I have a table and I'm trying to access a specific location that is passed in as a String. What is the easiest way to use the string to access the correct location?
Example, if the table looks like this:
a.b1 = true
a.b2.c1 = true
a.b2.c2 = false
a.b3 = true
How can I change a.b2.c2 to true given a location 'a.b2.c2' as a string.
If you have just a single level, you can use square-brace indexing:
function setSingle(obj, key, value)
obj[key] = value
end
setSingle(a, "b1", "foo")
print(a.b1) --> foo
If you have multiple, you need to do several iterations of this indexing. You can use a loop to do that:
function setMultiple(obj, keys, value)
for i = 1, #keys - 1 do
obj = obj[keys[i]]
end
-- Merely "obj = value" would affect only this local variable
-- (as above in the loop), rather than modify the table.
-- So the last index has to be done separately from the loop:
obj[keys[#keys]] = value
end
setMultiple(a, {"b2", "c1"}, "foo")
print(a.b2.c1) --> foo
You can use string.gmatch to parse a properly formatted list of keys. [^.]+ will match "words" made of non-period symbols:
function parseDots(str)
local keys = {}
for key in str:gmatch "[^.]+" do
table.insert(keys, key)
end
return keys
end
Putting this all together,
setMultiple(a, parseDots("b2.c2"), "foo")
print(a.b2.c2) --> foo
One issue you may run into is that you cannot create new tables with this function; you will have to create the containing table before you can create any keys in it. For example, beforing ading "b4.c3" you would have to add "b4".
You can use loadstring to build the statement you want to execute as a string.
a = { b2 = {} }
a.b1 = true
a.b2.c1 = true
a.b2.c2 = false
a.b3 = true
str = "a.b2.c2"
loadstring(str .. " = true")()
print(a.b2.c2)
Related
I'm new to lua, and I'm having trouble with a basic sort-by-bool-condition thing for entries in a table.
`local tblFormReturn = {
{
['Name'] = 'Spike',
['Year'] = '10',
['House'] = 'Holmes',
['Form Returned'] = true
},
{
['Name'] = 'Elvis',
['Year'] = '11',
['House'] = 'Shaw',
['Form Returned'] = true
},
{
['Name'] = 'Michael',
['Year'] = '10',
['House'] = 'Langley',
['Form Returned'] = false
},
{
['Name'] = 'Chang',
['Year'] = '11',
['House'] = 'Holmes',
['Form Returned'] = false
}
}`
Basically, I want to be able to take this table, and for each chunk, check whether the kid is in Holmes house (1) and if they have returned their form (2). My feeling is I need to run a for-loop in pairs based off the lua manual, but I'm confused as to how I can access these values, given each chunk is sort of a sub-table. My attempts have all been based around something like this.
for i,'Form Returned' in tblFormReturned('Form Returned') do
if 'Form Returned' == true then
if 'House' == 'Holmes' then
print ('Number of Holmes forms returned' +1)
end
end
end
I'm not sure how to make this work. Any help greatly appreciated.
A few things of note here.
When you quote something (indicated by using the single quotes), you effectively make it a string.
A for loop loops through a table using ipairs (indexed pairs, such as yours is) or pairs (used on dictionary tables). Dictionary tables are considered those that are have a defined key rather than an index key (e.g. tblPets = {dog = "Fido", cat = "Sassy", duck = "Quackers} - this would allow you to return tblPets.dog (or tblPets["dog"]) to get the value).
Your print statement to add a number does not work. You cannot add a number to a string. Instead, you will need to set a count as a variable and add to it, provided it is a number.
Lastly, you can also combine the if statements into one to make it easier.
formCount = 0 -- This initializes the variable formCount as an interger, starting with 0.
for i,v in ipairs(tblFormReturned) do -- This iterates through the table
if v["Form Returned"] and v.House == "Holmes" then -- Looks to see if the form returned is true and house is Holmes. Note that with boolean values, you do not have to see if it equals true or false. if v["Form Returned"] == true and this format returns the same answer.
formCount = formCount + 1 -- Adds 1 to the formCount
end -- end if statement
end -- end for loop
Hopefully this helps a little with understanding. If you have any questions, don't hesitate to ask for clarification.
I have a table something like this:
table = {milk, butter, cheese} -- without "Quotation marks"
I was searching for a way to check if a given value is in the table or not, and found this:
if table.hasValue(table, milk) == true then ...
but it returns nil, any reason why? (it says .hasValue is invalid) or can I get an alternative to check if value exists in that table? I tried several ways like:
if table.milk == true then ...
if table[milk] == true then ...
All of these returns nil or false.
you can try this
items = {milk=true, butter=true, cheese=true}
if items.milk then
...
end
OR
if items.butter == true then
...
end
A Lua table can act as an array or as an associative array (map).
There is no hasValue, but by using a table as an associative array you can easily implement it efficiently:
local table = {
milk = true,
butter = true,
cheese = true,
}
-- has milk?
if table.milk then
print("Has milk!")
end
if table.rocks then
print("Has rocks!")
end
You have a few options here.
One, is to create a set:
local set = {
foo = true,
bar = true,
baz = true
}
Then you check if either of these are in the table:
if set.bar then
The drawback to this approach is that you won't iterate over it in any specific order (pairs returns items in an arbitrary order).
Another option would be to use a function to check each value in a table. This'll be very slow in large tables, which brings us to back to a modification of the first option: A reverse lookup generator: (This is what I'd recommend doing -- unless your set is static)
local data = {"milk", "butter", "cheese"}
local function reverse(tbl, target)
local target = target or {}
for k, v in pairs(tbl) do
target[v] = k
end
return target
end
local revdata = reverse(data)
print(revdata.cheese, revdata.butter, revdata.milk)
-- Output: 3 2 1
This'll generate a set (with the added bonus of giving you the index where the value was in your original table). You can also put the reverse into the same table as the data was in, but this won't go well with numbers (and it'll be messy if you need to generate the reverse again).
If you write table = {milk=true, butter=true, cheese=true}, then you can use if table.milk == true then ....
I have the following string data that I receive as input:
"route1,1234,1,no~,,route2,1234,1,no~,"
It represents two "records" of data... where each record has 4 fields.
I've built code to parse this string into it's individual columns / fields.
But the part that isn't working is when I test to see if I have any duplicates in field 2. Field 2 is the one that currently has "1234" as the value.
Here's the code:
function string:split(delimiter)
local result = { }
local from = 1
local delim_from, delim_to = string.find( self, delimiter, from )
while delim_from do
table.insert( result, string.sub( self, from , delim_from-1 ) )
from = delim_to + 1
delim_from, delim_to = string.find( self, delimiter, from )
end
table.insert( result, string.sub( self, from ) )
return result
end
local check_for_duplicate_entries = function(route_data)
local route
local route_detail = {}
local result =true
local errtxt
local duplicate = false
print("received :" ..route_data)
route = string.gsub(route_data, "~,,", "~")
route = route:sub(1,string.len(route)-2)
print("route :" ..route)
-- break up in to an array
route = string.split(route,"~")
for key, value in pairs(route) do
route_detail[key] = string.split(value,",")
end
local list_of_second_column_only = {}
for key,value in pairs(route_detail) do
local temp = value[2]
print(temp .. " - is the value I'm checking for")
if list_of_second_column_only[temp] == nil then
print("i dont think it exists")
list_of_second_column_only[key] = value[2]
print(list_of_second_column_only[key])
else
--found a duplicate.
return true
end
end
return false
end
print(check_for_duplicate_entries("route1,1234,1,no~,,route2,1234,1,no~,"))
I think where I'm going wrong is the test:
if list_of_second_column_only[temp] == nil then
I think I'm checking for key with the value temp instead of a value with the value that temp contains. But I don't know how to fix the syntax.
Also, I'm wondering if there's a more efficient way to do this.
The number of "records" i receive as input is dynamic / unknown, as is the value of the second column in each record.
Thanks.
EDIT 1
The post I was trying to use as a reference is: Search for an item in a Lua list
In the answer, they show how to test for a record in the table by value, instead of looping through the entire table...
if items["orange"] then
-- do something
end
I was playing around to try and do something similar...
This should be a bit more efficient with only one table creation and less regex matching.
The match does require that you're only interested in dups in the second field.
local function check_for_duplicate_entries(route_data)
assert(type(route_data)=="string")
local field_set = {}
for route in route_data:gmatch"([^~]*)~,?,?" do
local field = route:match",([^,]*)"
if field_set[field] then
return true
else
field_set[field] = true
end
end
return false
end
Try this. It's doing the check on the value of the second field.
I haven't looked at the efficiency.
if list_of_second_column_only[value[2]] == nil then
print("i dont think it exists")
list_of_second_column_only[value[2]] = true
print(list_of_second_column_only[value[2]])
else
--found a duplicate.
return true
end
A slightly altered version of the below permitted me to filter unique field values out of a multi-dimensional table (dictionary style).
[ url ] http://rosettacode.org/wiki/Remove_duplicate_elements#Lua
items = {1,2,3,4,1,2,3,4,"bird","cat","dog","dog","bird"}`
flags = {}
io.write('Unique items are:')
for i=1,#items do
if not flags[items[i]] then
io.write(' ' .. items[i])
flags[items[i]] = true
end
end
io.write('\n')`
What I'm lost at is what 'if not ... then ... end' part actually does. To me this is sillyspeak but hey, it works ;-) Now i want to know what happens under the hood.
I hope multi-dimensional does not offend anyone, I'm referring to a table consisting of multiple row containing multiple key-value pairs on each row.
Here's the code i'm using, no brilliant adaptation but good enough to filter unique values on a fieldname
for i=1,#table,1 do
if not table2[table[i].fieldname] then
table2[table[i].fieldname] = true
end
end
for k,v in pairs(table2) do
print(k)
end
function findDuplicates(t)
seen = {} --keep record of elements we've seen
duplicated = {} --keep a record of duplicated elements
for i = 1, #t do
element = t[i]
if seen[element] then --check if we've seen the element before
duplicated[element] = true --if we have then it must be a duplicate! add to a table to keep track of this
else
seen[element] = true -- set the element to seen
end
end
return duplicated
end
The logic behind the if seen[element] then, is that we check if we've already seen the element before in the table. As if they key doesn't exist nill will be returned which is evaluated which is false (this is not the same as boolean false, there are two types of false in lua!).
You can use this function like so:
t = {'a','b','a','c','c','c','d'}
for key,_ in pairs(findDuplicates(t)) do
print(key)
end
However that function won't work with multidimensional tables, this one will however:
function findDuplicates(t)
seen = {} --keep record of elements we've seen
duplicated = {} --keep a record of duplicated elements
local function traverse(subt)
for i=1, #subt do
element = subt[i]
if type(element) == 'table' then
traverse(element)
else
if seen[element] then
duplicated[element] = true
else
seen[element] = true
end
end
end
end
traverse(t)
return duplicated
end
Example useage:
t = {'a',{'b','a'},'c',{'c',{'c'}},'d'}
for k,_ in pairs(findDuplicates(t)) do
print(k)
end
Outputs
a
c
t = {a='a',b='b',c='c',d='c',e='a',f='d'}
function findDuplicates(t)
seen = {}
duplicated = {}
for key,val in pairs(t) do
if seen[val] then
duplicated[val] = true
else
seen[val] = true
end
end
return duplicated
end
This works the same way as before but checks if the same value is associated with a different key and if so, makes note of that value as being duplicated.
Eventually this is the code which worked for me. I've been asked to post it as a separate answer so here goes.
for i=1,#table1,1 do
if not table2[table1[i].fieldname] then
table2[table1[i].fieldname] = true
end
end
for k,v in pairs(table2) do
print(k)
end
I have 2 functions in Lua which create a dictionary table and allow to check if a word exists:
local dictTable = {}
local dictTableSize = 0
function buildDictionary()
local path = system.pathForFile("wordlist.txt")
local file = io.open( path, "r")
if file then
for line in file:lines() do
dictTable[line] = true
dictTableSize = dictTableSize + 1
end
io.close(file)
end
end
function checkWord(word)
if dictTable[word] then
return(true)
else
return(false)
end
end
Now I want to be able to generate a couple of random words. But since the words are the keys, how can I pick some, given the dictTableSize.
Thanks
Just add a numerical index for each word to the dictionary while loading it:
function buildDictionary()
local path = system.pathForFile("wordlist.txt")
local file = io.open( path, "r")
if file then
local index = 1
for line in file:lines() do
dictTable[line] = true
dictTable[index] = line
index = index + 1
end
io.close(file)
end
end
Now you can get a random word like this:
function randomWord()
return dictTable[math.random(1,#dictTable)]
end
Side note: nil evaluates to false in Lua conditionals, so you could write checkWord like this:
function checkWord(word)
return dictTable[word]
end
Another side note, you'll get less polution of the global namespace if you wrap the dictionary functionality into an object:
local dictionary = { words = {} }
function dictionary:load()
local path = system.pathForFile('wordlist.txt')
local file = io.open( path, 'r')
if file then
local index = 1
for line in file:lines() do
self.words[line] = true
self.words[index] = line
index = index + 1
end
io.close(file)
end
end
function dictionary:checkWord(word)
return self.words[word]
end
function dictionary:randomWord()
return self.words[math.random(1,#self.words)]
end
Then you can say:
dictionary:load()
dictionary:checkWord('foobar')
dictionary:randomWord()
Probably two ways: you can keep the array with words and just do words[math.random(#words)] when you need to pick a random word (just make sure that the second one is different from the first).
The other way is to use next the number of times you need:
function findNth(t, n)
local val = next(t)
for i = 2, n do val = next(t, val) end
return val
end
This will return b for findNth({a = true, b = true, c = true}, 3) (the order is undefined).
You can avoid repetitive scanning by memoizing the results (at this point you will be better off using the first way).
this is a trade off that you have for using the word table the way you are. i would invert the word table once you load it, so that you can get references to words by index as well if you have to. something like this:
-- mimic your dictionary structure
local t = {
["asdf"] = true, ["wer"] = true, ["iweir"] = true, ["erer"] = true
}
-- function to invert your word table
function invert(tbl)
local t = {}
for k,_ in pairs(tbl) do
table.insert(t, k)
end
return t
end
-- now the code to grab random words
local idx1, idx2 = math.random(dictTableSize), math.random(dictTableSize)
local new_t = invert(t)
local word1, word2 = new_t[idx1], new_t[idx2]
-- word1 and word2 now have random words from your 'dictTable'