I was asked the following question in an interview. How can I sort a Dictionary by the key, in the order which is in an array list.
So for example I have a dictionary as follows
Dictionary<string, string> stringDict = new Dictionary<string, string>();
stringDict.Add("1", "One");
stringDict.Add("7", "Seven");
stringDict.Add("6", "Six");
stringDict.Add("2", "Two");
stringDict.Add("3", "Three");
stringDict.Add("5", "Five");
stringDict.Add("4", "Four");
And an array list as follows
ArrayList stringArList = new ArrayList();
stringArList.Add("1");
stringArList.Add("2");
stringArList.Add("3");
stringArList.Add("5");
stringArList.Add("6");
stringArList.Add("7");
stringArList.Add("4");
How can I sort the dictionary in the order it is in the array list?
Well you can't sort a Dictionary per se, but you can extract the key-values pairs as a list and sort those:
IEnumerable<KeyValuePair<string, string>> pairs =
stringDict.OrderBy(kvp => stringArList.IndexOf(kvp.Key));
But there's not a way to "traverse" dictionary items in any particular order.
You could create a SortedDictionary and provide an IComparer<string>
var d = new SortedDictionary<string, string>(stringDict,
new PositionComparer(stringArList));
With the Comparer implementation as:
public class PositionComparer : IComparer<string>
{
private ArrayList Keys {get; set;}
public PositionComparer(ArrayList keys)
{
Keys = keys;
}
public int Compare(string s1, string s2)
{
return Keys.IndexOf(s1).CompareTo(Keys.IndexOf(s2));
}
}
This will produce a list of the values sorted as required.
var sortedValues = stringDict.OrderBy(pair => stringArList.IndexOf(pair.Key))
.Select(pair => pair.Value)
.ToList();
As said many times before in this question, a C# Dictionary can not be sorted. This is inherent to the implementation. As you can read here, the dictionary is implemented using a Hashtable. This means that those items don't have any ordering. You can't say "give me the first element of this dictionary". There simply is no first, second or last element. The only thing you can say about an element in a dictionary is that it has a unique identifier which determines it's location in the dictionary (with a little help from the hash function).
When you have an element in an array for example, you can say "this element is the next element" or "this element is the previous element". Each element in an array has a previous and a next. This does not hold for dictionaries.
When you insert an item into the dictionary it will generate a hash (a fairly unique number) based on the key. For example, a very simple (and bad) hash of keys consisting of names would be to take the sum of each character presented as their ASCII value in the name, and then add those together. The result is a number, say 5, then we would insert the value in a store (an array for example) on position 5. If however, at position 5 is another value, which happens to have the same hash result you have a collision. How you solve these, and how you avoid these is what hashtables are all about. See the Wiki for more information on this.
When you request your value with your key someName it will hash that value and look it up at that position.
Hashtables are not as easy as I just explained, there is a lot too it. You can read more on the Wiki.
So the answer to "Sort this dictionary" is most definitely "No can do.". However, you can convert it to a sortable data structure like a list, or whatever and then sort it.
I replied with the following answer.
var list = from arElement in stringArList.ToArray().ToList()
join dict in stringDict on arElement equals dict.Key
select dict ;
But the interviewer didn't seem to be impressed.
original_dic.OrderBy returns IOrderedEnumerable, that you convert to a new dictionary.
var ordered_dic = original_dic.OrderBy(x => x.Key).ToDictionary(x=> x.Key, x=> x.Value);
For each item in the collection, I need to have a short string and a few int16 fields. I want to iterate through the collection using the string field (meaning I don't want to use numeric index to iterate).
The collection is at most about 10 items. Thanks for any suggestions.
I think Dictionary<string, List<int>> should work for you needs.
Dictionary<string, List<int>> dictionary = new Dictionary<string, List<int>>();
dictionary.Add("key", new List<int>{1,2,3,4});
...
If you use .NET 4 and the "few int16" are always the same number of values you might also want to consider the Tuple class as value in the dictionary:
var map = new Dictionary<string, Tuple<Int16, Int16, Int16>>();
map["Foo"] = Tuple.Create(1, 2, 3);
var values = map["Foo"];
Console.WriteLine("{0} {1} {2}", value.Item1, value.Item2, value.Item3);
I would use a Dictionary so you can index into it using an arbitrary key - string, in your case.
If you mean iterate as looping thru; then it really does not matter because all collections support foreach:
foreach (var item in collection) { ... }
However, if you mean iterate as indexing, then a Dictionary should do the job.
class SomeFields { public int a; public int b; ... }
var collection = new Dictionary<string, SomeFields>();
collection.Add("name", new SomeFields() { a = 1, b = 2 });
var fields = collection["name"];
If the items in the collection is small "10 items or less" then better for performance to use ListDictionary, If you are not sure about the elements count or if the element count maybe increase in the future then use HaybridDictionary.
Note that HybridDictionary mechanism is to use internally a ListDictionary while the collection is small, and then switching to a Hashtable when the collection gets large.
I want to build an Dictonary like this :
Dictionary<String, ArrayList> myDic = new Dictionary<String, ArrayList>();
in the end i want a structure like :
["blabla"] => array(1,2,3)
["foo"] => array(1,4,6,8)
.......
to build this i run in a loop and in every loop build some strings ,
first question :
how to check every time if this string exists
in the dictionary , if its not exists open a new entry in dictionary with one element in the array list, if exists only add another element to the array list
and another question:
how can i sort this dictionary according to number of elements in the array list(In descending order) like :
["foo"] => array(1,4,6,2,8)
["bar"] => array(4,6,2,8)
["bla"] => array(1,2,3)
["blo"] => array(1,2)
.......
thanks !
Use the right tool for the job. The data structure you want is called a "multi-dictionary" - that is a dictionary that maps from a key to a sequence of values, rather than from a key to a unique value.
The PowerCollections codebase contains an implementation of MultiDictionary that probably does what you want. I would use it rather than writing your own.
To sort the dictionary into a sequence of key/sequence pairs ordered by the length of the sequence, I would use a LINQ query with an "order by" clause. That seems like the easiest way to do it.
Instead of ArrayList you should use an array or List<T>. Assuming you have a Dictionary<string, int> called source this should work:
var items = source
.GroupBy(kvp => kvp.Key)
.Select(grp => new { Key = grp.Key, Items = grp.Select(kvp => kvp.Value).ToArray() })
.OrderByDescending(i => i.Items.Length);
To explain, Dictionary<TKey, TValue> implements IEnumerable<KeyValuePair<TKey, TValue>> so can be considered a sequence of key-value pairs. Group by groups the pairs by key and then Select creates a sequence of an anonymous type which contains the key and associated values in a Key and Items property respectively. This sequence is then ordered by the number of items in the Items array of each object.
If you want to order them by the length of the created array, you can't use a dictionary since they are not ordered.
To check if a key exists in a dictionary and use the value if it does, you can use TryGetValue:
ArrayList array;
if(!myDic.TryGetValue("blabla", out array))
{
array = new ArrayList();
myDic["blabla"] = array;
}
array.Add(42);
Would something like this work:
if (myDic.ContainsKey(myString))
myDic[myString].Add(myNumber);
else
myDic.Add(myString, new ArrayList(new int[] {myNumber}));
I have a dictionary which I need to keep updated with incoming data, after parsing the incoming data I have to check if there are any entries in the dictionary which are not present in the incoming data (incoming data when parsed is a list and I need to map it with the dictionary entries).
To avoid multiple loops to removed the entries, I ran a decrementing for loop for dictionary count, then I fetch the dictionary key of the index using ElementAt, then check if the entry is present in the incoming data if not then I remove that entry from the list. I did this because running the foreach loop on the dictionary keys and removing from it will raise and exception as the dictionary keys collection would be modified.
I wanted to understand that doing this will there be any impact on execution time. I want to understand what is the order of ElementAt operation.
ElementAt is useful if you need to provide indexing semantics and cannot guarantee that indexing semantics will be available in the underlying enumeration. It does use O(1) indexing when the enumeration acted upon is an IList<T> (which includes List and arrays), but otherwise is O(n)*, which makes it being used in a sequence over everything go from the O(n) operation it would be with a list to O(n * n).
If however you got a copy of the keys with dict.Keys.ToList() then you could safely foreach through that, as it won't be changed by changes to your dictionary.
What isn't clear is why you don't just replace the old dictionary with the new one, which would be considerably faster again (simple reference assignment).
*Update: In the .NET Core version of linq there are a greater range of cases where ElementAt() is O(1) such as the results of a Select() done on an IList<T>. Also OrderBy(…).ElementAt(…) is now O(n) rather than O(n log n) as the combined sequence is turned into a quick-select rather than a quicksort followed by an iteration.
Use "mark then remove" trick as a workaround for inability to modify collection while iterating.
var dict = new Dictionary<int, string>
{
{3, "kuku" },
{1, "zOl"}
};
var newKeys = new List<int> { 1, 2, 4 };
var toRemove = dict.Keys.Except(newKeys).ToList();
foreach (var k in toRemove)
dict.Remove(k);
ElementAt() does use the enumerator as pointed out, so if you want fastest index access you should use an array. Of course, that comes at the price of a fixed length, but if the size of the array is not constantly changing, it is feasible that Array.Resize() might be the way to go.
Except() seems like it would work here:
Dictionary<int, string> dict = new Dictionary<int, string>
{
{3, "kuku" },
{1, "zOl"}
};
IEnumerable<int> data = new List<int> { 1, 2, 4 };
IEnumerable<int> toRemove = dict.Keys.Except(data);
foreach(var x in toRemove)
dict.Remove(x);
I think that ElementAt() uses the enumerator to get to the required element.
It will be the same as:
object returnedElement = null;
int i = 0;
foreach (var obj in dictionary.Keys)
{
if (i++ == at)
{
returnedElement = obj;
break;
}
}
You can get a Dictionary of the matching entries in target (Dictionary) and source (List) as follows:
using System;
using System.Collections.Generic;
using System.Linq;
Dictionary<string, string> target = new Dictionary<string, string>();
List<string> source = new List<string>();
target.Add("a", "this is a");
target.Add("b", "this is b");
source.Add("a");
source.Add("c");
target = Enumerable.Select(target, n => n.Key).
Where(n => source.Contains(n)).ToDictionary(n => n, k => target[k]);
It's not clear to me if you want to include new entries from the List into the Dictionary - if so, I am not sure what the new entry values would be if you only have a List of new incoming data.
I often have to sort a dictionary (consisting of keys & values) by value. For example, I have a hash of words and respective frequencies that I want to order by frequency.
There is a SortedList which is good for a single value (say frequency), that I want to map back to the word.
SortedDictionary orders by key, not value. Some resort to a custom class, but is there a cleaner way?
Use LINQ:
Dictionary<string, int> myDict = new Dictionary<string, int>();
myDict.Add("one", 1);
myDict.Add("four", 4);
myDict.Add("two", 2);
myDict.Add("three", 3);
var sortedDict = from entry in myDict orderby entry.Value ascending select entry;
This would also allow for great flexibility in that you can select the top 10, 20 10%, etc. Or if you are using your word frequency index for type-ahead, you could also include StartsWith clause as well.
Use:
using System.Linq.Enumerable;
...
List<KeyValuePair<string, string>> myList = aDictionary.ToList();
myList.Sort(
delegate(KeyValuePair<string, string> pair1,
KeyValuePair<string, string> pair2)
{
return pair1.Value.CompareTo(pair2.Value);
}
);
Since you're targeting .NET 2.0 or above, you can simplify this into lambda syntax -- it's equivalent, but shorter. If you're targeting .NET 2.0 you can only use this syntax if you're using the compiler from Visual Studio 2008 (or above).
var myList = aDictionary.ToList();
myList.Sort((pair1,pair2) => pair1.Value.CompareTo(pair2.Value));
You could use:
var ordered = dict.OrderBy(x => x.Value).ToDictionary(x => x.Key, x => x.Value);
You can sort a Dictionary by value and save it back to itself (so that when you foreach over it the values come out in order):
dict = dict.OrderBy(x => x.Value).ToDictionary(x => x.Key, x => x.Value);
Sure, it may not be correct, but it works. Hyrum's Law means that this will very likely continue to work.
Looking around, and using some C# 3.0 features we can do this:
foreach (KeyValuePair<string,int> item in keywordCounts.OrderBy(key=> key.Value))
{
// do something with item.Key and item.Value
}
This is the cleanest way I've seen and is similar to the Ruby way of handling hashes.
On a high level, you have no other choice than to walk through the whole Dictionary and look at each value.
Maybe this helps:
http://bytes.com/forum/thread563638.html
Copy/Pasting from John Timney:
Dictionary<string, string> s = new Dictionary<string, string>();
s.Add("1", "a Item");
s.Add("2", "c Item");
s.Add("3", "b Item");
List<KeyValuePair<string, string>> myList = new List<KeyValuePair<string, string>>(s);
myList.Sort(
delegate(KeyValuePair<string, string> firstPair,
KeyValuePair<string, string> nextPair)
{
return firstPair.Value.CompareTo(nextPair.Value);
}
);
You'd never be able to sort a dictionary anyway. They are not actually ordered. The guarantees for a dictionary are that the key and value collections are iterable, and values can be retrieved by index or key, but there is no guarantee of any particular order. Hence you would need to get the name value pair into a list.
You do not sort entries in the Dictionary. Dictionary class in .NET is implemented as a hashtable - this data structure is not sortable by definition.
If you need to be able to iterate over your collection (by key) - you need to use SortedDictionary, which is implemented as a Binary Search Tree.
In your case, however the source structure is irrelevant, because it is sorted by a different field. You would still need to sort it by frequency and put it in a new collection sorted by the relevant field (frequency). So in this collection the frequencies are keys and words are values. Since many words can have the same frequency (and you are going to use it as a key) you cannot use neither Dictionary nor SortedDictionary (they require unique keys). This leaves you with a SortedList.
I don't understand why you insist on maintaining a link to the original item in your main/first dictionary.
If the objects in your collection had a more complex structure (more fields) and you needed to be able to efficiently access/sort them using several different fields as keys - You would probably need a custom data structure that would consist of the main storage that supports O(1) insertion and removal (LinkedList) and several indexing structures - Dictionaries/SortedDictionaries/SortedLists. These indexes would use one of the fields from your complex class as a key and a pointer/reference to the LinkedListNode in the LinkedList as a value.
You would need to coordinate insertions and removals to keep your indexes in sync with the main collection (LinkedList) and removals would be pretty expensive I'd think.
This is similar to how database indexes work - they are fantastic for lookups but they become a burden when you need to perform many insetions and deletions.
All of the above is only justified if you are going to do some look-up heavy processing. If you only need to output them once sorted by frequency then you could just produce a list of (anonymous) tuples:
var dict = new SortedDictionary<string, int>();
// ToDo: populate dict
var output = dict.OrderBy(e => e.Value).Select(e => new {frequency = e.Value, word = e.Key}).ToList();
foreach (var entry in output)
{
Console.WriteLine("frequency:{0}, word: {1}",entry.frequency,entry.word);
}
You could use:
Dictionary<string, string> dic= new Dictionary<string, string>();
var ordered = dic.OrderBy(x => x.Value);
return ordered.ToDictionary(t => t.Key, t => t.Value);
Or for fun you could use some LINQ extension goodness:
var dictionary = new Dictionary<string, int> { { "c", 3 }, { "a", 1 }, { "b", 2 } };
dictionary.OrderBy(x => x.Value)
.ForEach(x => Console.WriteLine("{0}={1}", x.Key,x.Value));
Sorting a SortedDictionary list to bind into a ListView control using VB.NET:
Dim MyDictionary As SortedDictionary(Of String, MyDictionaryEntry)
MyDictionaryListView.ItemsSource = MyDictionary.Values.OrderByDescending(Function(entry) entry.MyValue)
Public Class MyDictionaryEntry ' Need Property for GridViewColumn DisplayMemberBinding
Public Property MyString As String
Public Property MyValue As Integer
End Class
XAML:
<ListView Name="MyDictionaryListView">
<ListView.View>
<GridView>
<GridViewColumn DisplayMemberBinding="{Binding Path=MyString}" Header="MyStringColumnName"></GridViewColumn>
<GridViewColumn DisplayMemberBinding="{Binding Path=MyValue}" Header="MyValueColumnName"></GridViewColumn>
</GridView>
</ListView.View>
</ListView>
The other answers are good, if all you want is to have a "temporary" list sorted by Value. However, if you want to have a dictionary sorted by Key that automatically synchronizes with another dictionary that is sorted by Value, you could use the Bijection<K1, K2> class.
Bijection<K1, K2> allows you to initialize the collection with two existing dictionaries, so if you want one of them to be unsorted, and you want the other one to be sorted, you could create your bijection with code like
var dict = new Bijection<Key, Value>(new Dictionary<Key,Value>(),
new SortedDictionary<Value,Key>());
You can use dict like any normal dictionary (it implements IDictionary<K, V>), and then call dict.Inverse to get the "inverse" dictionary which is sorted by Value.
Bijection<K1, K2> is part of Loyc.Collections.dll, but if you want, you could simply copy the source code into your own project.
Note: In case there are multiple keys with the same value, you can't use Bijection, but you could manually synchronize between an ordinary Dictionary<Key,Value> and a BMultiMap<Value,Key>.
Actually in C#, dictionaries don't have sort() methods.
As you are more interested in sort by values,
you can't get values until you provide them key.
In short, you need to iterate through them using LINQ's OrderBy(),
var items = new Dictionary<string, int>();
items.Add("cat", 0);
items.Add("dog", 20);
items.Add("bear", 100);
items.Add("lion", 50);
// Call OrderBy() method here on each item and provide them the IDs.
foreach (var item in items.OrderBy(k => k.Key))
{
Console.WriteLine(item);// items are in sorted order
}
You can do one trick:
var sortedDictByOrder = items.OrderBy(v => v.Value);
or:
var sortedKeys = from pair in dictName
orderby pair.Value ascending
select pair;
It also depends on what kind of values you are storing: single (like string, int) or multiple (like List, Array, user defined class).
If it's single you can make list of it and then apply sort.
If it's user defined class, then that class must implement IComparable, ClassName: IComparable<ClassName> and override compareTo(ClassName c) as they are more faster and more object oriented than LINQ.
Required namespace : using System.Linq;
Dictionary<string, int> counts = new Dictionary<string, int>();
counts.Add("one", 1);
counts.Add("four", 4);
counts.Add("two", 2);
counts.Add("three", 3);
Order by desc :
foreach (KeyValuePair<string, int> kvp in counts.OrderByDescending(key => key.Value))
{
// some processing logic for each item if you want.
}
Order by Asc :
foreach (KeyValuePair<string, int> kvp in counts.OrderBy(key => key.Value))
{
// some processing logic for each item if you want.
}
Suppose we have a dictionary as:
Dictionary<int, int> dict = new Dictionary<int, int>();
dict.Add(21,1041);
dict.Add(213, 1021);
dict.Add(45, 1081);
dict.Add(54, 1091);
dict.Add(3425, 1061);
dict.Add(768, 1011);
You can use temporary dictionary to store values as:
Dictionary<int, int> dctTemp = new Dictionary<int, int>();
foreach (KeyValuePair<int, int> pair in dict.OrderBy(key => key.Value))
{
dctTemp.Add(pair.Key, pair.Value);
}
The easiest way to get a sorted Dictionary is to use the built in SortedDictionary class:
//Sorts sections according to the key value stored on "sections" unsorted dictionary, which is passed as a constructor argument
System.Collections.Generic.SortedDictionary<int, string> sortedSections = null;
if (sections != null)
{
sortedSections = new SortedDictionary<int, string>(sections);
}
sortedSections will contain the sorted version of sections
Sort and print:
var items = from pair in players_Dic
orderby pair.Value descending
select pair;
// Display results.
foreach (KeyValuePair<string, int> pair in items)
{
Debug.Log(pair.Key + " - " + pair.Value);
}
Change descending to acending to change sort order
A dictionary by definition is an unordered associative structure that contains only values and keys in a hashable way. In other words has not a previsible way to orderer a dictionary.
For reference read this article from python language.
Link
python data structures
Best way:
var list = dict.Values.OrderByDescending(x => x).ToList();
var sortedData = dict.OrderBy(x => list.IndexOf(x.Value));
The following code snippet sorts a Dictionary by values.
The code first creates a dictionary and then uses OrderBy method to sort the items.
public void SortDictionary()
{
// Create a dictionary with string key and Int16 value pair
Dictionary<string, Int16> AuthorList = new Dictionary<string, Int16>();
AuthorList.Add("Mahesh Chand", 35);
AuthorList.Add("Mike Gold", 25);
AuthorList.Add("Praveen Kumar", 29);
AuthorList.Add("Raj Beniwal", 21);
AuthorList.Add("Dinesh Beniwal", 84);
// Sorted by Value
Console.WriteLine("Sorted by Value");
Console.WriteLine("=============");
foreach (KeyValuePair<string, Int16> author in AuthorList.OrderBy(key => key.Value))
{
Console.WriteLine("Key: {0}, Value: {1}", author.Key, author.Value);
}
}
You can sort the Dictionary by value and get the result in dictionary using the code below:
Dictionary <<string, string>> ShareUserNewCopy =
ShareUserCopy.OrderBy(x => x.Value).ToDictionary(pair => pair.Key,
pair => pair.Value);
Given you have a dictionary you can sort them directly on values using below one liner:
var x = (from c in dict orderby c.Value.Order ascending select c).ToDictionary(c => c.Key, c=>c.Value);