Some people are not sure of the difference between ToLookup vs ToDictionary LINQ methods.
To break it down real simple:
- One is a look up (ToLookup)
- One is a dictionary (ToDictionary)
Duh. That’s what you’re probably thinking. The real difference is understanding what each of these data structures do.
Lookups
A lookup is a key value pair in which multiple keys of the same value can exist, therefore resulting in a “lookup” type of scenario. The perfect example for this is to assume that we have a List of Author objects. The author object contains the following:
- First Name
- Last Name
- Year their first book was published
- Social Security Number
Assume that we’d like to give the user the option to select the authors based upon the year they published their first book in a WPF app. We could do that like this:
// Returns a list of Authors var authors = authorService.FindAll(); // Turn the list into a lookup for later use var authorLookup = authors.ToLookup(k => k.YearFirstPublished, v => v); // Further down in the code // Find all authors who published their first book in 1969 // Returns an enumerable of Authors that match that lookup value var authorsPublishedInParticularYear = authorLookup[1969];
This will return all authors who published their first book in 1969. We could use this look up later in the application as well to look up other values.
The key thing to note here is that a Lookup can have multiple keys of the same value.
Dictionaries
A dictionary is a key value pair in which a key can only exist once within the dictionary. You cannot have two entries with the same key. This is the difference between the Dictionary and Lookup data structures.
Using the same Author object we described above we can find an author by their Social Security Number if we turned the list into a dictionary for fast lookups based upon a unique key (the social security number).
This will return a single Author object who’s social security number matches ‘555-55-5555’.
// Returns a list of Authors var authors = authorService.FindAll(); // Turn the list into a lookup for later use var authorDictionary = authors.ToDictionay(k => k.SocialSecurityNumber, v => v); // Further down in the code // Finds the author with 555-55-5555 as their Social Security Number var author = authorDictionary["555-55-5555"];
Conclusion
While dictionaries and lookups seem to be nearly the same data structure, they provide completely different functionality based upon the value the key possesses.
So a ToLookup is similar to a NameValueCollection?
So what's the difference between a lookup and a hashtable?
@Justin Chase: A lookup has a little more plumbing to it. Lookup(of TKey, TValue) is essentially Hashtable(ofTKey, IEnumerable(Of TValue)).
I see, but multiple keys can point to the same value? Sort of a reverse hashtable?
Does it supports Array with Oops?
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nice script you put up. very usefull
Shouldn't a lookup be described as having multiple values for the same key rather than multiple keys of the same value?
Nice Article Donn i am NavinKumar from India. i like to know many things like this concept can you please help me
Hi Donn,
i like this article its is very useful.
Yes, you’re correct. Since each key has a IEnumerable<tvalue> as it’s value.
There’s no reason why multiple keys ‘shouldn’t’ point to the same object.
This article is very interesting and more useful and I like this article very much. The difference linq tolookup vs todictionary is massive..
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I think the mapping is done by defining classes that correspond to the tables in the database, and containing all or a subset of the columns in the table as data members
Great post. Many thanks !!!
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Did you mean: “A lookup can have multiple values with the same key”?
I agree with you Tomasz
It’s All A Matter Of Point of View. It’s important to note that you can
input a lookup with duplicate keys, but that doesn’t mean that the lookup
contains duplicates afterwards. It’s similar to a collection where every
value of a duplicate key will be added. So you end with one unique key
and a sequence of values which can be empty(would also cause an
exception in a dictionary) or or contains one or multiple values.
Agree. It’s worth to update the post. Please.
Do you really mean ToLookUp allows to have multiple values associated with one key, or one key associated with multiple values, which ToDictionnary cannot allow?
I love the example…. I dislike the “var” word for examples………it’s much harder to follow
ILookup authorLookup = authors.ToLookup(k => k.YearFirstPublished, v => v);
IEnumerable authorsPublishedInParticularYear = authorLookup[1991];
var authorDictionary = authors.ToDictionary(k => k.SocialSecurityNumber, v => v);
Author foundAuthor1 = authorDictionary[“555-55-5555”];
Thanks for this topic, its very useful
Multiple people have already expressed the same befuddlement. Same key, multiple values.
Is the amortized lookup time constant?
Very simple: a lookup works like a Dictionary<KeyType, List>!