I want to take a List, and generate a Dictionary which maps each element to its index in the List. I can do this like so, for a List<string>:
var myList = new List<string>{ /* populate list */ };
var orderMap = new Dictionary<string, int>();
foreach (var element in myList)
{
orderMap[element] = myList.IndexOf(element);
}
Basically, I want to take a list like:
Apple
Banana
Orange
And return a map showing indices:
Apple -> 0
Banana -> 1
Orange -> 2
How can I do this with Linq? I think something like this should work:
orderMap = myList.Select( x => /* return a key value pair mapping x to myList.IndexOf(x) */ );
But I can't figure out the right syntax for it. Besides, can you refer to the list itself in the delegate used for Select?
While you can refer to the list within the delegate, it's not generally a good idea. You really want to use the overload of Select which provides the index as well as the value:
var dictionary = list.Select((value, index) => new { value, index })
.ToDictionary(p => p.value, p => p.index);
Note that this will throw an exception if you have any duplicate elements.
You could try the ToDictionary extension method:
int index = 0;
orderMap = myList.ToDictionary(x => x, x => index++);
Take a look at this overload of ToDictionary<TKey, TValue>(). It takes to functions to convert the input element into a Key and a Value.
e.g.
var myList = new List<string>{ /* populate list */ };
var orderMap = myList.ToDictionary(x => myList.IndexOf(x), x => x);
However, one problem with this is if the elements of myList aren't unique.
Related
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.
I am trying to do a sort on dictionary
class Program {
static void Main()
{
// Example dictionary.
var dictionary = new Dictionary<string, int>(5);
dictionary.Add("cat", 3);
dictionary.Add("dog", 1);
dictionary.Add("mouse", 0);
dictionary.Add("elephant", 2);
dictionary.Add("bird", 4);
var items = from pair in dictionary
orderby pair.Value ascending
select pair;
// Display results.
foreach (KeyValuePair<string, int> pair in items)
{
Console.WriteLine("{0}: {1}", pair.Key, pair.Value);
}
items = from pair in dictionary
orderby pair.Value descending
select pair;
} }
The result is
mouse
dog
elephant
cat
bird
But I need to exclude the first pair to sort the rest and to get this result
cat
mouse
dog
elephant
bird
Can I do anything from here?
var items = from pair in dictionary
orderby pair.Value ascending
select pair;
I would suggest using Linq
var result = dictionary
.Take(1) // Take first element
.Concat(dictionary // Skip first element and sort the rest on value.
.Skip(1)
.OrderBy(o=>o.Value))
.Select(x=>x.Key);
Output
cat ,
mouse ,
dog ,
elephant ,
bird
Check this Demo
dictionary.Take(1).Concat(dictionary.Skip(1).OrderBy({ logic }))
You get the idea I think
var firstItems = dictionary.Take(1);
var sortItems = from pair in dictionary.Skip(1)
orderby pair.Value ascending
select pair;
var items = firstItems.Concat(sortItems);
Beware that Dictionary does not guarantee orders.
According its implementation, orders will be messed up once after item removal.
exclude the first pair
I would suggest not to note your item by orders.
Try something like this
var result = YourList.OrderBy(mc => mc.SomePropToSortOn)
.ToDictionary(mc => mc.Key.ToString(),
mc => mc.Value.ToString(),
StringComparer.OrdinalIgnoreCase);
You can use LINQ .OrderBy:
dictionary.OrderBy(t => t.Value).ToDictionary(t => t.Key, t=> t.Value);
You can try to use this code:
var first = dictionary.Take(1);
var orderedRest = dictionary.Skip(1).OrderBy(p => p.Value);
var items = first.Union(orderedRest);
I already mentioniod that I think that a Dictionary is not the right collection type for this. In my opinion you should use List<KeyValuePair<string, int>>. The above code works with the list, too.
I have below code in c# 4.0.
//Dictionary object with Key as string and Value as List of Component type object
Dictionary<String, List<Component>> dic = new Dictionary<String, List<Component>>();
//Here I am trying to do the loping for List<Component>
foreach (List<Component> lstComp in dic.Values.ToList())
{
// Below I am trying to get first component from the lstComp object.
// Can we achieve same thing using LINQ?
// Which one will give more performance as well as good object handling?
Component depCountry = lstComp[0].ComponentValue("Dep");
}
Try:
var firstElement = lstComp.First();
You can also use FirstOrDefault() just in case lstComp does not contain any items.
http://msdn.microsoft.com/en-gb/library/bb340482(v=vs.100).aspx
Edit:
To get the Component Value:
var firstElement = lstComp.First().ComponentValue("Dep");
This would assume there is an element in lstComp. An alternative and safer way would be...
var firstOrDefault = lstComp.FirstOrDefault();
if (firstOrDefault != null)
{
var firstComponentValue = firstOrDefault.ComponentValue("Dep");
}
[0] or .First() will give you the same performance whatever happens.
But your Dictionary could contains IEnumerable<Component> instead of List<Component>, and then you cant use the [] operator. That is where the difference is huge.
So for your example, it doesn't really matters, but for this code, you have no choice to use First():
var dic = new Dictionary<String, IEnumerable<Component>>();
foreach (var components in dic.Values)
{
// you can't use [0] because components is an IEnumerable<Component>
var firstComponent = components.First(); // be aware that it will throw an exception if components is empty.
var depCountry = firstComponent.ComponentValue("Dep");
}
You also can use this:
var firstOrDefault = lstComp.FirstOrDefault();
if(firstOrDefault != null)
{
//doSmth
}
for the linq expression you can use like this :
List<int> list = new List<int>() {1,2,3 };
var result = (from l in list
select l).FirstOrDefault();
for the lambda expression you can use like this
List list = new List() { 1, 2, 3 };
int x = list.FirstOrDefault();
You can do
Component depCountry = lstComp
.Select(x => x.ComponentValue("Dep"))
.FirstOrDefault();
Alternatively if you are wanting this for the entire dictionary of values, you can even tie it back to the key
var newDictionary = dic.Select(x => new
{
Key = x.Key,
Value = x.Value.Select( y =>
{
depCountry = y.ComponentValue("Dep")
}).FirstOrDefault()
}
.Where(x => x.Value != null)
.ToDictionary(x => x.Key, x => x.Value());
This will give you a new dictionary. You can access the values
var myTest = newDictionary[key1].depCountry
Try this to get all the list at first, then your desired element (say the First in your case):
var desiredElementCompoundValueList = new List<YourType>();
dic.Values.ToList().ForEach( elem =>
{
desiredElementCompoundValue.Add(elem.ComponentValue("Dep"));
});
var x = desiredElementCompoundValueList.FirstOrDefault();
To get directly the first element value without a lot of foreach iteration and variable assignment:
var desiredCompoundValue = dic.Values.ToList().Select( elem => elem.CompoundValue("Dep")).FirstOrDefault();
See the difference between the two approaches: in the first one you get the list through a ForEach, then your element. In the second you can get your value in a straight way.
Same result, different computation ;)
There are a bunch of such methods:
.First .FirstOrDefault .Single .SingleOrDefault
Choose which suits you best.
var firstObjectsOfValues = (from d in dic select d.Value[0].ComponentValue("Dep"));
I would to it like this:
//Dictionary object with Key as string and Value as List of Component type object
Dictionary<String, List<Component>> dic = new Dictionary<String, List<Component>>();
//from each element of the dictionary select first component if any
IEnumerable<Component> components = dic.Where(kvp => kvp.Value.Any()).Select(kvp => (kvp.Value.First() as Component).ComponentValue("Dep"));
but only if it is sure that list contains only objects of Component class or children
Is there a built-in function for converting a string array into a dictionary of strings or do you need to do a loop here?
Assuming you're using .NET 3.5, you can turn any sequence (i.e. IEnumerable<T>) into a dictionary:
var dictionary = sequence.ToDictionary(item => item.Key,
item => item.Value)
where Key and Value are the appropriate properties you want to act as the key and value. You can specify just one projection which is used for the key, if the item itself is the value you want.
So for example, if you wanted to map the upper case version of each string to the original, you could use:
var dictionary = strings.ToDictionary(x => x.ToUpper());
In your case, what do you want the keys and values to be?
If you actually just want a set (which you can check to see if it contains a particular string, for example), you can use:
var words = new HashSet<string>(listOfStrings);
You can use LINQ to do this, but the question that Andrew asks should be answered first (what are your keys and values):
using System.Linq;
string[] myArray = new[] { "A", "B", "C" };
myArray.ToDictionary(key => key, value => value);
The result is a dictionary like this:
A -> A
B -> B
C -> C
IMO, When we say an Array we are talking about a list of values that we can get a value with calling its index (value => array[index]), So a correct dictionary is a dictionary with a key of index.
And with thanks to #John Skeet, the proper way to achieve that is:
var dictionary = array
.Select((v, i) => new {Key = i, Value = v})
.ToDictionary(o => o.Key, o => o.Value);
Another way is to use an extension method like this:
public static Dictionary<int, T> ToDictionary<T>(this IEnumerable<T> array)
{
return array
.Select((v, i) => new {Key = i, Value = v})
.ToDictionary(o => o.Key, o => o.Value);
}
If you need a dictionary without values, you might need a HashSet:
var hashset = new HashSet<string>(stringsArray);
What do you mean?
A dictionary is a hash, where keys map to values.
What are your keys and what are your values?
foreach(var entry in myStringArray)
myDictionary.Add(????, entry);
I'll assume that the question has to do with arrays where the keys and values alternate. I ran into this problem when trying to convert redis protocol to a dictionary.
private Dictionary<T, T> ListToDictionary<T>(IEnumerable<T> a)
{
var keys = a.Where((s, i) => i % 2 == 0);
var values = a.Where((s, i) => i % 2 == 1);
return keys
.Zip(values, (k, v) => new KeyValuePair<T, T>(k, v))
.ToDictionary(kv => kv.Key, kv => kv.Value);
}
Dictionary<int, string> dictionaryTest = new Dictionary<int, string>();
for (int i = 0; i < testArray.Length; i++)
{
dictionaryTest.Add(i, testArray[i]);
}
foreach (KeyValuePair<int, string> item in dictionaryTest)
{
Console.WriteLine("Array Position {0} and Position Value {1}",item.Key,item.Value.ToString());
}
The Question is not very clear, but Yes you can convert a string to Dictionary provided the string is delimited with some characters to support Dictionary<Key,Value> pair
So if a string is like a=first;b=second;c=third;d=fourth you can split it first based on ; then on = to create a Dictionary<string,string> the below extension method does the same
public static Dictionary<string, string> ToDictionary(this string stringData, char propertyDelimiter = ';', char keyValueDelimiter = '=')
{
Dictionary<string, string> keyValuePairs = new Dictionary<string, string>();
Array.ForEach<string>(stringData.Split(propertyDelimiter), s =>
{
if(s != null && s.Length != 0)
keyValuePairs.Add(s.Split(keyValueDelimiter)[0], s.Split(keyValueDelimiter)[1]);
});
return keyValuePairs;
}
and can use it like
var myDictionary = "a=first;b=second;c=third;d=fourth".ToDictionary();
since the default parameter is ; & = for the extension method.
You can create a dictionary from an IEnumerable<T>, including an array, via:
var dictionary = myEnumerable.ToDictionary(element => element.Key,
element => element.Value)
where Key and Value are the key and value you want to store in each dictionary element. Available in .NET Framework 3.5+/.NET Core 1.0+/.NET 5.0+. Official documentation.
If you want the dictionary values to be the elements from the original enumerable:
var dictionary = myEnumerable.ToDictionary(element => element.Key)
If you only need high-performance set operations, you may be able to use:
var words = new HashSet<string>(listOfStrings);
In simple terms, the HashSet class can be thought of as a Dictionary<TKey,TValue> collection without values. Official documentation.
(Note that a 'sequence' in an entirely unrelated object.
Originally submitted an existing answer edit but it was rejected by the author so posting separately, including with links to the official Microsoft documentation.)
I have the following scenario:
List 1 has 20 items of type TItem, List 2 has 5 items of the same type. List 1 already contains the items from List 2 but in a different state. I want to overwrite the 5 items in List 1 with the items from List 2.
I thought a join might work, but I want to overwrite the items in List 1, not join them together and have duplicates.
There is a unique key that can be used to find which items to overwrite in List 1 the key is of type int
You could use the built in Linq .Except() but it wants an IEqualityComparer so use a fluid version of .Except() instead.
Assuming an object with an integer key as you indicated:
public class Item
{
public int Key { get; set; }
public int Value { get; set; }
public override string ToString()
{
return String.Format("{{{0}:{1}}}", Key, Value);
}
}
The original list of objects can be merged with the changed one as follows:
IEnumerable<Item> original = new[] { 1, 2, 3, 4, 5 }.Select(x => new Item
{
Key = x,
Value = x
});
IEnumerable<Item> changed = new[] { 2, 3, 5 }.Select(x => new Item
{
Key = x,
Value = x * x
});
IEnumerable<Item> result = original.Except(changed, x => x.Key).Concat(changed);
result.ForEach(Console.WriteLine);
output:
{1:1}
{4:4}
{2:4}
{3:9}
{5:25}
LINQ isn't used to perform actual modifications to the underlying data sources; it's strictly a query language. You could, of course, do an outer join on List2 from List1 and select List2's entity if it's not null and List1's entity if it is, but that is going to give you an IEnumerable<> of the results; it won't actually modify the collection. You could do a ToList() on the result and assign it to List1, but that would change the reference; I don't know if that would affect the rest of your application.
Taking your question literally, in that you want to REPLACE the items in List1 with those from List2 if they exist, then you'll have to do that manually in a for loop over List1, checking for the existence of a corresponding entry in List2 and replacing the List1 entry by index with that from List2.
As Adam says, LINQ is about querying. However, you can create a new collection in the right way using Enumerable.Union. You'd need to create an appropriate IEqualityComparer though - it would be nice to have UnionBy. (Another one for MoreLINQ perhaps?)
Basically:
var list3 = list2.Union(list1, keyComparer);
Where keyComparer would be an implementation to compare the two keys. MiscUtil contains a ProjectionEqualityComparer which would make this slightly easier.
Alternatively, you could use DistinctBy from MoreLINQ after concatenation:
var list3 = list2.Concat(list1).DistinctBy(item => item.Key);
Here's a solution with GroupJoin.
List<string> source = new List<string>() { "1", "22", "333" };
List<string> modifications = new List<string>() { "4", "555"};
//alternate implementation
//List<string> result = source.GroupJoin(
// modifications,
// s => s.Length,
// m => m.Length,
// (s, g) => g.Any() ? g.First() : s
//).ToList();
List<string> result =
(
from s in source
join m in modifications
on s.Length equals m.Length into g
select g.Any() ? g.First() : s
).ToList();
foreach (string s in result)
Console.WriteLine(s);
Hmm, how about a re-usable extension method while I'm at it:
public static IEnumerable<T> UnionBy<T, U>
(
this IEnumerable<T> source,
IEnumerable<T> otherSource,
Func<T, U> selector
)
{
return source.GroupJoin(
otherSource,
selector,
selector,
(s, g) => g.Any() ? g.First() : s
);
}
Which is called by:
List<string> result = source
.UnionBy(modifications, s => s.Length)
.ToList();