c#, lists, collections and itterators

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Hi Guys,

For whatever reason, I'd like to hold a dictionary of type object, keyed by a string.
I'd then like to have a list of strings, that when accessed by an itterator instead of returning the string ... returns the object in the dictionary that is keyed by the string..

eg.

Dictionary 'dct' contains ... { { "i1","Mark" },{ "i2", "Fred" }, { "i3", "Harry" } }
List 'lst' contains { "i2", "i1", "i3" }

then ...

foreach( object a in lst ) {
Console.WriteLine( a )
}

would produce the result ...

Fred
Mark
Harry

How do I achieve this?

Many thanks in advance for your help

8^)
Mark

Posted 15-Mar-13 10:56am

MarkAGr

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Sergey Alexandrovich Kryukov 15-Mar-13 17:03pm

Did you try anything so far? The problems looks like more then trivial.
—SA

MarkAGr 15-Mar-13 17:15pm

I did it in VB6.0 a few years ago ...
And it was deceptively difficult.

I'm trying to produce a reference system that can be made persistent AND will port over more than one language ( and not necessarily any of the CLR/Microsoft languages either.) This example is just for c#.

Sergey Alexandrovich Kryukov 15-Mar-13 18:14pm

Why remembering the lame of VB6. It's good not to remember nightmares... In .NET, it's non-deceptively easy... :-)
—SA

MarkAGr 15-Mar-13 18:25pm

:D ... point accepted.

Sergey Alexandrovich Kryukov 15-Mar-13 18:34pm

Great. :-)

MarkAGr 15-Mar-13 18:40pm

As I'm generating the code, I suppose I shouldn't really need to worry ... just get the generator to do it .... BUT ... I still want to see a nice neat solution!

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**Zoltán Zörgő** · Answer 1 · 2013-03-15T11:26:00

You can have any reason to store things in a dictionary.
But the rest is quite confusing to me: why exactly that way?

You have several options, like these (LinqPad[^] script, but you can see the concept)

C#

Dictionary<string, string> dictionary = new Dictionary<string,string>() {{"i1","John"},{"i2","Paul"},{"i3","Gloria"}};
List<string> list = new List<string>() {"i1","i3"};

// Option A
foreach(string index in list)
{
    dictionary[index].Dump();
}

// Option B
(from index in list select dictionary[index]).Dump();

// Option C
(from index in list join element in dictionary on index equals element.Key select element.Value).Dump();

// Option D
foreach(var @object in dictionary.Where(element => list.Contains(element.Key)).Select(element => element.Value))
{
	@object.Dump();
}

But you can find some other options here too: http://stackoverflow.com/questions/200574/linq-equivalent-of-foreach-for-ienumerablet[^]

And here is a neater solution based on option D, by extracting the query in an extension method:

C#

public static class DictionaryExtension
{
    public static IEnumerable<T> Filter<T> (this Dictionary<string, T> dictionary, List<string> list)
    {
        return dictionary.Where(element => list.Contains(element.Key)).Select(element => element.Value);
    }
}
....
foreach(var @object in dictionary.Filter&lt;string&gt;(list))
    {
        @object.Dump();
    }

Matt T Heffron · Answer 2 · 2013-03-15T12:46:00

Your example of

C#

foreach (object a in lst){
  Console.WriteLine( a )
}

never actually references the Dictionary of interest.
I think an (extension) method that returns IEnumerable<T> would encapsulate what you seem to want:

C#

using System.Linq;
public static class ExtensionMethod
{
  public static IEnumerable<Tvalue> SubSet<Tkey, Tvalue>(this Dictionary<Tkey, Tvalue> dict, IEnumerable<Tkey> inner, Tvalue notFoundValue = default(Tvalue))
  {
    if (inner == null)
      throw new ArgumentNullException("inner");
    return inner.Select(item => {
      Tvalue val;
      if (!dict.TryGetValue(item, out val))
      {
        val = notFoundValue;
      }
      return val;
    });
  }
}

Notice that I've provided a way to specify the value to be returned if the key from the list is not found in the Dictionary.
So you would use this like:

C#

const string NotFound = "NotFound";            // could be string.Empty, etc.
foreach (var a in dict.SubSet(lst, NotFound)){ // the notFoundValue is optional
  Console.WriteLine( a )
}

Matt T Heffron · Answer 3 · 2013-03-15T13:24:00

Based on your comments above, I'll redo this without LINQ:

C#

public static class ExtensionMethods
{
  public static IEnumerable<Tvalue> SubSet<Tkey, Tvalue>(this Dictionary<Tkey, Tvalue> dict, IEnumerable<Tkey> inner, Tvalue notFoundValue = default(Tvalue))
  {
    if (dict == null)
      throw new ArgumentNullException("dict");
    if (inner == null)
      throw new ArgumentNullException("inner");
    foreach (Tkey item in inner)
    {
      Tvalue val;
      if (!dict.TryGetValue(item, out val))
      {
        val = notFoundValue;
      }
      yield return val;
    });
  }

  public static IEnumerable<Tvalue> SubSet<Tkey, Tvalue>(this Dictionary<Tkey, Tvalue> dict, IEnumerable<Tkey> inner)
  {
    return SubSet(dict, inner, default(Tvalue));
  }
}

This (and the previous Solution) are untested but it ought to work! ;). (Not even compiled. Caveat emptor!)

Sergey Alexandrovich Kryukov · Answer 4 · 2013-03-15T11:12:00

You don't really need the list; it looks totally redundant. The only possible reason to iterate by the keys is when you need to preserve the order of items, and the right order is represented by the order in the list, which is not the same as the order in the dictionary, [EDIT] of the list contains only a subset of the keys, as you correctly noted in your comment. It's very good that you know the purpose [END EDIT].

Nevertheless, you can do it. As I can see, your only problem is getting a value by a key. In your loop, your key is a and, as getting the dictionary value from the dictionary is the main feature of the dictionary, please look at MSDN help page and see how to do it. You can use the indexed property, using, in C#, '[]' syntax. And read about indexed properties in C# reference. This should be more than enough.

However, as I say, you are rather advised to ignore your list (see my first paragraphs) and iterate using foreach(var pair in yourDictionary) { /* ... */ }, where pair will be:
http://msdn.microsoft.com/en-us/library/5tbh8a42.aspx[^].

This is really all what you need, event more than that.

I hope you understand why I'm not showing any code. If you don't do such trivial things with your own hands, most likely, after looking at the code, your skills will remain on the same level, called "next to nothing", which I think is not what you want.

Thank you for understanding, good luck,

—SA

c#, lists, collections and itterators

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