I've found this function to remove duplicate values in linked list:
public static void deleteDups (LinkedListNode n){
Hashtable table = new Hashtable();
LinkedListNode previous = null;
while(n!=null){
if(table.containsKey(n.data)){
previous.next = n.next;
} else {
table.put(n.data, true);
previous = n;
}
n = n.next;
}
}
Why is better copy the element in an hash table and not to another structure like a different linked list?
Thanks
Because checking for the existence of an item is an O(N) operation in a linked-list, however it is O(1) for the hash-table. Performance is the reason.
if(table.containsKey(n.data))
this is where the current item is checked if it is seen before (a duplicate) and that operation would be costly when implemented via a linked-list.
Related
I'm looking for the fastest way to merge two unsorted collections based on a common id key.
Below O(N^2) implementation
for (Person per : pers) {
for (Data data : datas) {
if (per.getId().equals(data.getId())) {
per.getData().add(data);
}
}
}
I'm looking for the fastest possible way (and lowest memory footprint possible) to achieve this result, possibly O(N). Duplicates should be removed from per.getData(). For now, per.getData() is a HashSet
Any idea how this could be optimized ? I'm using java 11
Do one pass over persons to collect into a map for later O(1) lookup, then do one pass over data adding it to person:
Map<Object, Person> people = pers.stream()
.collect(Collectors.toMap(Person::getId, p -> p));
datas.forEach(d -> people.get(d.getId()).add(d));
If it’s possible for a data to have a matching person, filter out unmatched data:
datas.stream()
.filter(d -> people.containsKey(d.getId()))
.forEach(d -> people.get(d.getId()).add(d));
Both ways are O(m+n) (m people, n datas), because all map operations are O(1).
You mentioned that duplicates should be removed from person’s data. Being a HashSet (or any kind of Set), duplicates are automatically removed if equals() and hashCode() are coded properly for Data.
Here's a linear approach (O(n)) that is better than O(n^2) but will use memory.
Create a HashMap<personId, personObject> then loop on persons and insert them into the map.
Loop on Datas and check if the dataId is present in the HashMap. If it exists, get the personObject and add the dataObject to its HashSet.
HashMap<Integer, Person> mp = new HashMap<>();
for (Person per : pers) {
mp.put(per.getId(), per);
}
for (Data data : datas) {
if (mp.get(data.getId()) != null) {
Person person = mp.get(data.getId());
person.getData().add(data);
mp.put(person.getId(), person);
}
}
Please note that I am assuming that you're using Integers as Ids. You can change the code to suit your case.
Hello I am new to dart and trying to find an item by property name in a list of list.
class Product{
String id;
String title;
Product(this.id,this.title);
}
void main(){
List<List<Product>> allProdcuts=[
//New Prodcuts
[
Product("1","Hammer"),
Product("3","Nails"),
Product("2","Screws"),
],
futureItems,
//Old Prodcuts
[
Product("4","Rock"),
Product("5","Paper"),
Product("6","Scissor"),
],
//Rare Items
[
Product("7","Plank"),
Product("8","Wires"),
Product("9","Box"),
],
];
print(allProdcuts.where((itemsList)=>itemsList.contains((product)=>product.title='Wires')));
//Returns ()
}
I have tried using for a single List:
List<Product> futureItems= [
Product("101","Galactic Hammer"),
Product("301","Galactic Nails"),
Product("201","Galactic Screws"),
];
print(newProduct.firstWhere((p)=>p.title=='Hammer'));
//Instance of 'Product'
Also tried this:
print(allProdcuts.map((itemList)=>itemList.firstWhere((p)=>p.title=='Nails')));
// Bad state: No elementError: Bad state: No element.
But there is an element with the title='Nails'.I don't understand what I am doing wrong.
You are calling itemList.firstWhere((p)=>p.title=='Nails') on each list, also the ones with no element with title "Nails". Since firstWhere throws if there is no matching value, it does that for two of your three lists. Also, in the example, itemsList.contains(...) does not take a callback, so you are just checking whether a function is in the list, which it isn't. You might want to use any for that, but it won't solve the problem here.
To do this efficiently, I'd probably create helper function:
Product findByTitle(List<List<Product>> allProducts, String title) {
for (var products in allProducts) {
for (var product in products) {
if (product.title == title) return product;
}
}
// Or return `null`.
throw ArgumentError.value(title, "title", "No element with that title");
}
The return in the middle allows you to skip out of the double iteration the moment you have a match, something which is harder to do with firstWhere/map/forEach etc.
One alternative solutions would be:
var product = allProducts.expand((l) => l.where((p) => p.title == title)).first;
which finds all the products with the given title and flattens them into a single iterable, then picks the first one (if there are any). Because iterables are lazy, it will actually stop at the first match.
There are many ways to solve this.
One example is to use the forEach() method:
allProdcuts.forEach(
(List<Product> l)=>l.forEach(
(Product p){
if (p.title=="Nails")
print(p.id);
}
)
);
The for each method receives a function and applies this function to every element on the list. If you have a lists of lists, you can do this twice to get a function applied to each element of the sub lists.
The above code prints 3, which is the desired result.
Another solution would be to flatten the list first, so you can have an easier search later.
print(allProdcuts.any((innerListOfProducts) =>
innerListOfProducts.any((product) => product.title == 'Wires')));
This code will return true if 'Wires' is in the inner list, and false otherwise.
I am trying to use a HashMap of Lists of strings in Vala, but it seems the object lifecycle is biting me. Here is my current code:
public class MyClass : CodeVisitor {
GLib.HashTable<string, GLib.List<string>> generic_classes = new GLib.HashTable<string, GLib.List<string>> (str_hash, str_equal);
public override void visit_data_type(DataType d) {
string c = ...
string s = ...
if (! this.generic_classes.contains(c)) {
this.generic_classes.insert(c, new GLib.List<string>());
}
unowned GLib.List<string> l = this.generic_classes.lookup(c);
bool is_dup = false;
foreach(unowned string ss in l) {
if (s == ss) {
is_dup = true;
}
}
if ( ! is_dup) {
l.append(s);
}
}
Note that I am adding a string value into the list associated with a string key. If the list doesn't exist, I create it.
Lets say I run the code with the same values of c and s three times. Based some printf debugging, it seems that only one list is created, yet each time it is empty. I'd expect the list of have size 0, then 1, and then 1. Am I doing something wrong when it comes to the Vala memory management/reference counting?
GLib.List is the problem here. Most operations on GLib.List and GLib.SList return a modified pointer, but the value in the hash table isn't modified. It's a bit hard to explain why that is a problem, and why GLib works that way, without diving down into the C. You have a few choices here.
Use one of the containers in libgee which support multiple values with the same key, like Gee.MultiMap. If you're working on something in the Vala compiler (which I'm guessing you are, as you're subclassing CodeVisitor), this isn't an option because the internal copy of gee Vala ships with doesn't include MultiMap.
Replace the GLib.List instances in the hash table. Unfortunately this is likely going to mean copying the whole list every time, and even then getting the memory management right would be a bit tricky, so I would avoid it if I were you.
Use something other than GLib.List. This is the way I would go if I were you.
Edit: I recently added GLib.GenericSet to Vala as an alternative binding for GHashTable, so the best solution now would be to use GLib.HashTable<string, GLib.GenericSet<string>>, assuming you're okay with depending on vala >= 0.26.
If I were you, I would use GLib.HashTable<string, GLib.HashTable<unowned string, string>>:
private static int main (string[] args) {
GLib.HashTable<string, GLib.HashTable<unowned string, string>> generic_classes =
new GLib.HashTable<string, GLib.HashTable<unowned string, string>> (GLib.str_hash, GLib.str_equal);
for (int i = 0 ; i < 3 ; i++) {
string c = "foo";
string s = i.to_string ();
unowned GLib.HashTable<unowned string, string>? inner_set = generic_classes[c];
stdout.printf ("Inserting <%s, %s>, ", c, s);
if (inner_set == null) {
var v = new GLib.HashTable<unowned string, string> (GLib.str_hash, GLib.str_equal);
inner_set = v;
generic_classes.insert ((owned) c, (owned) v);
}
inner_set.insert (s, (owned) s);
stdout.printf ("container now holds:\n");
generic_classes.foreach ((k, v) => {
stdout.printf ("\t%s:\n", k);
v.foreach ((ik, iv) => {
stdout.printf ("\t\t%s\n", iv);
});
});
}
return 0;
}
It may seem hackish to have a hash table with the key and value having the same value, but this is actually a common pattern in C as well, and specifically supported by GLib's hash table implementation.
Moral of the story: don't use GLib.List or GLib.SList unless you really know what you're doing, and even then it's generally best to avoid them. TBH we probably would have marked them as deprecated in Vala long ago if it weren't for the fact that they're very common when working with C APIs.
Vala's new can be a little weird when used as a parameter. I would recommend assigning the new list to a temporary, adding it to the list, then letting it go out of scope.
I would also recommend using libgee. It has better handling of generics than GLib.List and GLib.HashTable.
Below is the code in question. I receive Object reference not set to an instance of an object. on the where clause inside the Linq query. However, this only happens after it goes through and builds my viewpage.
Meaning: If I step through using debugger, I can watch it pull the correct order I am filtering for, go to the correct ViewPage, fill in the model/table with the correct filtered item, and THEN it comes back to my Controller and shows me the error.
public ActionResult OrderIndex(string searchBy, string search)
{
var orders = repositoryOrder.GetOpenOrderList();
if (Request.QueryString["FilterOrderNumber"] != null)
{
var ordersFiltered = from n in orders
where n.OrderNumber.ToUpper().Contains(Request.QueryString["FilterOrderNumber"].ToUpper().ToString())
select n;
return View(ordersFiltered);
}
return View(orders);
}
its always better to manipulate your strings and other things outside the linq query ,
please refer : http://msdn.microsoft.com/en-us/library/bb738550.aspx
from the readability point of view also its not good ,
public ActionResult OrderIndex(string searchBy, string search)
{
var orders = repositoryOrder.GetOpenOrderList();
var orderNumber = Request.QueryString["FilterOrderNumber"];
if (!string.IsNullOrEmpty(orderNumber))
{
orderNumber = orderNumber.ToUpper();
var ordersFiltered = from n in orders
where n.OrderNumber.ToUpper().Contains(orderNumber)
select n;
return View(ordersFiltered);
}
return View(orders);
}
Your query is not being executed in your Action method because you don't have a ToList (or equivalent) added to your query. When your code returns, your query will be enumerated somewhere in your view and that's the point where the error occurs.
Try adding ToList to your query like this to force query execution in your action method:
var ordersFiltered = (from n in orders
where n.OrderNumber.ToUpper().Contains(Request.QueryString["FilterOrderNumber"].ToUpper().ToString())
select n).ToList();
What's going wrong is that a part of your where clause is null. This could be your query string parameter. Try moving the Request.QueryString part out of your query and into a temporary variable. If that's not the case make sure that your orders have an OrderNumber.
You both were right. Just separately.
This fixed my problem
var ordersFiltered = (from n in orders
where !string.IsNullOrEmpty(n.OrderNumber) && n.OrderNumber.ToUpper().Contains(Request.QueryString["FilterOrderNumber"].ToUpper().ToString())
select n);
I have an arraylist with few duplicate items. I need to know the count of each duplicated item. I am using 2.0 so cannot use linq.
I had posted a similar question earlier, but my question was not clear.
Thanks
Prady
I've done something in the past. My solution was to loop through the ArrayList and store the counts in a dictionary. Then loop though the dictionary to display the results:
ArrayList list = new ArrayList();
list.Add(1);
list.Add("test");
list.Add("test");
list.Add("test");
list.Add(2);
list.Add(3);
list.Add(2);
Dictionary<Object, int> itemCount = new Dictionary<object, int>();
foreach (object o in list)
{
if (itemCount.ContainsKey(o))
itemCount[o]++;
else
itemCount.Add(o, 1);
}
foreach (KeyValuePair<Object, int> item in itemCount)
{
if (item.Value > 1)
Console.WriteLine(item.Key + " count: " + item.Value);
}
Output:
test count: 3
2 count: 2
Edit
Realized I used the var keyword which is not a 2.0 feature. Replaced it with KeyValuePair.
Option 1: Sort the list and then count adjacently Equal items (requires you to override the Equals method for your class)
Option 2: Use your unique identifier (however you're defining two objects to be equal) as the key for a Dictionary and add each of your objects to that entry.
i needed something similar for a project a long time ago, and made a function for it
static Dictionary<object, int> GetDuplicates(ArrayList list, out ArrayList uniqueList)
{
uniqueList = new ArrayList();
Dictionary<object, int> dups = new Dictionary<object, int>();
foreach (object o in list)
{
if (uniqueList.Contains(o))
if (!dups.ContainsKey(o))
dups.Add(o, 2);
else
dups[o]++;
else
uniqueList.Add(o);
}
return dups;
}