Performance of Table Search vs. string.find in Lua - performance

Let's say that I want to store some alphanumeric data. I can either use a table:
t = { "a", "b", "c" }
or a string:
s = "abc"
When I want to test if 'b' is in my data set, I can test the table by saying:
function findInTable(table, element)
for i, v in ipairs(table) do
if v == element then return true end
end
return false
end
if findInTable(t, "b") then
--do stuff
or I can test the string by saying:
if s:find("b") then
--do stuff
Which of these methods is faster? I imagine that string.find is essentially doing the same thing as my findInTable function. I need to check data in this way on every draw for a game, so performance is critical. A lot of my data is being extracted from text files and it's easier to keep it in string format rather than using commas or some such as delimiters to organize it into table values.

Consider doing this:
t = { ["a"]=true, ["b"]=true, ["c"]=true }
Then to test if a string s is in t, simply do
if t[s] then ...

I do similar things between two frames in LÖVE [love2d].
And without benchmarking it i can say: Its fast enough
( Frames decreases with many drawable objects only )
Moreover i would like to add the table functions to the table t as methods...
t = setmetatable({"a", "b", "c"}, {__index = table})
print(t[t:concat():find("b")])
-- Output: b
-- False Output: nil
-- And nil never happens if...
print(t[t:concat():find("d")] or t[#t])
-- Output: c
-- ..fallback to a default with: or
...check it out or change this in...
https://www.lua.org/demo.html
...to figure out how it works.

Related

Sorting a Lua table by key

I have gone through many questions and Google results but couldn't find the solution.
I am trying to sort a table using table.sort function in Lua but I can't figure out how to use it.
I have a table that has keys as random numeric values. I want to sort them in ascending order. I have gone through the Lua wiki page also but table.sort only works with the table values.
t = { [223]="asd", [23]="fgh", [543]="hjk", [7]="qwe" }
I want it like:
t = { [7]="qwe", [23]="fgh", [223]="asd", [543]="hjk" }
You cannot set the order in which the elements are retrieved from the hash (which is what your table is) using pairs. You need to get the keys from that table, sort the keys as its own table, and then use those sorted keys to retrieve the values from your original table:
local t = { [223]="asd", [23]="fgh", [543]="hjk", [7]="qwe" }
local tkeys = {}
-- populate the table that holds the keys
for k in pairs(t) do table.insert(tkeys, k) end
-- sort the keys
table.sort(tkeys)
-- use the keys to retrieve the values in the sorted order
for _, k in ipairs(tkeys) do print(k, t[k]) end
This will print
7 qwe
23 fgh
223 asd
543 hjk
Another option would be to provide your own iterator instead of pairs to iterate the table in the order you need, but the sorting of the keys may be simple enough for your needs.
What was said by #lhf is true, your lua table holds its contents in whatever order the implementation finds feasible. However, if you want to print (or iterate over it) in a sorted manner, it is possible (so you can compare it element by element). To achieve this, you can do it in the following way
for key, value in orderedPairs(mytable) do
print(string.format("%s:%s", key, value))
end
Unfortunately, orderedPairs is not provided as a part of lua, you can copy the implementation from here though.
The Lua sort docs provide a good solution
local function pairsByKeys (t, f)
local a = {}
for n in pairs(t) do table.insert(a, n) end
table.sort(a, f)
local i = 0 -- iterator variable
local iter = function () -- iterator function
i = i + 1
if a[i] == nil then return nil
else return a[i], t[a[i]]
end
end
return iter
end
Then you traverse the sorted structure
local t = { b=1, a=2, z=55, c=0, qa=53, x=8, d=7 }
for key,value in pairsByKeys(t) do
print(" " .. tostring(key) .. "=" .. tostring(value))
end
There is no notion of order in Lua tables: they are just sets of key-value pairs.
The two tables below have exactly the same contents because they contain exactly the same pairs:
t = { [223] = "asd" ,[23] = "fgh",[543]="hjk",[7]="qwe"}
t = {[7]="qwe",[23] = "fgh",[223] = "asd" ,[543]="hjk"}

Ruby // Random number between range, ensure uniqueness against others existing stored ones

Currently trying to generate a random number in a specific range;
and ensure that it would be unique against others stored records.
Using Mysql. Could be like an id, incremented; but can't be it.
Currently testing other existing records in an 'expensive' manner;
but I'm pretty sure that there would be a clean 1/2 lines of code to use
Currently using :
test = 0
Order.all.each do |ord|
test = (0..899999).to_a.sample.to_s.rjust(6, '0')
if Order.find_by_number(test).nil? then
break
end
end
return test
Thanks for any help
Here your are my one-line solution. It is also the quicker one since calls .pluck to retrieve the numbers from the Order table. .select instantiates an "Order" object for every record (that is very costly and unnecessary) while .pluck does not. It also avoids to iterate again each object with a .map to get the "number" field. We can avoid the second .map as well if we convert, using CAST in this case, to a numeric value from the database.
(Array(0...899999) - Order.pluck("CAST('number' AS UNSIGNED)")).sample.to_s.rjust(6, '0')
I would do something like this:
# gets all existing IDs
existing_ids = Order.all.select(:number).map(&:number).map(&:to_i)
# removes them from the acceptable range
available_numbers = (0..899999).to_a - existing_ids
# choose one (which is not in the DB)
available_numbers.sample.to_s.rjust(6, '0')
I think, you can do something like below :
def uniq_num_add(arr)
loop do
rndm = rand(1..15) # I took this range as an example
# random number will be added to the array, when the number will
# not be present
break arr<< "%02d" % rndm unless arr.include?(rndm)
end
end
array = []
3.times do
uniq_num_add(array)
end
array # => ["02", "15", "04"]

refactor this ruby database/sequel gem lookup

I know this code is not optimal, any ideas on how to improve it?
job_and_cost_code_found = false
timberline_db['SELECT Job, Cost_Code FROM [JCM_MASTER__COST_CODE] WHERE [Job] = ? AND [Cost_Code] = ?', job, clean_cost_code].each do |row|
job_and_cost_code_found = true
end
if job_and_cost_code_found == false then
info = linenum + "," + id + ",,Employees default job and cost code do not exist in timberline. job:#{job} cost code:#{clean_cost_code}"
add_to_exception_output_file(info)
end
You're breaking a lot of simple rules here.
Don't select what you don't use.
You select a number of columns, then completely ignore the result data. What you probably want is a count:
SELECT COUNT(*) AS cost_code_count FROM [JCM_MASTER__COST_CODE] WHERE [Job] = ? AND [Cost_Code] = ?'
Then you'll get one row that will have either a zero or non-zero value in it. Save this into a variable like:
job_and_cost_codes_found = timberline_db[...][0]['cost_code_count']
Don't compare against false unless you need to differentiate between that and nil
In Ruby only two things evaluate as false, nil and false. Most of the time you will not be concerned about the difference. On rare occasions you might want to have different logic for set true, set false or not set (nil), and only then would you test so specifically.
However, keep in mind that 0 is not a false value, so you will need to compare against that.
Taking into account the previous optimization, your if could be:
if job_and_cost_codes_found == 0
# ...
end
Don't use then or other bits of redundant syntax
Most Ruby style-guides spurn useless syntax like then, just as they recommend avoiding for and instead use the Enumerable class which is far more flexible.
Manipulate data, not strings
You're assembling some kind of CSV-like line in the end there. Ideally you'd be using the built-in CSV library to do the correct encoding, and libraries like that want data, not a string they'd have to parse.
One step closer to that is this:
line = [
linenum,
id,
nil,
"Employees default job and cost code do not exist in timberline. job:#{job} cost code:#{clean_cost_code}"
].join(',')
add_to_exception_output_file(line)
You'd presumably replace join(',') with the proper CSV encoding method that applies here. The library is more efficient when you can compile all of the data ahead of time into an array-of-arrays, so I'd recommend doing that if this is the end goal.
For example:
lines = [ ]
# ...
if (...)
# Append an array to the lines to write to the CSV file.
lines << [ ... ]
end
Keep your data in a standard structure like an Array, a Hash, or a custom object, until you're prepared to commit it to its final formatted or encoded form. That way you can perform additional operations on it if you need to do things like filtering.
It's hard to refactor this when I'm not exactly sure what it's supposed to be doing, but assuming that you want to log an error when there's no entry matching a job & code pair, here's what I've come up with:
def fetch_by_job_and_cost_code(job, cost_code)
timberline_db['SELECT Job, Cost_Code FROM [JCM_MASTER__COST_CODE] WHERE [Job] = ? AND [Cost_Code] = ?', job, cost_code]
end
if fetch_by_job_and_cost_code(job, clean_cost_code).none?
add_to_exception_output_file "#{linenum},#{id},,Employees default job and cost code do not exist in timberline. job:#{job} cost code:#{clean_cost_code}"
end

Associatively sorting a table by value in Lua

I have a key => value table I'd like to sort in Lua. The keys are all integers, but aren't consecutive (and have meaning). Lua's only sort function appears to be table.sort, which treats tables as simple arrays, discarding the original keys and their association with particular items. Instead, I'd essentially like to be able to use PHP's asort() function.
What I have:
items = {
[1004] = "foo",
[1234] = "bar",
[3188] = "baz",
[7007] = "quux",
}
What I want after the sort operation:
items = {
[1234] = "bar",
[3188] = "baz",
[1004] = "foo",
[7007] = "quux",
}
Any ideas?
Edit: Based on answers, I'm going to assume that it's simply an odd quirk of the particular embedded Lua interpreter I'm working with, but in all of my tests, pairs() always returns table items in the order in which they were added to the table. (i.e. the two above declarations would iterate differently).
Unfortunately, because that isn't normal behavior, it looks like I can't get what I need; Lua doesn't have the necessary tools built-in (of course) and the embedded environment is too limited for me to work around it.
Still, thanks for your help, all!
You seem to misunderstand something. What you have here is a associative array. Associative arrays have no explicit order on them, e.g. it's only the internal representation (usually sorted) that orders them.
In short -- in Lua, both of the arrays you posted are the same.
What you would want instead, is such a representation:
items = {
{1004, "foo"},
{1234, "bar"},
{3188, "baz"},
{7007, "quux"},
}
While you can't get them by index now (they are indexed 1, 2, 3, 4, but you can create another index array), you can sort them using table.sort.
A sorting function would be then:
function compare(a,b)
return a[1] < b[1]
end
table.sort(items, compare)
As Komel said, you're dealing with associative arrays, which have no guaranteed ordering.
If you want key ordering based on its associated value while also preserving associative array functionality, you can do something like this:
function getKeysSortedByValue(tbl, sortFunction)
local keys = {}
for key in pairs(tbl) do
table.insert(keys, key)
end
table.sort(keys, function(a, b)
return sortFunction(tbl[a], tbl[b])
end)
return keys
end
items = {
[1004] = "foo",
[1234] = "bar",
[3188] = "baz",
[7007] = "quux",
}
local sortedKeys = getKeysSortedByValue(items, function(a, b) return a < b end)
sortedKeys is {1234,3188,1004,7007}, and you can access your data like so:
for _, key in ipairs(sortedKeys) do
print(key, items[key])
end
result:
1234 bar
3188 baz
1004 foo
7007 quux
hmm, missed the part about not being able to control the iteration. there
But in lua there is usually always a way.
http://lua-users.org/wiki/OrderedAssociativeTable
Thats a start. Now you would need to replace the pairs() that the library uses. That could be a simples as pairs=my_pairs. You could then use the solution in the link above
PHP arrays are different from Lua tables.
A PHP array may have an ordered list of key-value pairs.
A Lua table always contains an unordered set of key-value pairs.
A Lua table acts as an array when a programmer chooses to use integers 1, 2, 3, ... as keys. The language syntax and standard library functions, like table.sort offer special support for tables with consecutive-integer keys.
So, if you want to emulate a PHP array, you'll have to represent it using list of key-value pairs, which is really a table of tables, but it's more helpful to think of it as a list of key-value pairs. Pass a custom "less-than" function to table.sort and you'll be all set.
N.B. Lua allows you to mix consecutive-integer keys with any other kinds of keys in the same table—and the representation is efficient. I use this feature sometimes, usually to tag an array with a few pieces of metadata.
Coming to this a few months later, with the same query. The recommended answer seemed to pinpoint the gap between what was required and how this looks in LUA, but it didn't get me what I was after exactly :- which was a Hash sorted by Key.
The first three functions on this page DID however : http://lua-users.org/wiki/SortedIteration
I did a brief bit of Lua coding a couple of years ago but I'm no longer fluent in it.
When faced with a similar problem, I copied my array to another array with keys and values reversed, then used sort on the new array.
I wasn't aware of a possibility to sort the array using the method Kornel Kisielewicz recommends.
The proposed compare function works but only if the values in the first column are unique.
Here is a bit enhanced compare function to ensure, if the values of a actual column equals, it takes values from next column to evaluate...
With {1234, "baam"} < {1234, "bar"} to be true the items the array containing "baam" will be inserted before the array containing the "bar".
local items = {
{1004, "foo"},
{1234, "bar"},
{1234, "baam"},
{3188, "baz"},
{7007, "quux"},
}
local function compare(a, b)
for inx = 1, #a do
-- print("A " .. inx .. " " .. a[inx])
-- print("B " .. inx .. " " .. b[inx])
if a[inx] == b[inx] and a[inx + 1] < b[inx + 1] then
return true
elseif a[inx] ~= b[inx] and a[inx] < b[inx] == true then
return true
else
return false
end
end
return false
end
table.sort(items,compare)

Changing one table seems to change the other

I'm writing a lua script, and one of the things it does is copy a table into a table of tables, and apply a couple transformations to it. What's odd though is when i go to use one of those tables later (and modify some of it's properties), changes will also seem to show up in other tables! Code:
-- thanks to http://stackoverflow.com/questions/1283388/lua-merge-tables/1283608#1283608
-- tableMerge:
-- merges two tables, with the data in table 2 overwriting the data in table 1
function tableMerge(t1, t2)
for k,v in pairs(t2) do
if type(v) == "table" then
if type(t1[k] or false) == "table" then
tableMerge(t1[k] or {}, t2[k] or {})
else
t1[k] = v
end
else
t1[k] = v
end
end
return t1
end
--tableCopy:
--takes a table and returns a complete copy including subtables.
function tableCopy(t)
return tableMerge({}, t)
end
local t1 = { a = 1, b = true, c = "d", e = { f = 2 } }
local t2 = tableCopy(t1)
t2.b = false
t2.e.f = 1
print(t1.b) -- prints true as it should
print(t1.e.f) -- prints 1!
[removed other code for reasons of the information it contains, and this a good reproduction of the bug]
so is it a bug in my code or something? i can't figure it out....
This is how Lua tables work - they don't get copied around, only references to the tables are passed to functions or stored in tables instead. If you are familiar with .NET terminology, you could say that tables are "reference types". Observe:
function modtable(t)
t.hello = "world"
end
local t = { hello = "no!"; }
modtable(t)
print(t.hello)
This prints "world" because the modtable function gets a reference to the table and not a copy. The same thing happens when you try to store table in another table
local t = { hello = "no!"; }
local bigT = { innerTable = t; }
bigT.innerTable.hello = "world"
print(t.hello)
t.hello = "double world";
print(bigT.innerTable.hello);
This will print
world
double world
because t and bigT.innerTable are essentially the same table.
If you want copies of the tables that you can modify independently from one-another you can write a small function to duplicate a table
function deep_copy_table(t)
local result = {}
for k,v in pairs(t)
do
if (type(v) == "table")
then
result[k] = deep_copy_table(v)
else
result[k] = v
end
end
return result
end
local t = { hello = "no!"; }
local bigT = { innerTable = deep_copy_table(t); }
bigT.innerTable.hello = "world"
print(t.hello)
t.hello = "double world";
print(bigT.innerTable.hello);
This will print "no!" and "world" - t and bigT.innerTable are different tables now.
OK, sorry for the double answer - the previous answer is still good, although it's an answer to a different question.
I spotted the problem in your code, it's the tableMerge function as expected:
if type(v) == "table" then
if type(t1[k] or false) == "table" then
tableMerge(t1[k] or {}, t2[k] or {})
else
t1[k] = v -- this is the problematic line
end
else
So, if t1[k] is not a table, you just assign v to it and you end up with a reference to v instead of a copy. This effectively makes your tableCopy function shallow copy instead of deep copy. Since you are overwriting t1[k] anyway, this seems like a good implementation of the tableMerge function:
function tableMerge(t1, t2)
for k,v in pairs(t2) do
if type(v) == "table" then
if type(t1[k]) ~= "table" then -- if t1[k] is not a table, make it one
t1[k] = {}
end
tableMerge(t1[k], t2[k])
else
t1[k] = v
end
end
return t1
end
This should hopefully fix the problem at hand.
Some other random thoughts on your code, if you don't mind:
in the original implementation of mergeTable: type(t1[k] or false). The t1[k] or false trickery is pointless here, type() can handle nil values just fine (it will return "nil" for nil)
t2[k] can never be nil. Tables in Lua cannot contain nil as a value, pairs() will never return a pair with nil as key or value. The t2[k] or false is again kind of pointless.
I spotted this line near the end of the first script: table.remove(v.Weapons). I'm guessing you need second parameter for the call to table.remove
Oh, and one final piece of advice: don't just drop 200 lines of code and expect people to debug it for you - no one will bother. Try to narrow down the problematic code. You already suspected the tableMerge function, if you had taken it out by itself and ran a simple test on it you would have spotted the problem.
Happy coding!

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