I'm attempting to use a table as a means to do two things at once. For example:
s = passengers -- user input
t = {[3] = car, [7] = bus, [24] = plane, [45] = train}
for k,v in ipairs t do
if s = k then
z = v * 10 -- cost per person
end
end
Now this is extremely basic for what I'm trying to do. I have a list of about 12 items that each have their own number. I want to know if I can do what I did above with the table and provide each of the 12 items with their own key value and then use that ? This key value would represent each items particular, unique number. Also, can I then use that key's value in a later equation, such as above?
If your keys are unique, your data structure. The point of a table key is direct access to the corresponding value.
This has the same effect as your loop:
local v = t[s] -- value for s or nil if s is not a key
if v != nil then
z = v * 10
end
(Or, more exactly the same: local v = rawget(t,s) to account for cases where t has an __index metamethod.)
If we can assume that v will never be false (which would cause an error at false * 10) then it can be written more naturally (which skips that error):
local v = t[s]
if v then
z = v * 10
end
Related
Does someone know a solution to save the key and the values to an table? My idea does not work because the length of the table is 0 and it should be 3.
local newstr = "3 = Hello, 67 = Hi, 2 = Bye"
a = {}
for k,v in newstr:gmatch "(%d+)%s*=%s*(%a+)" do
--print(k,v)
a[k] = v
end
print(#a)
The output is correct.
run for k,v in pairs(a) do print(k,v) end to check the contents of your table.
The problem is the length operator which by default cannot be used to get the number of elements of any table but a sequence.
Please refer to the Lua manual: https://www.lua.org/manual/5.4/manual.html#3.4.7
When t is a sequence, #t returns its only border, which corresponds to
the intuitive notion of the length of the sequence. When t is not a
sequence, #t can return any of its borders. (The exact one depends on
details of the internal representation of the table, which in turn can
depend on how the table was populated and the memory addresses of its
non-numeric keys.)
Only use the length operator if you know t is a sequence. That's a Lua table with integer indexes 1,..n without any gap.
You don't have a sequence as you're using non-numeric keys only. That's why #a is 0
The only safe way to get the number of elements of any table is to count them.
local count = 0
for i,v in pairs(a) do
count = count + 1
end
You can put #Piglet' code in the metatable of a as method __len that is used for table key counting with length operator #.
local newstr = "3 = Hello, 67 = Hi, 2 = Bye"
local a = setmetatable({},{__len = function(tab)
local count = 0
for i, v in pairs(tab) do
count = count + 1
end
return count
end})
for k,v in newstr:gmatch "(%d+)%s*=%s*(%a+)" do
--print(k,v)
a[k] = v
end
print(#a) -- puts out: 3
The output of #a with method __len even is correct if the table holds only a sequence.
You can check this online in the Lua Sandbox...
...with copy and paste.
Like i do.
As you can tell I'm a beginner in lua. I am trying to understand a function I'm stuck at what the following code segment does?
It is used in the following code snippet in the last line:
function classify(txt_dir, img_dir, cls_list)
local acc = 0.0
local total = 0.0
local fea_img = {}
local fea_txt = {}
for fname in io.lines(cls_list) do
local imgpath = img_dir .. '/' .. fname .. '.t7'
local txtpath = txt_dir .. '/' .. fname .. '.t7'
fea_img[#fea_img + 1] = extract_img(imgpath)
fea_txt[#fea_txt + 1] = extract_txt(txtpath)
end
for i = 1,#fea_img do
-- loop over individual images.
for k = 1,fea_img[i]:size(1) do
local best_match = 1
local best_score = -math.huge
for j = 1,#fea_txt do
local cur_score = torch.dot(fea_img[i][{k,{}}], fea_txt[j])
From my understanding, fea_img is a lua table. Is the line fea_img[i][{k,{}}] some sort of slicing for the value for the key 'i' in the table fea_img?
I tried searching for more examples and found this being used here too (last line):
for i = 1,nsamples,batchsize do
-- indices
local lasti = math.min(i+batchsize-1,nsamples)
local m = lasti - i + 1
-- k-means step, on minibatch
local batch = x[{ {i,lasti},{} }]
Any help on this would be really appreciated. Thank you!
In lua you can access a specific index on a table in multiple ways. Like these two examples
local myValue = someTable.theIndex
-- or
local myOtherValue = someTable[2]
So the construct you see here is to access some values from a (nested) table.
Also in lua you can use anything except nil as a index, so even tables are possible.
The line
fea_img[i][{k,{}}]
Can be extended to this:
local index1 = i -- i in this case is your loop variable
local index2 = { k , { } } -- This creates a table with 2 values, the first one will be the vaule of the var k, the second one is an empty table
local value1 = fea_img[index1] -- This will get you a table
local value2 = value1[index2] -- This will get the same as: fea_img[i][{k,{}}]
Correction and Addition:
As Nicol Bolas already said in the comments: The index must be an exact match. Which means it literally has to be the same table, which is not the case for the presented code from you. Either you dropped code you thought is unnecessary or fea_img has some some kind of metatable on it.
In the case of
local k = 2
local table1 = {k, { } }
local table2 = {k, { } }
table2 and table1 do have the exact same content. But they are not the same table. Which will lead to nil always being retrieved if one is used to store data in a table and the other is used to get it back.
Syntactically, t[k] is indexing a table with a key. Normally, if there is a record in the table with the key k, its value is returned. Nothing more, nothing less.
If fea_img[i] was a normal table, {k,{}} would always return nil, since table indices are resolved based on their identity ({k,{}} is always a new table). Based on your code, I have to conclude that the elements of fea_img (i.e. what extract_img returns) are not normal tables.
In Lua, you can override the indexing operation using a metatable. If you index a value that has a metatable with __index, it will be used if there is no matching record in the table:
local t = {}
setmetatable(t, {
__index = function(t, k)
return k
end
})
print(t[{}])
This table has a metatable associated with it, which is used in the indexing operation. In this case __index returns the key, but whatever library you are using might provide more complex behaviour.
This is specific to the library you are using, not something related to the Lua syntax.
Assume I have a function that returns multiple values. I happen to be working with LÖVE's (Image):getDimensions. This returns two values, which I know to be width,height. I want to assign them to a new table, as an array. I would like named (string) keys. So for instance, I would like to assign the return values of the getDimensions() function to a new table with keys width and height, respectively.
I know the following works...
image = {}
image.data = love.graphics.newImage('myimage.png')
image.size = {}
image.size.width, image.size.height = image.data:getDimensions()
I'm wondering if there is any sort of syntactic sugar I can use, or any use of standard library functions that will allow a syntax more along the lines of...
image.size = { width, height = image.data:getDimensions() }
I know the above line does not work, along with many variations I've tried, including various attempts to use unpack(). I'm new to Lua (~2 days in now), so maybe there is another standard function or best practice that I'm unaware of that will associate a table of keys to an array-like table. Thanks!
You can write your own functions:
local function set_fields(tab, fields, ...)
-- fields is an array of field names
-- (use empty string to skip value at corresponging position)
local values = {...}
for k, field in ipairs(fields) do
if field ~= "" then
tab[field] = values[k]
end
end
return tab
end
local function get_fields(tab, fields)
local values = {}
for k, field in ipairs(fields) do
values[k] = tab[field]
end
return (table.unpack or unpack)(values, 1, #fields)
end
Usage example #1:
image.size = set_fields({}, {"width", "height"}, image.data:getDimensions())
Usage example #2:
Swap the values on-the-fly!
local function get_weekdays()
return "Sun", "Mon", "Tue", "Wed", "Thu", "Fri", "Sat"
end
-- we want to save returned values in different order
local weekdays = set_fields({}, {7,1,2,3,4,5,6}, get_weekdays())
-- now weekdays contains {"Mon", "Tue", "Wed", "Thu", "Fri", "Sat", "Sun"}
Usage example #3:
local function get_coords_x_y_z()
return 111, 222, 333 -- x, y, z of the point
end
-- we want to get the projection of the point on the ground plane
local projection = {y = 0}
-- projection.y will be preserved, projection.x and projection.z will be modified
set_fields(projection, {"x", "", "z"}, get_coords_x_y_z())
-- now projection contains {x = 111, y = 0, z = 333}
Usage example #4:
If require("some_module") returns a module with plenty of functions inside, but you need only a few of them:
local bnot, band, bor = get_fields(require("bit"), {"bnot", "band", "bor"})
Using a class construct, I have come up with the following...
Size = {}
Size.mt = {}
Size.prototype = { width = 0, height = 0 }
function Size.new (dimensions)
local size = setmetatable({}, Size.mt)
if dimensions ~= nil and type(dimensions) == 'table' then
for k,v in pairs(dimensions) do
size[k] = v
end
end
return size
end
Size.mt.__index = function (t, k)
if k == 'width' or k == 'height' then
rawset(t, k, Size.prototype[k])
return t[k]
else
return nil
end
end
Size.mt.__newindex = function (t, k, v)
if k == 1 or k == 'width' then
rawset(t, 'width', v)
elseif k == 2 or k == 'height' then
rawset(t, 'height', v)
end
end
Then I can initialize a Size object in a number of ways
Using multiple return values:
image.size = Size.new{image.data:getDimensions()}
image.size = Size.new(table.pack(image.data:getDimensions())
Using default values:
image.size = Size.new()
image.size = Size.new{}
image.size = Size.new({})
Using mixed array and hash tables:
image.size = Size.new({height=20, width=30})
image.size = Size.new({height=20, 30})
There are pros and cons to this approach vs. Egor's (utility function), which is what I was considering doing if there wasn't a simple syntax trick or an existing function that I was unaware of.
Pros:
(personal) learning experience with OO constructs in Lua
I can limit the number of actual keys on the table, while allowing 'synonyms' for those keys to be added by expanding the accepted values in the if/else logic of __index and __newindex
Explicit definition of fields in the table, without needing to worry about syncing a table of keys with a table of values (as with a general purpose utility function)
Cons
would need to repeat this pattern for each data structure where I wanted this behavior
costly, a lot of overhead for what amounts to a very small difference to the consumer
I'm sure I can make this approach more robust in time, but I would appreciate any feedback.
I want to make sure that kD points which I generate randomly don't happen to be identical. To test this efficiently, I want to use a set (at least that's what I would do in C++).
I know that in Lua tables can be used as sets by inserting the object as value with any non-nil key (typically just true or an integer for multisets).
But the problem is that tables in Lua are by default just compared w.r.t. their address, which is of course different even if all the vector components (in my case) are equal.
So I thought I'd implement a equal and/or less than metamethod for my points. But this didn't work (see code below). Can anybody help?
local k = 3
local mt = {}
mt.__eq = function( a, b )
for dim = 1, k do
if a[dim] ~= b[dim] then return false end
end
return true
end
mt.__lt = function( a, b )
for dim = 1, k do
if a[dim] < b[dim] then
return true
elseif a[dim] > b[dim] then
return false
end
end
return false -- equal
end
local set = {}
local p1 = { 1, 2, 3 }
setmetatable( p1, mt )
set[p1] = true
local p2 = { 1, 2, 3 }
setmetatable( p2, mt )
set[p2] = true -- should just overwrite the old value
print( "p1 == p2 --> "..tostring( p1 == p2 ) )
print( "p1 < p2 --> "..tostring( p1 < p2 ) )
local setSize = 0
for _, _ in pairs( set ) do
setSize = setSize + 1
end
print( "Size of the set: "..setSize )
Lua has no facilities for allowing what you're trying to do. That is, there's no metamethod trickery that can make two different pieces of userdata or two different tables map to the same index to the table indexing system.
You'll have to do this manually. You can effectively write your own set, by keeping a list of vectors sorted. table.sort can do the job, since you already have an appropriate __lt metamethod.
If anyone is interested: I have come up with another work-around that has worked for me.
One can use a set of x-coordinates and instead of associating the set items with just "true" or "1", the respective values are sets themselves (of y-coordinates); and so on.
This leads to a tree-like structure where the path to each leaf represents a point (sequence of coordinates) and points with the same first coordinates have common parent nodes. Also all points are unique.
I am starting to learn Lua from Programming in Lua (2nd edition)
I didn't understand the following in the book. Its very vaguely explained.
a.) w={x=0,y=0,label="console"}
b.) x={math.sin(0),math.sin(1),math.sin(2)}
c.) w[1]="another field"
d.) x.f=w
e.) print (w["x"])
f.) print (w[1])
g.) print x.f[1]
When I do print(w[1]) after a.), why doesn't it print x=0
What does c.) do?
What is the difference between e.) and print (w.x)?
What is the role of b.) and g.)?
You have to realize that this:
t = {3, 4, "eggplant"}
is the same as this:
t = {}
t[1] = 3
t[2] = 4
t[3] = "eggplant"
And that this:
t = {x = 0, y = 2}
is the same as this:
t = {}
t["x"] = 0
t["y"] = 2
Or this:
t = {}
t.x = 0
t.y = 2
In Lua, tables are not just lists, they are associative arrays.
When you print w[1], then what really matters is line c.) In fact, w[1] is not defined at all until line c.).
There is no difference between e.) and print (w.x).
b.) creates a new table named x which is separate from w.
d.) places a reference to w inside of x. (NOTE: It does not actually make a copy of w, just a reference. If you've ever worked with pointers, it's similar.)
g.) Can be broken up in two parts. First we get x.f which is just another way to refer to w because of line d.). Then we look up the first element of that table, which is "another field" because of line c.)
There's another way of creating keys in in-line table declarations.
x = {["1st key has spaces!"] = 1}
The advantage here is that you can have keys with spaces and any extended ASCII character.
In fact, a key can be literally anything, even an instanced object.
function Example()
--example function
end
x = {[Example] = "A function."}
Any variable or value or data can go into the square brackets to work as a key. The same goes with the value.
Practically, this can replace features like the in keyword in python, as you can index the table by values to check if they are there.
Getting a value at an undefined part of the table will not cause an error. It will just give you nil. The same goes for using undefined variables.
local w = {
--[1] = "another field"; -- will be set this value
--["1"] = nil; -- not save to this place, different with some other language
x = 0;
y = 0;
label = "console";
}
local x = {
math.sin(0);
math.sin(1);
math.sin(2);
}
w[1] = "another field" --
x.f = w
print (w["x"])
-- because x.f = w
-- x.f and w point one talbe address
-- so value of (x.f)[1] and w[1] and x.f[1] is equal
print (w[1])
print ((x.f)[1])
print (x.f[1])
-- print (x.f)[1] this not follows lua syntax
-- only a function's has one param and type of is a string
-- you can use print "xxxx"
-- so you print x.f[1] will occuur error
-- in table you can use any lua internal type 's value to be a key
-- just like
local t_key = {v=123}
local f_key = function () print("f123") end
local t = {}
t[t_key] = 1
t[f_key] = 2
-- then t' key actualy like use t_key/f_key 's handle
-- when you user t[{}] = 123,
-- value 123 related to this no name table {} 's handle