This question already has answers here:
How can I find a specific element in a List<T>?
(8 answers)
Closed 6 years ago.
I have a list containing the following structure.
class CompareDesignGroup
{
string FieldId;
string Caption;
}
The list is containing items of the above structure.
Is it possible to retrieve an element of the list if FieldId is known?
You can use the Find method on the generic list class. The find method takes a predicate that lets you filter/search the list for a single item.
List<CompareDesignGroup> list = // ..;
CompareDesignGroup item = list.Find(c => c.FieldId == "SomeFieldId");
item will be null if there is no matching item in the list.
If you need to find more than one item you can use the FindAll method:
List<CompareDesignGroup> list = // ..;
List<CompareDesignGroup> result= list.FindAll(c => c.FieldId == "SomeFieldId");
You can use LINQ like this:
CompareDesignGroup result = yourList.FirstOrDefault(x => x.FieldId == yourKnownId);
If you use the FirstOrDefault method the result will be null when list doesn't contain a record with a known id. So before using result check if it is not null.
There are a plethora of methods to find an item inside a list.
LINQ provides extensions method useful to work with collections that does not provide their own search features (or when you do not have the collection itself but a generic interface like IEnumerable<T>). If you have a List<CompareDesignGroup> object and you'll work on that object you can use the methods provided by that class (specialized methods are almost always faster than LINQ methods, they know collection's internal structure and does not have to rely on many abstraction layers).
In all examples I'll perform a culture invariant and case sensitive comparison for FieldId to a hypothetical id parameter. This may not be what you need and you may have to change according to your requirements.
Using List<T>
Given a list declared as:
List<CompareDesignGroup>() list = new List<CompareDesignGroup>();
To find first element that matches the search criteria (it'll return null if no items have been found):
CompareDesignGroup item = list.Find(
x => String.Equals(x.FieldId, id, StringComparison.InvariantCulture));
To find all the elements that matches the search criteria:
List<CompareDesignGroup> items = list.FindAll(
x => String.Equals(x.FieldId, id, StringComparison.InvariantCulture));
Using IEnumerable<T> (or IList<T>, for example)
Given a list declared as:
IEnumerable<CompareDesignGroup> list = ...
To find first element that matches the search criteria (null if no items have been found):
CompareDesignGroup item = list.FirstOrDefault(
x => String.Equals(x.FieldId, id, StringComparison.InvariantCulture));
To find the first element that matches the search criteria (or throw an exception if no items have been found):
CompareDesignGroup item = list.First(
x => String.Equals(x.FieldId, id, StringComparison.InvariantCulture));
To find all elements that matches the search criteria:
IEnumerable<CompareDesignGroup> item = list.Where(
x => String.Equals(x.FieldId, id, StringComparison.InvariantCulture));
There are many LINQ extensions methods, I suggest to take a look to them all to find the one that better suits your needs.
You can use Where and then you can use FirstOrDefault. That is an LINQ expression.
var ls = new List<CompareDesignGroup>();
var result = ls.Where(a => a.FieldId=="123").FirstOrDefault();
Or SingleOrDefault to get the item you want. Like this:
var ls = new List<CompareDesignGroup>();
var result = ls.Where(a => a.FieldId=="123").SingleOrDefault()
Or even simpler:
var result = ls.SingleOrDefault(a => a.FieldId=="123");
var result2 = ls.FirstOrDefault(a => a.FieldId=="123");
Yes. Use LINQ or the built-in functionalities of List.
List<CompareDesignGroup> listData = new List<CompareDesignGroup>(); // init the data
var result = listData.Where(x=> String.Equals(x.FieldID,"FIELDID KNOWN VALUE"); // gets all data
var first = listData.FirstOrDefault(x=> String.Equals(x.FieldID,"FIELDID KNOWN VALUE"); // gets first search result
Related
I have been over a few StackOverflow articles about this (this in particular)
and for some reason my case is different. I've used Tony the Lion's answer to attempt to get a list of objects that have different property values, without success. This, however does work:
List<Task> changedTasksWorking = new List<Task>();
for (int x = 0; x < originalTaskList.Count; x++)
{
if (originalTaskList[x].ActiveFlag != newTaskList[x].ActiveFlag)
{
changedTasksWorking.Add(newTaskList[x]);
}
}
The following is what I thought would provide me the same result. But where the returned list should equal 1, it instead equals zero. When I flip the property comparison to != and remove the nor condition on the inner list, I get ALL the objects of the list instead:
List<Task> notWork = oL.Where(o => newL.Any(n => o.ActiveFlag != n.ActiveFlag)).ToList();
I feel like I'm taking crazy pills. Looking at the above one-liner that should give me what I'm asking for. Perhaps I have misunderstood how the LINQ methods Where and Any are interacting.
Your proposed LINQ approach is completely different from what you seem to actually be trying to do. In particular, according to your original example, you have two lists that are exactly in sync with each other. I.e. they have the same number of elements, and each element from one list corresponds exactly to the same element in the same position in the other list.
Your LINQ code, on the other hand, looks at each element in one list at a time, and for each of those elements, searches the other list for one that has a property value that doesn't match. In other words, if the newL list has elements of all possible values of ActiveFlag then of course it will return all elements of oL, because for each element in oL, LINQ is able to find an element in newL where the property value doesn't match.
There are at least a couple of obvious alternatives using LINQ that will actually work:
Use the overload for Where() that passes the index to the predicate delegate:
List<Task> changedTasks = newTaskList
.Where((n, i) => n.ActiveFlag != originalTaskList[i].ActiveFlag).ToList();
Use Enumerable.Zip() to pair up elements in a new sequence and filter that:
List<Task> changedTasks = originalTaskList
.Zip(newTaskList, (o, n) => o.ActiveFlag != n.ActiveFlag ? n : null)
.Where(n => n != null).ToList();
Either of those should work fine.
This question already has answers here:
Linq Select All Items Matching Array
(2 answers)
Linq filter List<string> where it contains a string value from another List<string>
(4 answers)
Closed 4 years ago.
How can I create LINQ expression to find elements from collection contains names from string array?
string[] names = ["John", "Hanna", "Bill", "Donald"];
I've created expression like below but it is not correct. How can I fix that?
result = (x => x.CompanyEmployeeName.Contains(names));
If you want check if names contains x.CompanyEmployeeName, you'll want to use:
result = something.Where(x => names.Contains(x.CompanyEmployeeName));
let myCollection be the collection of a custom class having a property Name. you have to get all objects from that collection based on the condition that object's name should be available in the names array. Then You can try this:
var filteredItems = myCollection.Where(x=> names.Any(y=>y == x.Name));
I have added a working example here
In your LINQ, you should have a collection first.
Ex: if you have a list: listCompanyEmployee then you can use bellow expression:
var result = listCompanyEmployee.Where(x => names.Contains(x.CompanyEmployeeName));
You need to reverse it. Check if the names contains the employee
var result = db.CompanyEmployee.Where(x => names.Contains(x.CompanyEmployeeName));
One other option, which is prefered if the list in context are larger then your sample data, is to use Join
var result = db.CompanyEmployee.Join(names, x=> x.CompanyEmployeeName, n => n, (x,n)=> n);
You can use Array.Exists. Example if you want to check if names contains CompanyEmployeeName:
result = something.Where(x => Array.Exists(names, name => name == x.CompanyEmployeeName));
and you can use Array.IndexOf like:
result = something.Where(x => Array.IndexOf(names, x.CompanyEmployeeName) != -1);
Background: I have two Collections of different types of objects with different name properties (both strings). Objects in Collection1 have a field called Name, objects in Collection2 have a field called Field.
I needed to compare these 2 properties, and get items from Collection1 where there is not a match in Collection2 based on that string property (Collection1 will always have a greater or equal number of items. All items should have a matching item by Name/Field in Collection2 when finished).
The question: I've found answers using Lists and they have helped me a little(for what it's worth, I'm using Collections). I did find this answer which appears to be working for me, however I would like to convert what I've done from query syntax (if that's what it's called?) to a LINQ query. See below:
//Query for results. This code is what I'm specifically trying to convert.
var result = (from item in Collection1
where !Collection2.Any(x => x.ColumnName == item.FieldName)
select item).ToList();
//** Remove items in result from Collection1**
//...
I'm really not at all familiar with either syntax (working on it), but I think I generally understand what this is doing. I'm struggling trying to convert this to LINQ syntax though and I'd like to learn both of these options rather than some sort of nested loop.
End goal after I remove the query results from Collection1: Collection1.Count == Collection2 and the following is true for each item in the collection: ItemFromCollection1.Name == SomeItemFromCollection2.Field (if that makes sense...)
You can convert this to LINQ methods like this:
var result = Collection1.Where(item => !Collection2.Any(x => x.ColumnName == item.FieldName))
.ToList();
Your first query is the opposite of what you asked for. It's finding records that don't have an equivalent. The following will return all records in Collection1 where there is an equivalent:
var results=Collection1.Where(c1=>!Collection2.Any(c2=>c2.Field==c1.Name));
Please note that this isn't the fastest approach, especially if there is a large number of records in collection2. You can find ways of speeding it up through HashSets or Lookups.
if you want to get a list of non duplicate values to be retained then do the following.
List<string> listNonDup = new List<String>{"6","1","2","4","6","5","1"};
var singles = listNonDup.GroupBy(n => n)
.Where(g => g.Count() == 1)
.Select(g => g.Key).ToList();
Yields: 2, 4, 5
if you want a list of all the duplicate values then you can do the opposite
var duplicatesxx = listNonDup.GroupBy(s => s)
.SelectMany(g => g.Skip(1)).ToList();
I know this is simple, but my mind is playing tricks on me right now. If we have a flat list of objects with the properties GroupSortIndex and ItemSortIndex (within the group) and we want to find the first item in the list, what's the Linq/lambda for that?
About all I can think of is (meta, not literal code...)
var soughtItem = Source.OrderBy(ItemSortIndex).OrderBy(GroupSortIndex).ToList()[0]
...but that just looks so wrong to me for some reason.
Read post : Default Extension methods to get difference between first and firstordefault
you can use FirstOrDefualt() or First() function
var soughtItem = Source.OrderBy(ItemSortIndex).
ThenBy(GroupSortIndex).FirstOrDefualt();
if(soughtItem !=null)//advantage of using firstordefault
{
}
its better to use FirstOrDefualt because if there is no data it will return null intead of excetipn
You can use IOrderedEnumerable.ThenBy (Note: an IOrderedEnumerable is returned from IEnumerable.OrderBy):
var firstItem = source.OrderBy(s => s.GroupSortIndex)
.ThenBy(s => s.ItemSortIndex)
.First();
This orders first by the group and then by the item. You should use FirstOrDefault if the sequence can be empty. Otherwise First raises an exception.
(i've assumed that you want to order first by group and then by the item instead, since the ItemSortIndex is the index of the item within the group(as mentioned))
var soughtItem = Source
.OrderBy(ItemSortIndex)
.ThenBy(GroupSortIndex).First();
If ItemSortIndex and GroupSortIndex are properties instead of functions, then you need:
var soughtItem = Source
.OrderBy(i => ItemSortIndex)
.ThenBy(i => GroupSortIndex).First();
I have a linq query that is querying over IEnumberable. When I have a matching element for my where clause I would like to know the position of the element in the IEnumberable.
var result = from e in elements
where (string) e.Attribute("class") == "something"
select e.Position();
The e.Position() of course does not compile. The value of e.Position() would be the position of the selected element in the elements IEnumberable.
Any ideas on how to do this?
You need to use the overloaded Select method that allows for an index since that capability is not available in query syntax.
elements.Select((e, i) => new { Element = e, Index = i })
.Where(item => (string)item.Element.Attribute("class") == "something")
.Select(item => item.Index);
If you're using .NET 4.0 then you can use the (new) Zip method and write the same thing using the query syntax as well. It creates some temporary objects, so it isn't as efficient, but some people may find it more readable:
var result = from e in elements.Zip
(Enumerable.Range(0, elements.Count()), Tuple.Create)
where (string)e.Item1.Attribute("class") == "something"
select e.Item2;
It 'zips' the input collection with a generated sequence of numbers (with the same range as is the length of the collection). Then you can store the combined value either using the Tuple class (that's what I did) or you could use anonymous type.