foreach(var item in myObservalbleCollection)
{
//samoe coding
}
Above code will loop the items in the observable class from the beginning. I want to loop the this object from the last Item
ex 9,8,7,6,5,4,3,2,1,0 instead of 1,2,3,4,5,6,7,8,9...
Please give me a solution to do this in c#
Instead of using foreach you can use a for loop running from the end of the collection and to the start:
for (var i = myObservableCollection.Count - 1; i >= 0; i -= 1) {
var item = myObservableCollection[i];
// Process item
}
This is possible because ObservableCollection<T> implement IList<T> which provides both the count of the elements in the collection and indexed access to these elements in addition to the forward teration provided by IEnumerable<T> used by foreach. This is the most efficient solution.
You can also use LINQ Reverse which is available for any collection implementing IEnumerable<T>:
foreach (var item in myObservableCollection.Reverse()) {
// Process item
}
This will actually copy all the elements in the collection to a new array before iterating them in reverse order. While less efficient than the first solution it should not matter in most cases.
You could reverse the observablecollection and do the loop
var myObservalbleCollection = new ObservableCollection<YourType>(collection.Reverse());
foreach(var item in myObservalbleCollection)
{
//some coding
}
Related
For now, the best I could think of is:
bool oneMoreTime = true;
while (oneMoreTime)
{
ItemType toDelete=null;
oneMoreTime=false;
foreach (ItemType item in collection)
{
if (ShouldBeDeleted(item))
{
toDelete=item;
break;
}
}
if (toDelete!=null)
{
collection.Remove(toDelete);
oneMoreTime=true;
}
}
I know that I have at least one extra variable here, but I included it to improve the readability of the algorithm.
The "RemoveAll" method is best.
Another common technique is:
var itemsToBeDeleted = collection.Where(i=>ShouldBeDeleted(i)).ToList();
foreach(var itemToBeDeleted in itemsToBeDeleted)
collection.Remove(itemToBeDeleted);
Another common technique is to use a "for" loop, but make sure you go backwards:
for (int i = collection.Count - 1; i >= 0; --i)
if (ShouldBeDeleted(collection[i]))
collection.RemoveAt(i);
Another common technique is to add the items that are not being removed to a new collection:
var newCollection = new List<whatever>();
foreach(var item in collection.Where(i=>!ShouldBeDeleted(i))
newCollection.Add(item);
And now you have two collections. A technique I particularly like if you want to end up with two collections is to use immutable data structures. With an immutable data structure, "removing" an item does not change the data structure; it gives you back a new data structure (that re-uses bits from the old one, if possible) that does not have the item you removed. With immutable data structures you are not modifying the thing you're iterating over, so there's no problem:
var newCollection = oldCollection;
foreach(var item in oldCollection.Where(i=>ShouldBeDeleted(i))
newCollection = newCollection.Remove(item);
or
var newCollection = ImmutableCollection<whatever>.Empty;
foreach(var item in oldCollection.Where(i=>!ShouldBeDeleted(i))
newCollection = newCollection.Add(item);
And when you're done, you have two collections. The new one has the items removed, the old one is the same as it ever was.
Just as I finished typing I remembered that there is lambda-way to do it.
collection.RemoveAll(i=>ShouldBeDeleted(i));
Better way?
A forward variation on the backward for loop:
for (int i = 0; i < collection.Count; )
if (ShouldBeDeleted(collection[i]))
collection.RemoveAt(i)
else
i++;
You cannot delete from a collection inside a foreach loop (unless it is a very special collection having a special enumerator). The BCL collections will throw exceptions if the collection is modified while it is being enumerated.
You could use a for loop to delete individual elements and adjust the index accordingly. However, doing that can be error prone. Depending on the implementation of the underlying collection it may also be expensive to delete individual elements. For instance deleting the first element of a List<T> will copy all the remaning elements in the list.
The best solution is often to create a new collection based on the old:
var newCollection = collection.Where(item => !ShouldBeDeleted(item)).ToList();
Use ToList() or ToArray() to create the new collection or initialize your specific collection type from the IEnumerable returned by the Where() clause.
The lambda way is good. You could also use a regular for loop, you can iterate lists that a for loop uses within the loop itself, unlike a foreach loop.
for (int i = collection.Count-1; i >= 0; i--)
{
if(ShouldBeDeleted(collection[i])
collection.RemoveAt(i);
}
I am assuming that collection is an arraylist here, the code might be a bit different if you are using a different data structure.
Scenario
I have a system that holds races, each race has a unique list of members on that race. (the list is a List< T > )
I want users to be able to remove a member (if they ARE this member) from the list of members on that race.
Problem
I'm trying to get the following code to work:
foreach (string item in hillracing.searchRaces(RaceID).RaceList) // Loop through List with foreach.
{
if (item == SelectedItem)
{
item = null;
}
}
I can't edit the variable because it is in a foreach loop, how would I achieve this another way?
You can just store it and remove it form the collection afterwards.
var toRemove = null;
foreach (string item in hillracing.searchRaces(RaceID).RaceList) // Loop through List with foreach.
{
if (item == SelectedItem)
{
toRemove = item;
break; //Can break here if you're sure there's only one SelectedItem
}
}
hillracing.searchRaces(RaceID).Racelist.Remove(toRemove);
though in this case you could also just use hillracing.searchRaces(RaceID).Racelist.Remove(SelectedItem); and you won't use the foreach loop at all.
You can't modify collection that you are looping using foreach loop. The collection used in foreach is immutable. This is by design.
The foreach statement is used to iterate through the collection to get
the information that you want, but can not be used to add or remove
items from the source collection to avoid unpredictable side effects.
If you need to add or remove items from the source collection, use a
for loop.
Using Linq you shouldn't need to loop to find the entry you want to nullify...
// Use of Single() here assumes the object definitely exists.
// Use SingleOrDefaul() if there is a chance it might not exist.
var item = hillracing.searchRaces(RaceID)
.RaceList
.Where(x => x.Item == SelectedItem).Single();
item = null;
Edit: Since you've changed the requirement to remove the item from the list, I think you'd just call the Remove method`with the found item. So the code becomes
// Use of Single() here assumes the object definitely exists.
// Use SingleOrDefaul() if there is a chance it might not exist.
var item = hillracing.searchRaces(RaceID)
.RaceList
.Where(x => x.Item == SelectedItem).Single();
hillracing.searchRaces(RaceID).RaceList.Remove(item);
You can't do that in a foreach loop. If it's an IList/IList<T> which allows random access, like an array or list, you can use a for-loop:
List<string> = hillracing.searchRaces(RaceID).RaceList;
for(int i = 0; i < list.Count; i++)
{
if(list[i] == SelectedItem)
list[i] = null;
}
So you can't add or remove items in the foreach but you also can't replace the reference. The object refers to the original value so you could modify the object(if strings werent immutable) but you can't replace the reference itself in a foreach. This is related.
Use the existing Remove()-method to search and remove the item for you:
hillracing.searchRaces(RaceID).RaceList.Remove(SelectedItem);
I am trying to remove object while I am iterating through Collection. But I am getting exception. How can I achieve this?
Here is my code :
foreach (var gem in gems)
{
gem.Value.Update(gameTime);
if (gem.Value.BoundingCircle.Intersects(Player.BoundingRectangle))
{
gems.Remove(gem.Key); // I can't do this here, then How can I do?
OnGemCollected(gem.Value, Player);
}
}
foreach is designed for iterating over a collection without modifing it.
To remove items from a collection while iterating over it use a for loop from the end to the start of it.
for(int i = gems.Count - 1; i >=0 ; i--)
{
gems[i].Value.Update(gameTime);
if (gems[i].Value.BoundingCircle.Intersects(Player.BoundingRectangle))
{
Gem gem = gems[i];
gems.RemoveAt(i); // Assuming it's a List<Gem>
OnGemCollected(gem.Value, Player);
}
}
If it's a dictionary<string, Gem> for example, you could iterate like this:
foreach(string s in gems.Keys.ToList())
{
if(gems[s].BoundingCircle.Intersects(Player.BoundingRectangle))
{
gems.Remove(s);
}
}
The easiest way is to do what #IV4 suggested:
foreach (var gem in gems.ToList())
The ToList() will convert the Dictionary to a list of KeyValuePair, so it will work fine.
The only time you wouldn't want to do it that way is if you have a big dictionary from which you are only removing relatively few items and you want to reduce memory use.
Only in that case would you want to use one of the following approaches:
Make a list of the keys as you find them, then have a separate loop to remove the items:
List<KeyType> keysToRemove = new List<KeyType>();
foreach (var gem in gems)
{
gem.Value.Update(gameTime);
if (gem.Value.BoundingCircle.Intersects(Player.BoundingRectangle))
{
OnGemCollected(gem.Value, Player);
keysToRemove.Add(gem.Key);
}
}
foreach (var key in keysToRemove)
gems.Remove(key);
(Where KeyType is the type of key you're using. Substitute the correct type!)
Alternatively, if it is important that the gem is removed before calling OnGemCollected(), then (with key type TKey and value type TValue) do it like this:
var itemsToRemove = new List<KeyValuePair<TKey, TValue>>();
foreach (var gem in gems)
{
gem.Value.Update(gameTime);
if (gem.Value.BoundingCircle.Intersects(Player.BoundingRectangle))
itemsToRemove.Add(gem);
}
foreach (var item in itemsToRemove)
{
gems.Remove(item.Key);
OnGemCollected(item.Value, Player);
}
As the other answers say, a foreach is designed purely for iterating over a collection without modifying it as per the documenation:
The foreach statement is used to iterate through the collection to get
the desired information, but should not be used to change the contents
of the collection to avoid unpredictable side effects.
in order to do this you would need to use a for loop (storing the items of the collection you need to remove) and remove them from the collection afterwards.
However if you are using a List<T> you could do this:
lines.RemoveAll(line => line.FullfilsCertainConditions());
After going through all the answers, and being equally good. I faced a challenge where I had to modify a List and what I ended up doing worked quite well for me. So just in case anyone finds it useful. Can someone provide me feedback on how efficient it might be.
Action removeFromList;
foreach(var value in listOfValues){
if(whatever condition to remove is){
removeFromList+=()=>listOfValues.remove(value);
}
}
removeFromList?.Invoke();
removeFromList = null;
You should use the for loop instead of the foreach loop. Please refer here
Collections support foreach statement using Enumarator. Enumerators can be used to read the data in the collection, but they cannot be used to modify the underlying collection. If changes are made to the collection, such as adding, modifying, or deleting elements, the enumerator is irrecoverably invalidated and the next call to MoveNext or Reset throws an InvalidOperationException.
Use for loop for collection modifying.
i want to know what is the best way to delete elements from a List in c# while doing a foreach.
here is a code sample. first i create a list with some elements and then delte one:
List<int> foo = new List<int>();
foo.Add(1);
foo.Add(2);
foo.Add(3);
foreach (int i in foo)
{
if (i==2)
{
foo.Remove(i);
}
}
when i run this, i get a InvalidOperationException but how to solve this with a performant way?
If you must remove entries while enumerating, walk the list in backward direction, and remove items that you need to remove.
for (var i = foo.Count-1 ; i >= 0 ; i--) {
if (MustBeRemoved(foo[i])) {
foo.RemoveAt(i);
}
}
Note that this is not required in case of your post, where you know the values that need to be removed.
foo.RemoveAll(x => x == 2);
In case you decide not to use for and foreach ;)
I assume your actual use case is more complicated than what you have laid out. So let's assume you actually have some condition at play that applies to each element and that multiple elements can satisfy. We'll call that a predicate.
List<T> exposes a RemoveAll method that allows you to supply a predicate. Any item that matches that predicate is then removed. For example
Func<int, bool> isEven = i => i % 2 == 0;
List<int> ints = ...
ints.RemoveAll(item => isEven(item));
// ints will only contain odd numbers
Other approaches to consider would be walking over the list backwards in a for loop and removing by index, building a second list containing the items to delete, and then in a second loop over the second list, remove items from the first. Or you could just write a query to construct a new sequence containing the items that you wish to keep.
Why do you need a loop?
foo.Remove(2);
I think the best way is to iterate backwards using a simple for loop.
for(int i = foo.Count-1; i>=0; i--)
if(foo[i]==2) foo.RemoveAt(i);
change your foreach as below
foreach (int i in new List<int>(foo))
If you want to remove arbitrary elements, and not just one, you can use RemoveAll and specify a predicate:
foo.RemoveAll(element => (element == 2));
You can add the items that you want to remove to a temporary list, then remove them after the loop:
List<int> foo = new List<int>();
foo.Add(1);
foo.Add(2);
foo.Add(3);
List<int> remove = new List<int>();
foreach (int i in foo) {
if (i==2) {
remove.Add(i);
}
}
foreach (int i in remove) {
foo.Remove(i);
}
You can't edit a list while you're iterating it.
Consider:
List<int> foo;
int[] bar = foo.ToArray();
foreach(int i in bar)
{
if (i == 2)
{
foo.Remove(i);
}
}
But beware: you should walk this list backwards, because removing an item from the foo list will mean the bar list no longer aligns with it. (If you don't walk backwards, you'll have to keep track of the count of removals and adjust the index passed to the remove call!)
I have a loop that iterates through elements in a list. I am required to remove elements from this list within the loop based on certain conditions. When I try to do this in C#, I get an exception. apparently, it is not allowed to remove elements from the list which is being iterated through. The problem was observed with a foreach loop. Is there any standard way to get around this problem?
Note : One solution I could think of is to create a copy of the list solely for iteration purpose and to remove elements from the original list within the loop. I am looking for a better way of dealing with this.
When using List<T> the ToArray() method helps in this scenario vastly:
List<MyClass> items = new List<MyClass>();
foreach (MyClass item in items.ToArray())
{
if (/* condition */) items.Remove(item);
}
The alternative is to use a for loop instead of a foreach, but then you have to decrement the index variable whenever you remove an element i.e.
List<MyClass> items = new List<MyClass>();
for (int i = 0; i < items.Count; i++)
{
if (/* condition */)
{
items.RemoveAt(i);
i--;
}
}
If your list is an actual List<T> then you can use the built-in RemoveAll method to delete items based on a predicate:
int numberOfItemsRemoved = yourList.RemoveAll(x => ShouldThisItemBeDeleted(x));
You could use LINQ to replace the initial list by a new list by filtering out items:
IEnumerable<Foo> initialList = FetchList();
initialList = initialList.Where(x => SomeFilteringConditionOnElement(x));
// Now initialList will be filtered according to the condition
// The filtered elements will be subject to garbage collection
This way you don't have to worry about loops.
You can use integer indexing to remove items:
List<int> xs = new List<int> { 1, 2, 3, 4 };
for (int i = 0; i < xs.Count; ++i)
{
// Remove even numbers.
if (xs[i] % 2 == 0)
{
xs.RemoveAt(i);
--i;
}
}
This can be weird to read and tough to maintain, though, especially if the logic in the loop gets any more complex.
Another trick is to loop through the list backwards.. removing an item won't affect any of the items you are going to encounter in the rest of the loop.
I'm not recommending this or anything else though. Everything you need this for can probably be done using LINQ statements to filter the list on your requirements.
You can iterate with foreach this way:
List<Customer> custList = Customer.Populate();
foreach (var cust in custList.ToList())
{
custList.Remove(cust);
}
Note: ToList on the list of variables, this iterates through the list created by the ToList but removes the items from the original list.
Hope this helps.
The recommended solution is to put all your elements you want to remove in a separate list and after the first loop, put a second loop where you iterate over the remove-list and remove those elements form the first list.
The reason you get an error is because you're using a foreach loop. If you think about how a foreach loop works this makes sense. The foreach loop calls the GetEnumerator method on the List. If you where to change the number of elements in the List, the Enumerator the foreach loop holds wouldn't have the correct number of elements. If you removed an element a null exception error would be thrown, and if you added an element the loop would miss an item.
If you like Linq and Lamda expressions I would recommend Darin Dimitrov solution, otherwise I would use the solution provided by Chris Schmich.