How to iterate only distinct string values by custom substring equality

Question

Similar to this question, I'm trying to iterate only distinct values of sub-string of given strings, for example:

List<string> keys = new List<string>()
{
    "foo_boo_1",
    "foo_boo_2,
    "foo_boo_3,
    "boo_boo_1"
}

The output for the selected distinct values should be (select arbitrary the first sub-string's distinct value):

foo_boo_1 (the first one)
boo_boo_1

I've tried to implement this solution using the IEqualityComparer with:

public class MyEqualityComparer : IEqualityComparer<string>
{
    public bool Equals(string x, string y)
    {            
        int xIndex = x.LastIndexOf("_"); 
        int yIndex = y.LastIndexOf("_");
        if (xIndex > 0 && yIndex > 0)
            return x.Substring(0, xIndex) == y.Substring(0, yIndex);
        else
            return false;
    }

    public int GetHashCode(string obj)
    {
        return obj.GetHashCode();
    }
}

foreach (var key in myList.Distinct(new MyEqualityComparer()))
{
    Console.WriteLine(key)    
}

But the resulted output is:

foo_boo_1
foo_boo_2
foo_boo_3
boo_boo_1

Using the IEqualityComparer How do I remove the sub-string distinct values (foo_boo_2 and foo_boo_3)?

*Please note that the "real" keys are a lot longer, something like "1_0_8-B153_GF_6_2", therefore I must use the LastIndexOf.

score 1 · Answer 1 · answered Mar 12 '20 at 10:34

1

Your GetHashCode method in your equality comparer is returning the hash code for the entire string, just make it hash the substring, for example:

public int GetHashCode(string obj)
{
    var index = obj.LastIndexOf("_");
    return obj.Substring(0, index).GetHashCode();
}

answered Mar 12 '20 at 10:34

DavidG

113,891
12
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Hey @DavidG, thanks, What about the `Equals(...)`? can I leave it as NotImplementedException? – Shahar Shokrani Mar 12 '20 at 10:35
The `Equals` method won't even be called because all your strings are different. Make this change and it will be. – DavidG Mar 12 '20 at 10:37
When the Equals get called? – Shahar Shokrani Mar 12 '20 at 10:40
You know, you can debug this code and work that out for yourself right? – DavidG Mar 12 '20 at 10:42
I've already tried to put a breakpoint inside the `Equals(...)` but it won't break. when I make it as `NotImplementedException` the program crash. It seem to work only when the `Equals(...)` refer the `GetHashCode(...)`, thanks. – Shahar Shokrani Mar 12 '20 at 10:51

score 1 · Accepted Answer · edited Mar 12 '20 at 10:59

Your current implementation has some flaws:

Both Equals and GetHashCode must never throw exception (you have to check for null)
If Equals returns true for x and y then GetHashCode(x) == GetHashCode(y). Counter example is "abc_1" and "abc_2".

The 2nd error can well cause Distinct return incorrect results (Distinct first compute hash).

Correct code can be something like this

public class MyEqualityComparer : IEqualityComparer<string> {
  public bool Equals(string x, string y) {            
    if (ReferenceEquals(x, y))
      return true;
    else if ((null == x) || (null == y))
      return false;

    int xIndex = x.LastIndexOf('_'); 
    int yIndex = y.LastIndexOf('_');

    if (xIndex >= 0)         
      return (yIndex >= 0) 
        ? x.Substring(0, xIndex) == y.Substring(0, yIndex)
        : false;
    else if (yIndex >= 0)         
      return false;
    else
      return x == y; 
  }

  public int GetHashCode(string obj) {
    if (null == obj)  
      return 0;

    int index = obj.LastIndexOf('_');

    return index < 0 
      ? obj.GetHashCode() 
      : obj.Substring(0, index).GetHashCode();
  }
}

Now you are ready to use it with Distinct:

   foreach (var key in myList.Distinct(new MyEqualityComparer())) {
     Console.WriteLine(key)    
   }

Hey @dmitry, great answer. can you please explain why the breakpoint does not break inside the `Equals(...)`? — Shahar Shokrani, Mar 12 '20 at 10:53
@Shahar Shokrani: `Distinct` compares `x` and `y` in **2** stages: 1st it compares *hashes*; if hash codes are different there's not need to call `Equals` and only if hash codes are *equal* it runs `Equals`. Our hashes are good, and it can well appear we don't need `Equals` at all — Dmitry Bychenko, Mar 12 '20 at 11:00

score 1 · Answer 3 · edited Jun 20 '20 at 09:12

1

For a more succinct solution that avoids using a custom IEqualityComparer<>, you could utilise GroupBy. For example:

var keys = new List<string>()
{
    "foo_boo_1",
    "foo_boo_2",
    "foo_boo_3",
    "boo_boo_1"
};

var distinct = keys
    .Select(k => new
    {
        original = k,
        truncated = k.Contains("_") ? k.Substring(0, k.LastIndexOf("_")) : k
    })
    .GroupBy(k => k.truncated)
    .Select(g => g.First().original);

This outputs:

foo_boo_1

boo_boo_1

edited Jun 20 '20 at 09:12

Community

1
1

answered Mar 12 '20 at 11:10

Oliver

8,794
2
40
60

Nice but what about index is -1 – Shahar Shokrani Mar 12 '20 at 11:44

How to iterate only distinct string values by custom substring equality

3 Answers3