I have below code -
var refNosToOrder = new int[9] {1,2,3,4,5,6,7,8,9}
var orderedList = lst.OrderBy(x=>x.RefNo==7)
.ThenBy(y=> refNosToOrder.Contains(y.RefNo)).ToList();
lst is list of class object containing int property - RefNo : i.e. List<SampleClass>
class SampleClass
{
public int RefNo {get;set;}
}
lst contains all the unsorted data of RefNo:
lst = 2,4,6,9,7,5,8,1,3
What I want to do -
First I want to order lst by keeping first element as - 7; then for the rest of the list, it should be ordered as the array refNosToOrder
i.e. Final output I am expecting to be -
7,1,2,3,4,5,6,8,9
With the above code -
var orderedList = lst.OrderBy(x=>x.RefNo==7)
.ThenBy(y=> refNosToOrder.Contains(y.RefNo)).ToList();
It is giving - 2,4,6,9,7,5,8,1,3 i.e. this code is not at all ordering the list.
Contains returns a boolean of whether an element is in a list or not, which won't be very helpful here. Instead, you could sort by the index of that element:
var orderedList =
lst.OrderBy(x => x.RefNo != 7)
.ThenBy(y => Array.IndexOf(refNosToOrder, y.RefNo))
.ToList();
EDIT:
Following up on Jeroen Mostert's comment, this sorting has quadratic complexity. For large refNosToOrder it may be more efficient to first convert the array to a dictionary of orders and then use it for the sorting:
var orderDict =
Enumerable.Range(0, refNosToOrder.Length).ToDictionary(i => refNosToOrder[i]);
var orderedList =
lst.OrderBy(x => x.RefNo != 7).ThenBy(y => orderDict[y.RefNo]).ToList();
Related
I have a list of parameters like this:
public class parameter
{
public string name {get; set;}
public string paramtype {get; set;}
public string source {get; set;}
}
IEnumerable<Parameter> parameters;
And a array of strings i want to check it against.
string[] myStrings = new string[] { "one", "two"};
I want to iterate over the parameter list and check if the source property is equal to any of the myStrings array. I can do this with nested foreach's but i would like to learn how to do it in a nicer way as i have been playing around with linq and like the extension methods on enumerable like where etc so nested foreachs just feel wrong. Is there a more elegant preferred linq/lambda/delegete way to do this.
Thanks
You could use a nested Any() for this check which is available on any Enumerable:
bool hasMatch = myStrings.Any(x => parameters.Any(y => y.source == x));
Faster performing on larger collections would be to project parameters to source and then use Intersect which internally uses a HashSet<T> so instead of O(n^2) for the first approach (the equivalent of two nested loops) you can do the check in O(n) :
bool hasMatch = parameters.Select(x => x.source)
.Intersect(myStrings)
.Any();
Also as a side comment you should capitalize your class names and property names to conform with the C# style guidelines.
Here is a sample to find if there are match elements in another list
List<int> nums1 = new List<int> { 2, 4, 6, 8, 10 };
List<int> nums2 = new List<int> { 1, 3, 6, 9, 12};
if (nums1.Any(x => nums2.Any(y => y == x)))
{
Console.WriteLine("There are equal elements");
}
else
{
Console.WriteLine("No Match Found!");
}
If both the list are too big and when we use lamda expression then it will take a long time to fetch . Better to use linq in this case to fetch parameters list:
var items = (from x in parameters
join y in myStrings on x.Source equals y
select x)
.ToList();
list1.Select(l1 => l1.Id).Intersect(list2.Select(l2 => l2.Id)).ToList();
var list1 = await _service1.GetAll();
var list2 = await _service2.GetAll();
// Create a list of Ids from list1
var list1_Ids = list1.Select(l => l.Id).ToList();
// filter list2 according to list1 Ids
var list2 = list2.Where(l => list1_Ids.Contains(l.Id)).ToList();
I have a list which I get from a database. The structure looks like (which I'm representing with JSON as it's easier for me to visualise)
{id:1
value:"a"
},
{id:1
value:"b"
},
{id:1
value:"c"
},
{id:2
value:"t"
}
As you can see, I have 2 unique ID's, ID 1 and 2. I want to group by the ID. The end result I'd like is
{id:1,
values:["a","b","c"],
},
{id:2,
values["g"]
}
Is this possible with Linq? At the moment, I have a massive complex foreach, which first sorts the list (by ID) and then detects if it's already been added etc but this monstrous loop made me realise I'm doing wrong and honestly, it's too embarrassing to share.
You can group by the item Id and have the resulting type be a Dictionary<int, List<string>>
var result = myList.GroupBy(item => item.Id)
.ToDictionary(item => item.Key,
item => item.Select(i => i.Value).ToList());
You can either use GroupBy method on IEnumerable to create IGrouping object that contains a key and grouped objects or you can use ToLookupto create exactly what you want in result:
yourList.ToLookup(m => m.id, m => m.value);
This creates a hashed collection of keys with their values.
For more information please see below post:
https://www.c-sharpcorner.com/UploadFile/d3e4b1/practical-usage-of-using-tolookup-method-in-linq-C-Sharp/
Just a little more detail to emphasize the difference between the ToLookup approach and the GroupBy approach:
// class definition
public class Item
{
public long Id { get; set; }
public string Value { get; set; }
}
// create your list
var items = new List<Item>
{
new Item{Id = 0, Value = "value0a"},
new Item{Id = 0, Value = "value0b"},
new Item{Id = 1, Value = "value1"}
};
// this approach results in a List<string> (a collection of the values)
var lookup = items.ToLookup(i => i.Id, i => i.Value);
var groupOfValues = lookup[0].ToList();
// this approach results in a List<Item> (a collection of the objects)
var itemsGroupedById = items.GroupBy(i => i.Id).ToList();
var groupOfItems = itemsGroupedById[0].ToList();
So, if you want to work with values only after grouping, then you could take the first approach; if you want to work with objects after grouping, you could take the second approach. And, these are just a couple example implementations, there are plenty of ways to accomplish your goal.
First convert to a Lookup then select into a list, like so:
var groups = list
.ToLookup
(
item => item.ID,
item => item.Value
)
.Select
(
item => new
{
ID = item.Key,
Values = item.ToList()
}
)
.ToList();
The resulting JSON looks like this:
[{"ID":1,"Values":["a","b","c"]},{"ID":2,"Values":["t"]}]
Link to working example on DotNetFiddle.
The basic question
I have:
IEnumerable<string> listA
var listB (this is an anonymous list generated by a LINQ query)
I want to query a list of objects that contain listA to see if they match to listB:
someObjectList.Where(x => x.listA == listB)
The comparison doesn't work - so how do I ensure that both lists are the same type for comparison?
The detailed question
I am grouping a larger list into a subset that contains a name and related date(s).
var listGroup = from n in list group n by new
{ n.NAME } into d
select new
{
NAME = d.Key.NAME, listOfDates = from x in d select new
{ Date = x.DATE } };
I have a object to hold the values for further processing:
class SomeObject
{
public SomeObject()
{
_listOfDates = new List<DateTime>();
}
private IEnumerable<DateTime> _listOfDates;
public IEnumerable<DateTime> ListOfDates
{
get { return _listOfDates; }
set { _listOfDates = value; }
}
}
I am then iterating over the listGroup and adding into a generic List<> of SomeObject:
foreach(var item in listGroup)
{
SomeObject so = new SomeObject();
// ...do some stuff
if (some match occurs then add into List<SomeObject>)
}
As I iterate through then I want to check the existing List<SomeOjbect> for matches:
var record = someObjectList.Where(x => x.NAME == item.NAME &&
x.ListOfDates == item.listOfDates)
.SingleOrDefault();
The problem is that comparing x.ListOfDates against item.listOfDates doesn't work.
There is no compiler error but I suspect that the returned value lists are different. How to I get the lists to commonize so they can be compared?
Update #1
This seems to work to get the listOfDates into a similar format:
IEnumerable<DateTime> tempList = item.listOfDates.Select(x => x.DATE).ToList()
Then I followed the 'SequenceEqual' suggestion from #Matt Burland
You can just compare one IEnumerable<DateTime> to another IEnumerable<DateTime>, you need to compare the sequence. Luckily, there's Enumerable.SequenceEquals (in both static and extension method flavors) which should work here.
So something like:
var record = someObjectList
.Where(x => x.NAME == item.NAME && x.ListOfDates.SequenceEquals(item.listOfDates))
.SingleOrDefault();
Ridiculously simple question that for the life of me I cant figure out. How do I 'get' at the values returned by GroupBy?
Take simple example below. I want to print out the first value that occurs more than once. Looking at the output in the watch window (image below) it sort of suggests that list3[0][0] might get at "one". But it gives me an error.
Note, I'm looking for the general solution - ie understanding what GroupBy returns.
Also, I would like to use the watch window to help me figure out for my self how I would access variables (as I find much of MSDN reference incomprehensible) - is this possible?
var list1 = new List<String>() {
"one", "two", "three", "one", "two"};
var list3 = list1
.GroupBy(x => x)
.Where(x => x.Count() > 1)
.ToList();
Console.WriteLine("list3[0][0]=" + list3[0][0]); //error
While the VS debugger shows you an "index" number because the underlying type is a collection, the grouping is exposed as an IGrouping<T> that does not have an indexer. If you just want the first item in the first group do:
Console.WriteLine("list3[0][0] =" + list3.First().First());
If you want to see all if the items you cam loop through the groupings:
int gi = 0, ii = 0;
foreach(var g in list3)
{
foreach(item i in g)
{
Console.WriteLine("list3[{0}][{1}] = {2}", gi, ii, i);
ii++;
}
gi++;
}
You are looking for the .Key property, as GroupBy returns an IEnumerable containing IGrouping elements.
If you look at the documentation of GroupBy you'll see it returns a IEnumerable<IGrouping<TKey, TSource>>.
IGrouping<TKey,TSource> has a single property Key and itself inherits IEnumerable<TElement>.
So you can enumerate over the list of items returned from a call to GroupBy and each element will have a Key property (which is whatever you grouped by) as well as enumerate each item (which will be the list of items grouped together)
Hopefully this demonstrates a bit clearer. Given a class:
public class Person
{
public string Name{get;set;}
public int Age{get;set;}
}
And a list:
var people = new List<Person>{
new Person{Name="Jamie",Age=35},
new Person{Name="Bob",Age=45},
new Person{Name="Fred",Age=35},
};
Grouping and enumerating as follows:
var groupedByAge = people.GroupBy(x => x.Age);
foreach(var item in groupedByAge)
{
Console.WriteLine("Age:{0}", item.Key);
foreach(var person in item)
{
Console.WriteLine("{0}",person.Name);
}
}
Gives this output:
Age:35
Jamie
Fred
Age:45
Bob
Live example: http://rextester.com/OWPR50756
GroupBy return an IEnumerable<IGrouping<TKey, TSource>> where each IGrouping<TKey, TElement> object contains a sequence of objects and a key it's not a Multidimensional Array which can be accessed by index [][].
To access the first element try this
static void Main(string[] args)
{
var list1 = new List<String>() {
"one", "two", "three", "one", "two"};
var list3 = list1
.GroupBy(x => x)
.Where(x => x.Count() > 1)
.ToList();
Console.WriteLine("list3[0][0]=" + list3[0].ToList()[0].ToString());
//OR Console.WriteLine("list3[0][0]=" + list3[0].First());
}
I have an interesting problem, and I can't seem to figure out the lambda expression to make this work.
I have the following code:
List<string[]> list = GetSomeData(); // Returns large number of string[]'s
List<string[]> list2 = GetSomeData2(); // similar data, but smaller subset
List<string[]> newList = list.FindAll(predicate(string[] line){
return (???);
});
I want to return only those records in list in which element 0 of each string[] is equal to one of the element 0's in list2.
list contains data like this:
"000", "Data", "more data", "etc..."
list2 contains data like this:
"000", "different data", "even more different data"
Fundamentally, i could write this code like this:
List<string[]> newList = new List<string[]>();
foreach(var e in list)
{
foreach(var e2 in list2)
{
if (e[0] == e2[0])
newList.Add(e);
}
}
return newList;
But, i'm trying to use generics and lambda's more, so i'm looking for a nice clean solution. This one is frustrating me though.. maybe a Find inside of a Find?
EDIT:
Marc's answer below lead me to experiment with a varation that looks like this:
var z = list.Where(x => list2.Select(y => y[0]).Contains(x[0])).ToList();
I'm not sure how efficent this is, but it works and is sufficiently succinct. Anyone else have any suggestions?
You could join? I'd use two steps myself, though:
var keys = new HashSet<string>(list2.Select(x => x[0]));
var data = list.Where(x => keys.Contains(x[0]));
If you only have .NET 2.0, then either install LINQBridge and use the above (or similar with a Dictionary<> if LINQBridge doesn't include HashSet<>), or perhaps use nested Find:
var data = list.FindAll(arr => list2.Find(arr2 => arr2[0] == arr[0]) != null);
note though that the Find approach is O(n*m), where-as the HashSet<> approach is O(n+m)...
You could use the Intersect extension method in System.Linq, but you would need to provide an IEqualityComparer to do the work.
static void Main(string[] args)
{
List<string[]> data1 = new List<string[]>();
List<string[]> data2 = new List<string[]>();
var result = data1.Intersect(data2, new Comparer());
}
class Comparer : IEqualityComparer<string[]>
{
#region IEqualityComparer<string[]> Members
bool IEqualityComparer<string[]>.Equals(string[] x, string[] y)
{
return x[0] == y[0];
}
int IEqualityComparer<string[]>.GetHashCode(string[] obj)
{
return obj.GetHashCode();
}
#endregion
}
Intersect may work for you.
Intersect finds all the items that are in both lists.
Ok re-read the question. Intersect doesn't take the order into account.
I have written a slightly more complex linq expression that will return a list of items that are in the same position (index) with the same value.
List<String> list1 = new List<String>() {"000","33", "22", "11", "111"};
List<String> list2 = new List<String>() {"000", "22", "33", "11"};
List<String> subList = list1.Select ((value, index) => new { Value = value, Index = index})
.Where(w => list2.Skip(w.Index).FirstOrDefault() == w.Value )
.Select (s => s.Value).ToList();
Result: {"000", "11"}
Explanation of the query:
Select a set of values and position of that value.
Filter that set where the item in the same position in the second list has the same value.
Select just the value (not the index as well).
Note I used:
list2.Skip(w.Index).FirstOrDefault()
//instead of
list2[w.Index]
So that it will handle lists of different lengths.
If you know the lists will be the same length or list1 will always be shorter then list2[w.Index] would probably a bit faster.