C# equivalent of C++ map<string,double>

Question

I want to keep some totals for different accounts. In C++ I'd use STL like this:

map<string,double> accounts;

// Add some amounts to some accounts.
accounts["Fred"] += 4.56;
accounts["George"] += 1.00;
accounts["Fred"] += 1.00;

cout << "Fred owes me $" << accounts['Fred'] << endl;

Now, how would I do the same thing in C# ?

Solution 1

Roughly:-

var accounts = new Dictionary<string, double>();

// Initialise to zero...

accounts["Fred"] = 0;
accounts["George"] = 0;
accounts["Fred"] = 0;

// Add cash.
accounts["Fred"] += 4.56;
accounts["George"] += 1.00;
accounts["Fred"] += 1.00;

Console.WriteLine("Fred owes me ${0}", accounts["Fred"]);

Solution 2

Dictionary<string, double> accounts;

Solution 3

Although System.Collections.Generic.Dictionary matches the tag "hashmap" and will work well in your example, it is not an exact equivalent of C++'s std::map - std::map is an ordered collection.

If ordering is important you should use SortedDictionary.

Solution 4

You want the Dictionary class.

Solution 5

Dictionary is the most common, but you can use other types of collections, e.g. System.Collections.Generic.SynchronizedKeyedCollection, System.Collections.Hashtable, or any KeyValuePair collection

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Comments

[Adam Pierce]

This is very close to what I need, the only drawback is I do not know what the names of the accounts will be ahead of time.

[XXXXX]

You don't need to know them ahead of time. The examples use constant strings for brevity, but you can use string objects.

[Adam Pierce]

Perhaps I should clarify that comment by saying "I do not know the names, or how many names I will have". In this answer, if I added accounts["Ron"] += 2.50;, it would throw an exception. In reality, I'll be throwing an XML file at it with lots of names and numbers.

[Adam Pierce]

Well, I've already done it with Dictionary now but the XML is real simple, just a list of tags like this: <transaction name="Fred" amount="5.20" />

[Alastair Pitts]

Actually no, if you use the index and attempt to set a non-existent key, it will actually create the object for you with the specified key. An exception will only be thrown on the get operation. Look here on remarks: msdn.microsoft.com/en-us/library/9tee9ht2.aspx

[XXXXX]

Erebus is correct: You can arbitrarily add things to a Dictionary without knowing at compile time what or how many elements will be in the dictionary.

[Alex]

Why people so much like var?

[DarkWanderer]

@Alex: because var is much shorter than IParallelEnumerable<Record<string,XmlSerializer>> :)

[Kevin Depue]

Dictionaries in c# are not equivalent to stl::map's just so you know - c# dictionaries are hash tables whereas stl::map's are red-black trees, the underlying algorithms are totally different.

[Marcus Mangelsdorf]

Note, that the accounts["Fred"] = 0;in this example is equivalent to accounts.Add("Fred", 0); (See dotnetfiddle.net/q3UteL for both variants) Also: *When you use Dictionary.Add( Key, Value ) to add a new KeyValuePair an ArgumentException will be thrown if the Key already exists in the dictionary. *Using the indexer (Dictionary[ Key ]) you get an implicit add-or-update behavior since the Key will automatically be added if it is not contained in the Dictionary already otherwise the corresponding value will simply be updated. *Consider using decimal for currency