C# Dictionary Map

Question

I have a dictionary:

   Dictionary<ICD_Map2, string> maps = new Dictionary<ICD_Map2, string>();

    public class ICD_Map2
    {
        public string call_type {get; set; }
        public string destination{get; set;} 

    }

maps.Add(new ICD_Map2() {call_type = "Mobile SMS", destination = "Australia"},"Local Text");
maps.Add(new ICD_Map2() {call_type = "Mobile SMS", destination = "International"},"International Text");

So what I want is, when I pass two variables:

Case 1 variable1 = "Mobile SMS" && variable2 = "Australia" I want a function to return "Local Text"

Case 2 "International Text" depending on my input variables matching ICD_Map2 definition "Mobile SMS" and "International".`

How do I construct this mapping function to return the First of the set from a set ofresults (if there are more than one)? This is a very simplified example, I have over 100 mappings.

A dictionary, when used correctly, does not have more than 1 result - it is a unique-key to value mapping (the values do not need to be unique). — Marc Gravell, Oct 29 '12 at 08:23

score 3 · Answer 1 · answered Oct 29 '12 at 08:26

3

Although there are quite a few ways you could achieve this, the quickest and simplest I'd presonally use is LINQ's FirstOrDefault like this:

string var1 = "Mobile SMS";
string var2 = "Australia";

var item = maps.FirstOrDefault(e => e.Key.call_type == var1 && e.Key.destination == var2);
string result = (item == null) ? "No value" : item.Value;

In this case, if there is no corresponding match, you'd end up with result being null.

answered Oct 29 '12 at 08:26

Paul Aldred-Bann

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This doesn't make any use of the fact that it is a dictionary, though – Marc Gravell Oct 29 '12 at 08:46
@MarcGravell I absolutely agree, I was just answering the most basic component of the question. – Paul Aldred-Bann Oct 29 '12 at 09:06

Marc Gravell · Accepted Answer · 2012-10-29T08:33:03.943

To use a dictionary, the key needs to support equality operations. For example:

public class ICD_Map2 : IEquatable<ICD_Map2>
{
    public ICD_Map2(string callType, string destination) {
        CallType = callType;
        Destination = destination;
    }
    public override int GetHashCode() {
        int result = 17;
        result = -13 * result +
            (CallType == null ? 0 : CallType.GetHashCode());
        result = -13 * result +
            (Destination == null ? 0 : Destination.GetHashCode());
        return result;
    }
    public override bool Equals(object other) {
        return Equals(other as ICD_Map2);
    }
    public bool Equals(ICD_Map2 other) {
        if(other == null) return false;
        if(other == this) return true;
        return CallType == other.CallType && Destination == other.Destination;
    }
    public string CallType {get; private set; }
    public string Destination{get; private set;} 
}

Note making it read-only is intentional: mutable keys are going to cause huge problems - avoid that.

Now you can use this as a key, for example:

var key = new ICD_Map2("Mobile SMS", "Australia");
string result;
if(maps.TryGetValue(key, out result)) {
    Console.WriteLine("found: " + result);
}

The reverse lookup is problematic, and cannot be optimised unless you have a second dictionary. A simple operation (performance O(n)) would be:

string result = "International Text";
var key = (from pair in maps
           where pair.Value == result
           select pair.Key).FirstOrDefault();
if(key != null) {
    Console.WriteLine("found: " + key);
}

Putting it all together:

static void Main()
{
    Dictionary<ICD_Map2, string> maps = new Dictionary<ICD_Map2, string> {
        {new ICD_Map2 ("Mobile SMS", "Australia"),"Local Text"},
        {new ICD_Map2 ("Mobile SMS", "International"),"International Text"}
    };

    // try forwards lookup
    var key = new ICD_Map2("Mobile SMS", "Australia");
    string result;
    if (maps.TryGetValue(key, out result))
    {
        Console.WriteLine("found: " + result);
    }

    // try reverse lookup (less efficient)
    result = "International Text";
    key = (from pair in maps
               where pair.Value == result
               select pair.Key).FirstOrDefault();
    if (key != null)
    {
        Console.WriteLine("found: " + key);
    }
}

score 2 · Answer 3 · answered Oct 29 '12 at 08:27

Build custom comparer:

public class CusComparer: IEqualityComparer<ICD_Map2>
{
    public bool Equals(ICD_Map2 x, ICD_Map2 y)
    {
        return x.call_type.Equals(y.call_type) 
              && x.destination.Equals(y.destination);
    }

    public int GetHashCode(ICD_Map2 obj)
    {
        return obj.call_type.GetHashCode() 
                 ^ obj.destination.GetHashCode();
    }
}

Remember that Dictionary has another overload constructor with accepting IEqualityComparer:

var maps = new Dictionary<ICD_Map2, string>(new CusComparer());

maps.Add(new ICD_Map2() {
              call_type = "Mobile SMS", 
              destination = "Australia"},
         "Local Text");

maps.Add(new ICD_Map2() {
            call_type = "Mobile SMS", 
            destination = "International"},
         "International Text");

So you can get:

 var local = maps[new ICD_Map2() {
                     call_type = "Mobile SMS", 
                     destination = "Australia"}];

score 0 · Answer 4 · edited Oct 29 '12 at 08:48

0

I think this should work, will double check in a sec:

maps.Where(a => (String.IsNullOrEmpty(var1) || String.Compare(a.Key.call_type, var1) == 0) 
             && (String.IsNullOrEmpty(var2) || String.Compare(a.Key.destination, var2) == 0))
    .FirstOrDefault().Value`

edited Oct 29 '12 at 08:48

cuongle

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answered Oct 29 '12 at 08:22

Mike Trusov

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it does make it pointless to use a dictionary, though... since this doesn't benefit from *any* of the dictionary code – Marc Gravell Oct 29 '12 at 08:23
@MarcGravell Out of curiosity, what else could he use to resemble a mapping between ICD_Map objects and strings? – Mohammad Banisaeid Oct 29 '12 at 08:26
@MohammadBanisaeid he could make the key equatable, he could use a custom comparer, he could use tuples (which have inbuilt equatable implementation), he could use anonymous types, etc – Marc Gravell Oct 29 '12 at 08:38
@Mike - also - why use `string.Compare` here? the obvious method to use would be `string.Equals`...? – Marc Gravell Oct 29 '12 at 08:38
@Mark - RE it being **pointless to sue a dictionary**: yes, performance-wise this is crap, but it takes a fraction of time to implement compared to alternatives :P RE **string.Compare**: it's quite possible he'd want to treat variable-case strings the equal, and there's also CultureInfo to consider (may or may not be applicable in this case), generally when comparing strings I find that using String.Compare and specifying all the possible options explicitly is the safest way to go. – Mike Trusov Oct 29 '12 at 08:43
@Marc I prefer Compare for reasons listed here: http://blogs.msdn.com/b/bclteam/archive/2007/05/31/string-compare-string-equals-josh-free.aspx ... which might be out of date... according to http://stackoverflow.com/questions/44288/differences-in-string-compare-methods-in-c-sharp you're absolutely right though, bad habits are bad I guess – Mike Trusov Oct 29 '12 at 08:58

score 0 · Answer 5 · answered Oct 29 '12 at 08:24

0

You have to implement IEquitable interface on ICD_Map2 and override GetHashCode function (it's better to override Equals(object) too even though it's not necessary as Dictionary uses generic IEquitable interface to find the key)

answered Oct 29 '12 at 08:24

Anton Sorokin

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C# Dictionary Map

5 Answers5