) return {...} end,...)
end
-- Same as before.
function transpose(...)
return zip(unpack(...))
end
Usage example:
for _, row in pairs(transpose({{1,2},{3,4},{5}})) do
for _, col in pairs(row) do io.write(col .. ' ') end
io.write('\n')
end
-- Output: 1 3 5
-- 2 4
The {5} in your example is being ignored because of this line:
if table.getn(arg_list) < arg_length then return new_array end
What you may want to do instead is break out of the loop only when arg_list is empty.
This will then give the result you want provided that the rows are monotonically increasing in length.
For the more general case, when later rows may be shorter than earlier ones
(e.g. {{1,2},{3,4,5},{6}}), you will need to keep track of the row lengths to allow for holes. This can be done by adding an optional argument (and extra return value) to map to indicate the maximum index i for which func(array[i]) was evaluated:
function map(func, array, len)
local new_array = {}
len = len or #array
for i=1,len do
new_array[i] = func(array[i])
end
return new_array, len
end
function mapn(func, ...)
local new_array = {}
local i=1
local arg_length = select('#', ...)
local args = {...}
while true do
local arg_list, num_results = map(function(arr) return arr[i] end, args, arg_length)
if not next(arg_list) then return new_array end
new_array[i] = func(unpack(arg_list, 1, num_results))
i = i+1
end
end
function zip(...)
return mapn(function(...) return {...} end,...)
end
function transpose(...)
return zip(unpack(...))
end
I'm trying, as an exercise, to make a set implementation in Lua. Specifically I want to take the simplistic set implementation of Pil2 11.5 and grow it up to include the ability to insert values, delete values, etc.
Now the obvious way to do this (and the way that works) is this:
Set = {}
function Set.new(l)
local s = {}
for _, v in ipairs(l) do
s[v] = true
end
return s
end
function Set.insert(s, v)
s[v] = true
end
ts = Set.new {1,2,3,4,5}
Set.insert(ts, 5)
Set.insert(ts, 6)
for k in pairs(ts) do
print(k)
end
As expected I get the numbers 1 through 6 printed out. But those calls to Set.insert(s, value) are really rather ugly. I'd much rather be able to call something like ts:insert(value).
My first attempt at a solution to this looked like this:
Set = {}
function Set.new(l)
local s = {
insert = function(t, v)
t[v] = true
end
}
for _, v in ipairs(l) do
s[v] = true
end
return s
end
ts = Set.new {1,2,3,4,5}
ts:insert(5)
ts:insert(6)
for k in pairs(ts) do
print(k)
end
This works mostly fine until you see what comes out of it:
1
2
3
4
5
6
insert
Very obviously the insert function, which is a member of the set table, is being displayed. Not only is this even uglier than the original Set.insert(s, v) problem, it's also prone to some serious trouble (like what happens if "insert" is a valid key someone is trying to enter?). It's time to hit the books again. What happens if I try this instead?:
Set = {}
function Set.new(l)
local s = {}
setmetatable(s, {__call = Set.call})
for _, v in ipairs(l) do
s[v] = true
end
return s
end
function Set.call(f)
return Set[f]
end
function Set.insert(t, v)
t[v] = true
end
ts = Set.new {1,2,3,4,5}
ts:insert(5)
ts:insert(6)
for k in pairs(ts) do
print(k)
end
Now the way I'm reading this code is:
When I call ts:insert(5), the fact that insert doesn't exist to be called means that the ts metatable is going to be searched for "__call".
The ts metatable's "__call" key returns Set.call.
Now Set.call is called with the name insert which causes it to return the Set.insert function.
Set.insert(ts, 5) is called.
What's really happening is this:
lua: xasm.lua:26: attempt to call method 'insert' (a nil value)
stack traceback:
xasm.lua:26: in main chunk
[C]: ?
And at this point I'm stumped. I have absolutely no idea where to go from here. I hacked around for an hour with varying degrees of increasingly desperate variations on this code but the end result is that I have nothing that works. What undoubtedly obvious thing am I overlooking at this point?
Now the way I'm reading this code is:
When I call ts:insert(5), the fact that insert doesn't exist to be called means that the ts metatable is going to be searched for "__call".
There's your problem. The __call metamethod is consulted when the table itself is called (ie, as a function):
local ts = {}
local mt = {}
function mt.__call(...)
print("Table called!", ...)
end
setmetatable(ts, mt)
ts() --> prints "Table called!"
ts(5) --> prints "Table called!" and 5
ts"String construct-call" --> prints "Table called!" and "String construct-call"
Object-oriented colon-calls in Lua such as this:
ts:insert(5)
are merely syntactic sugar for
ts.insert(ts,5)
which is itself syntactic sugar for
ts["insert"](ts,5)
As such, the action that is being taken on ts is not a call, but an index (the result of ts["insert"] being what is called), which is governed by the __index metamethod.
The __index metamethod can be a table for the simple case where you want indexing to "fall back" to another table (note that it is the value of the __index key in the metatable that gets indexed and not the metatable itself):
local fallback = {example = 5}
local mt = {__index = fallback}
local ts = setmetatable({}, mt)
print(ts.example) --> prints 5
The __index metamethod as a function works similarly to the signature you expected with Set.call, except that it passes the table being indexed before the key:
local ff = {}
local mt = {}
function ff.example(...)
print("Example called!",...)
end
function mt.__index(s,k)
print("Indexing table named:", s.name)
return ff[k]
end
local ts = {name = "Bob"}
setmetatable(ts, mt)
ts.example(5) --> prints "Indexing table named:" and "Bob",
--> then on the next line "Example called!" and 5
For more information on metatables, consult the manual.
You said:
Now the way I'm reading this code is:
When I call ts:insert(5), the fact that insert doesn't
exist to be called means that the ts metatable is going
to be searched for "__call".
The ts metatable's "__call" key returns Set.call.
Now Set.call is called with the name insert which causes
it to return the Set.insert function.
Set.insert(ts, 5) is called.
No, what happens is this:
When insert isn't found directly in the ts object, Lua looks for __index in its metatable.
If it is there and it is a table, Lua will search for insert there.
If it is there and it is a function, it will call it with the original table (ts in this case) and the key being searched for (insert).
If it isn't there, which is the case, it is considered nil.
The error you're having is because you don't have __index set in your metatable, so you are effectively calling a nil value.
This can be solved by pointing __index to some table, namely Set, if you're going to store your methods there.
As for __call, it is used for when you call the object as a function. Ie:
Set = {}
function Set.new(l)
local s = {}
setmetatable(s, {__index=Set, __call=Set.call})
for _, v in ipairs(l) do
s[v] = true
end
return s
end
function Set.call(s, f)
-- Calls a function for every element in the set
for k in pairs(s) do
f(k)
end
end
function Set.insert(t, v)
t[v] = true
end
ts = Set.new {1,2,3,4,5}
ts:insert(5)
ts:insert(6)
ts(print) -- Calls getmetatable(ts).__call(ts, print),
-- which means Set.call(ts, print)
-- The way __call and __index are set,
-- this is equivalent to the line above
ts:call(print)
Set = {}
function Set.new(l)
local s = {}
setmetatable(s, {__index=Set})
for _, v in ipairs(l) do
s[v] = true
end
return s
end
function Set.call(f)
return Set[f]
end
function Set.insert(t, v)
t[v] = true
end
ts = Set.new {1,2,3,4,5}
ts:insert(5)
ts:insert(6)
for k in pairs(ts) do
print(k)
end
I modified your first version and this version would offer the features I think you are looking for.
Set = {}
Set.__index = Set
function Set:new(collection)
local o = {}
for _, v in ipairs(collection) do
o[v] = true
end
setmetatable(o, self)
return o
end
function Set:insert(v)
self[v] = true
end
set = Set:new({1,2,3,4,5})
print(set[1]) --> true
print(set[10]) --> nil
set:insert(10)
print(set[10]) --> true