I'm manipulating entries inside a DoubleLinkedQueue via the DoubleLinkedQueueElement.append/prepend methods. This results in the new elements being inserted into the queue, but fails to update the length, and the toList() method results in an error being thrown.
I understand queues are only supposed to have elements added at the start/end, but it looks like the interface should allow for adding in the middle via the entries. I find it hard to believe such a common/well understood data structure would have a bug at this point - so am I using DoubleLinkedQueues incorrectly? Is there another data structure that I should be using? I'm looking to merge values from another iterable into my own sorted iterable - a SplayTreeSet might get me there in n log n time, but a simple merge should get me there in linear time...
Example of code that acts unexpectedly:
main() {
var q = new DoubleLinkedQueue<int>.from([1]);
q.firstEntry().prepend(0);
print('length: ${q.length}');
int i = 0;
for (var qi in q){
print('${i++}: $qi');
}
}
Output:
length: 1
0: 0
1: 1
It looks like the length getter is only pointing to an internal counter. This is done because counting the elements everytime might take very long for long lists.
The internal counter is only updated if you use the methods that directly operate on the list instead of using the prepend method of an element. In your example you should use q.addFirst(0); which results in the length being updates. The .prepend() method just inserts a new element and changes the pointer. Which results in correct traversation of the elements, but the counter is wrong anyway.
Unfortunately it looks like you cannot insert elements in the middle of the list, nor can you make the list recount the elements. You should consider creating a bug over at www.dartbug.com.
// Update:
toList() throws an error because there are more elements than length.
Related
I'm modding a game. I'd like to optimize my code if possible for a frequently called function. The function will look into a dictionary table (consisting of estimated 10-100 entries). I'm considering 2 patterns a) direct reference and b) lookup with ipairs:
PATTERN A
tableA = { ["moduleName.propertyName"] = { some stuff } } -- the key is a string with dot inside, hence the quotation marks
result = tableA["moduleName.propertyName"]
PATTERN B
function lookup(type)
local result
for i, obj in ipairs(tableB) do
if obj.type == "moduleName.propertyName" then
result = obj
break
end
end
return result
end
***
tableB = {
[1] = {
type = "moduleName.propertyName",
... some stuff ...
}
}
result = lookup("moduleName.propertyName")
Which pattern should be faster on average? I'd expect the 'native' referencing to be faster (it is certainly much neater), but maybe this is a silly assumption? I'm able to sort (to some extent) tableB in a order of frequency of the lookups whereas (as I understand it) tableA will have in Lua random internal order by default even if I declare the keys in proper order.
A lookup table will always be faster than searching a table every time.
For 100 elements that's one indexing operation compared to up to 100 loop cycles, iterator calls, conditional statements...
It is questionable though if you would experience a difference in your application with so little elements.
So if you build that data structure for this purpose only, go with a look-up table right away.
If you already have this data structure for other purposes and you just want to look something up once, traverse the table with a loop.
If you have this structure already and you need to look values up more than once, build a look up table for that purpose.
Our task is create a table, and read values to the table using a loop. Print the values after the process is complete. - Create a table. - Read the number of values to be read to the table. - Read the values to the table using a loop. - Print the values in the table using another loop. for this we had written code as
local table = {}
for value in ipairs(table) do
io.read()
end
for value in ipairs(table) do
print(value)
end
not sure where we went wrong please help us. Our exception is
Input (stdin)
3
11
22
abc
Your Output (stdout)
~ no output ~
Expected Output
11
22
abc
Correct Code is
local table1 = {}
local x = io.read()
for line in io.lines() do
table.insert(table1, line)
end
for K, value in ipairs(table1) do
print(value)
end
Let's walk through this step-by-step.
Create a table.
Though the syntax is correct, table is a reserved pre-defined global name in Lua, and thus cannot should not be declared a variable name to avoid future issues. Instead, you'll need to want to use a different name. If you're insistent on using the word table, you'll have to distinguish it from the function global table. The easiest way to do this is change it to Table, as Lua is a case-sensitive language. Therefore, your table creation should look something like:
local Table = {}
Read values to the table using a loop.
Though Table is now established as a table, your for loop is only iterating through an empty table. It seems your goal is to iterate through the io.read() instead. But io.read() is probably not what you want here, though you can utilize a repeat loop if you wish to use io.read() via table.insert. However, repeat requires a condition that must be met for it to terminate, such as the length of the table reaching a certain amount (in your example, it would be until (#Table == 4)). Since this is a task you are given, I will not provide an example, but allow you to research this method and use it to your advantage.
Print the values after the process is complete.
You are on the right track with your printing loop. However, it must be noted that iterating through a table always returns two results, an index and a value. In your code, you would only return the index number, so your output would simply return:
1
2
3
4
If you are wanting the actual values, you'll need a placeholder for the index. Oftentimes, the placeholder for an unneeded variable in Lua is the underscore (_). Modify your for loop to account for the index, and you should be set.
Try modifying your code with the suggestions I've given and see if you can figure out how to achieve your end result.
Edited:
Thanks, Piglet, for corrections on the insight! I'd forgotten table itself wasn't a function, and wasn't reserved, but still bad form to use it as a variable name whether local or global. At least, it's how I was taught, but your comment is correct!
Obviously playing with dart and still new to it and implementing tic tac toe. So I have a list of lists of winning positions and want to check if any of the lists is sublist of the inputed moves list. Sounds pretty easy right, so I apply for each element of inner list I wanna check my condition. What are the dart ways to achieve this. This isn't working because map expect function of type void and I return some calls to print for each element of every inner list obviously. Why it isn't showing me any mistake at all.
I tried with .forEach and .takeWhile examples but coudn't get it working.
List <List<int>> winMoves = new List.generate(8, (i) => new List(3));
winMoves[0]=[1,2,3];
winMoves[1]=[4,5,6];
winMoves[2]=[7,8,9];
winMoves[3]=[1,4,7];
winMoves[4]=[2,5,8];
winMoves[5]=[3,6,9];
winMoves[6]=[1,5,9];
winMoves[7]=[3,5,7];
winMoves.map((list) => (list.forEach((el) => print(el))));
import 'package:collection/collection.dart' show ListEquality;
...
const eq = const ListEquality<int>();
print(winMoves.indexWhere((list) => eq.equals(list, [1, 5, 9])));
Depending on what result you want you can also use where, firstWhere, any, ...
There are many ways to solve this problem, depending on what the exact desired result is.
You have a list of elements, and you want to check something about these elements (in this case the elements are lists too, but that's not important to begin with).
If you want to know whether any of the elements satisfies your check, you can use:
bool hasWin = winMoves.any((moves) => ...check...);
If you want to find an element which satisfies your check, you can use:
var win = winMoves.firstWhere((moves) => ...check..., orElse: null);
If you want to find all the winners:
var wins = winMoves.where((moves) => ...check...).toList(); // `where` is lazy.
How to write the check is a separate issue. In this case you seem to want, as Günther Zöchbauer has already written, something like
(moves) => moves.every(expectedMoves.contains)
Your example is not giving you any warnings or errors because it's correct, and it doesn't actually do anything. The map function is lazy, it creates an Iterable which won't do anything until you start iterating. If you call .toList() on it, it'll create a List<void>. Not very useful, but valid in Dart 2.
Also, your initial winMoves initialization is overly complex. You can just do:
List<List<int>> winMoves = []..length = 8; // for growable list, or
List<List<int>> winMoves = List(8); // for fixed-length list.
You actively fill the list with lists, then immediately overwrite those values with new lists in the following lines.
I recently started checking new Java 8 features.
I've come across this forEach iterator-which iterates over the Collection.
Let's take I've one ArrayList of type <Integer> having values= {1,2,3,4,5}
list.forEach(i -> System.out.println(i));
This statement iteates over a list and prints the values inside it.
I'd like to know How am I going to specify that I want it to iterate over some specific values only.
Like, I want it to start from 2nd value and iterate it till 2nd last value. or something like that- or on alternate elements.
How am I going to do that?
To iterate on a section of the original list, use the subList method:
list.subList(1, list.length()-1)
.stream() // This line is optional since List already has a foreach method taking a Consumer as parameter
.forEach(...);
This is the concept of streams. After one operation, the results of that operation become the input for the next.
So for your specific example, you can follow #Joni's command. But if you're asking in general, then you can create a filter to only get the values you want to loop over.
For example, if you only wanted to print the even numbers, you could create a filter on the streams before you forEached them. Like this:
List<Integer> intList = Arrays.asList(1,2,3,4,5);
intList.stream()
.filter(e -> (e & 1) == 0)
.forEach(System.out::println);
You can similarly pick out the stuff you want to loop over before reaching your terminal operation (in your case the forEach) on the stream. I suggest you read this stream tutorial to get a better idea of how they work: http://winterbe.com/posts/2014/07/31/java8-stream-tutorial-examples/
i'm using riak to store json documents right now, and i want to sort them based on some attribute, let's say there's a key, i.e
{
"someAttribute": "whatever",
"order": 1
}
so i want to sort the documents based on the "order".
I am currently retrieving the documents in riak with the erlang interface. i can retrieve the document back as a string, but i dont' really know what to do after that. i'm thinking the map function just reduces the json document itself, and in the reduce function, i'd make a check to see whether the item i'm looking at has a higher "order" than the head of the rest of the list, and if so append to beginning, and then return a lists:reverse.
despite my ideas above i've had zero results after almost an entire day, i'm so confused with the erlang interface in riak. can someone provide insight on how to write this map/reduce function, or just how to parse the json document?
As far as I know, You do not have access to Input list in Map. You emit from Map a document as 1 element list.
Inputs (all the docs to handle as {Bucket, Key}) -> Map (handle single doc) -> Reduce (whole list emitted from Map).
Maps are executed per each doc on many nodes whereas Reduce is done once on so called coordinator node (the one where query was called).
Solution:
Define Inputs (as a list or bucket)
Retrieve Value in Map and emit whole doc or {Id, Val_to_sort_by)
Sort in Reduce (using regular list:keysort)
This is not a map reduce solution but you should check out Riak Search.
so i "solved" the problem using javascript, still can't do it using erlang.
here is my query
{"inputs":"test",
"query":[{"map":{"language":"javascript",
"source":"function(value, keyData, arg){ var data = Riak.mapValuesJson(value)[0]; var obj = {}; obj[data.order] = data; return [ obj ];}"}},
{"reduce":{"language":"javascript",
"source":"function(values, arg){ return [ values.reduce(function(acc, item){ for(var order in item){ acc[order] = item[order]; } return acc; }) ];}",
"keep":true}}
]
}
so in the map phase, all i do is create a new array, obj, with the key as the order, and the value as the data itself. so visually, the obj is like this
{"1":{"firstName":"John","order":1}
in the reduce phase, i'm just putting it in the accumulator, so basically that's the sort if you think about it, because when you're done, everything will be put in order for you. so i put 2 json documents for testing, one is above, the ohter is just firstName: Billie, order 2. and here is my result for the query above
[{"1":{"firstName":"John","order":1},"2":{"firstName":"Billie","order":2}}]
so it works! . but i still need to do this in ERLANG, any insights?