I'm wondering if the index of an array can be given a name in C# instead of the default index value. What I'm basically looking for is the C# equivalent of the following PHP code:
$array = array(
"foo" => "some foo value",
"bar" => "some bar value",
);
Cheers.
PHP blends the concept of arrays and the concept of dictionaries (aka hash tables, hash maps, associative arrays) into a single array type.
In .NET and most other programming environments, arrays are always indexed numerically. For named indices, use a dictionary instead:
var dict = new Dictionary<string, string> {
{ "foo", "some foo value" },
{ "bar", "some bar value" }
};
Unlike PHP's associative arrays, dictionaries in .NET are not sorted. If you need a sorted dictionary (but you probably don't), .NET provides a sorted dictionary type.
In an array, no. However, there is the very useful Dictionary class, which is a collection of KeyValuePair objects. It's similar to an array in that it is an iterable collection of objects with keys, but more general in that the key can be any type.
Example:
Dictionary<string, int> HeightInInches = new Dictionary<string, int>();
HeightInInches.Add("Joe", 72);
HeightInInches.Add("Elaine", 60);
HeightInInches.Add("Michael", 59);
foreach(KeyValuePair<string, int> person in HeightInInches)
{
Console.WriteLine(person.Key + " is " + person.Value + " inches tall.");
}
MSDN Documentation for Dictionary<TKey, TValue>
Look at Hashtable in C#. This is the data structure that does what you want in C#.
You could use a Dictionary<string, FooValue> or similar type or collection type or, if you must stick to array, define an Enum with your labels.
Related
What I need is something like an array but letting me to assign an element to whatever an index at any time and check if there is already a value assigned to particular index approximately like
MyArray<string> a = new MyArray<string>();
a[10] = "ten";
bool isTheFifthElementDefined = a[5] != null; // false
Perhaps Dictionary<int, string> with its ContainsKey method could do, but isn't there a more appropriate data structure if I want an ordered collection with numeric keys only?
I am also going to need to iterate through the defined elements (with foreach or linq preferably) accessing both the value and the key of current element.
As you mentioned Dictionary seems more appropriate for this.But you can do it with generic lists,for example, when you are creating your list you can specify an element count,and you can give a default temporary value for all your elements.
List<string> myList = new List<string>(Enumerable.Repeat("",5000));
myList[2300] = "bla bla bla..";
For int:
List<int> myList = new List<int>(Enumerable.Repeat(0,5000));
For custom type:
List<MyClass> myList = new List<MyClass>(Enumerable.Repeat(new MyClass(), 100));
Ofcourse It is not the best solution...
Note: Also you can use SortedList instead of Dictionary if you want an ordered collection by keys:
SortedList<TKey, TValue> : Represents a collection of key/value pairs that are sorted by key based on the associated IComparer implementation.
If you need key/value pairs you cannot use a list, you'll need a Dictionary.
The implementation is pretty snappy so don't be too afraid about performance (as long as you don't put too much values in it).
You can iterate over it with
foreach(KeyValuePair<int, string> kvp in dict)
{
}
If you need to order it you can use a list:
List<int> ordered = new List(dict.Keys);
ordered.Sort();
foreach(int key in ordered)
{
}
I do not have a lot of experience with C#, yet I am used of working with associative arrays in PHP.
I see that in C# the List class and the Array are available, but I would like to associate some string keys.
What is the easiest way to handle this?
Thx!
Use the Dictionary class. It should do what you need.
Reference is here.
So you can do something like this:
IDictionary<string, int> dict = new Dictionary<string, int>();
dict["red"] = 10;
dict["blue"] = 20;
A dictionary will work, but .NET has associative arrays built in. One instance is the NameValueCollection class (System.Collections.Specialized.NameValueCollection).
A slight advantage over dictionary is that if you attempt to read a non-existent key, it returns null rather than throw an exception. Below are two ways to set values.
NameValueCollection list = new NameValueCollection();
list["key1"] = "value1";
NameValueCollection list2 = new NameValueCollection()
{
{ "key1", "value1" },
{ "key2", "value2" }
};
Suppose I have an array of strings like :
myArray["hello", "my", "name", "is", "marco"]
to access to this variable, I have to put an integer as index. So if I wanto to extract the third element I just do :
myArray[2]
Now, I'd like to use label instead of integer.
So for example somethings like :
myArray["canada"]="hello";
myArray["america"]="my";
myArray["brazil"]="name";
myArray["gosaldo"]="is";
myArray["italy"]="marco";
How can I do this on C#? Is it possible? Thanks
That's called an associative array, and C# doesn't support them directly. However, you can achieve exactly the same the effect with a Dictionary<TKey, TValue>. You can add values with the Add method (which will throw an exception if you try to add an already existing key), or with the indexer directly, as below (this will overwrite the existing value if you use the same key twice).
Dictionary<string, string> dict = new Dictionary<string, string>();
dict["canada"] = "hello";
dict["america"] = "my";
dict["brazil"] = "name";
dict["gosaldo"] = "is";
dict["italy"] = "marco";
C# has a Dictionary class (and interface) to deal with this sort of storage. For example:
Dictionary<string, string> dict = new Dictionary<string, string>();
dict.Add("canada", "hello");
dict.Add("america", "my");
dict.Add("brazil", "name");
dict.Add("gosaldo", "is");
Here are the docs: http://msdn.microsoft.com/en-us/library/xfhwa508.aspx
With a Dictionary you will be able to set the "key" for each item as a string, and and give them string "values". For example:
Dictionary<string, string> dic = new Dictionary<string, string>();
dic.Add("canada", "hello");
You're looking for an associative array and I think this question is what you're looking for.
I currently have a WebSocket between JavaScript and a server programmed in C#. In JavaScript, I can pass data easily using an associative array:
var data = {'test': 'val',
'test2': 'val2'};
To represent this data object on the server side, I use a Dictionary<string, string>, but this is more 'typing-expensive' than in JavaScript:
Dictionary<string, string> data = new Dictionary<string,string>();
data.Add("test", "val");
data.Add("test2", "val2");
Is there some kind of literal notation for associative arrays / Dictionarys in C#?
You use the collection initializer syntax, but you still need to make a new Dictionary<string, string> object first as the shortcut syntax is translated to a bunch of Add() calls (like your code):
var data = new Dictionary<string, string>
{
{ "test", "val" },
{ "test2", "val2" }
};
In C# 6, you now have the option of using a more intuitive syntax with Dictionary as well as any other type that supports indexers. The above statement can be rewritten as:
var data = new Dictionary<string, string>
{
["test"] = "val",
["test2"] = "val2"
};
Unlike collection initializers, this invokes the indexer setter under the hood, rather than an appropriate Add() method.
While, the dictionary initializer answer is totally correct, there is another approach to this that I would point out (but I might not recommend it). If your goal is to provide terse API usage, you could use anonymous objects.
var data = new { test1 = "val", test2 = "val2"};
The "data" variable is then of an "unspeakable" anonymous type, so you could only pass this around as System.Object. You could then write code that can transform an anonymous object into a dictionary. Such code would rely on reflection, which would potentially be slow. However, you could use System.Reflection.Emit, or System.Linq.Expressions to compile and cache a delegate that would make subsequent calls much faster.
Asp.net MVC APIs use this technique in a number of places that I've seen. A lot of the Html Helpers have overloads that accept either an object or a dictionary. I assume the goal of their API design is the same as what you are after; terse syntax at the method call.
Use Dictionary Literals (C#9 proposal) [rejected] or the new syntax (beginning with C#9)
C#9 introduces a simpler syntax to create initialized Dictionary<TKey,TValue> objects without having to specify either the Dictionary type name or the type parameters. The type parameters for the dictionary are inferred using the existing rules used for array type inference.
// C# 1..8
var x = new Dictionary <string,int> () { { "foo", 4 }, { "bar", 5 }};
// C# 9
var x = ["foo":4, "bar": 5];
This synthax makes the work with dictionaries in C# simpler and removing the redundant code.
You can follow the issue on GitHub (and here is the milestone for C#9).
Edit: This proposal is currently rejected:
[...] We think there are a number of interesting use cases around initializing data, particularly for things like immutable dictionaries. We don't find the existing syntax for initializing a dictionary that onerous, nor do we see it as a frequent pattern in code that would benefit much from a language feature. We thing that the general area of initializing data should be looked at again after we do records and withers. [...]
current milestone:
Note that beginning with C# 9.0, constructor invocation expressions are target-typed. That is, if a target type of an expression is known, you can omit a type name, as the following example shows:
Dictionary<int, List<int>> lookup = new()
{
[1] = new() {1, 2, 3},
[2] = new() {5, 8, 3},
[5] = new() {1, 0, 4}
};
As the preceding example shows, you always use parentheses in a target-typed new expression.
If a target type of a new expression is unknown (for example, when you use the var keyword), you must specify a type name.
MSDN
Using DynamicObject, it is not that difficult to create a simpler dictionary initializer.
Imagine you want to call the following method
void PrintDict(IDictionary<string, object> dict) {
foreach(var kv in dict) {
Console.WriteLine (" -> " + kv.Key + " = " + kv.Value);
}
}
using a literal syntax like
var dict = Dict (Hello: "World", IAm: "a dictionary");
PrintDict (dict);
This can be accomplished by creating a dynamic object like this
dynamic Dict {
get {
return new DynamicDictFactory ();
}
}
private class DynamicDictFactory : DynamicObject
{
public override bool TryInvoke (InvokeBinder binder, object[] args, out object result)
{
var res = new Dictionary<string, object> ();
var names = binder.CallInfo.ArgumentNames;
for (var i = 0; i < args.Length; i++) {
var argName = names [i];
if(string.IsNullOrEmpty(argName)) throw new ArgumentException();
res [argName] = args [i];
}
result = res;
return true;
}
}
I read here that SortedDictionary does not allow indexed retrieval unlike SortedList.
Then how can I get the nameAddr["C"] correctly in the following code snippet?
SortedDictionary<string, string> nameAddr = new SortedDictionary<string, string>();
nameAddr.Add("X", "29");
nameAddr.Add("A", "30");
nameAddr.Add("C", "44");
Console.WriteLine(nameAddr["C"]);
That's indexing by key. SortedList allows indexing by "index of key", e.g. nameAddr.Values[1] would return "44".
(The collection doesn't allow indexing of name/value pair, just of each of Keys and Values separately.)
For example:
var list = new SortedList<string, string>
{
{ "X", "29" },
{ "A", "30" },
{ "C", "44" },
};
Console.WriteLine(list.Keys[1]); // Prints "C"
Console.WriteLine(list.Values[1]); // Prints "44"
SortedList internally uses an array as the data structure for storage and then just sorts the array as needed to keep the items in order. Since it uses an array the items can be accessed using a numeric index like you would for any array.
SortedDictionary internally uses a red-black binary search tree to keep the items in order. The concept is completely different. There is no array and no analog for retrieving the items by a numeric index. Your only option is to use the key portion of the key-value pair that was added to the dictionary.
With that said. Your code looks correct to me. That is the only way to retrieve items from the dictionary (other than using than Values collection, but that will not give you the numeric indexing ability either).