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.

Shahar Shokrani · Accepted Answer · 2020-03-12 10:59:07Z

1

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)    
   }

edited Mar 12, 2020 at 10:59

Shahar Shokrani

8,93010 gold badges60 silver badges106 bronze badges

answered Mar 12, 2020 at 10:47

Dmitrii Bychenko

188k20 gold badges178 silver badges231 bronze badges

Sign up to request clarification or add additional context in comments.

2 Comments

Shahar Shokrani Over a year ago

Hey @dmitry, great answer. can you please explain why the breakpoint does not break inside the Equals(...)?

Dmitrii Bychenko Over a year ago

@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

DavidG · Accepted Answer · 2020-03-12 10:34:14Z

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, 2020 at 10:34

DavidG

120k13 gold badges232 silver badges239 bronze badges

5 Comments

Shahar Shokrani Over a year ago

Hey @DavidG, thanks, What about the Equals(...)? can I leave it as NotImplementedException?

DavidG Over a year ago

The Equals method won't even be called because all your strings are different. Make this change and it will be.

Shahar Shokrani Over a year ago

When the Equals get called?

DavidG Over a year ago

You know, you can debug this code and work that out for yourself right?

Shahar Shokrani Over a year ago

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.

Community · Accepted Answer · 2020-06-20 09:12:55Z

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, 2020 at 9:12

CommunityBot

11 silver badge

answered Mar 12, 2020 at 11:10

Oliver

9,0322 gold badges45 silver badges63 bronze badges

1 Comment

Shahar Shokrani Over a year ago

Nice but what about index is -1

Collectives™ on Stack Overflow

How to iterate only distinct string values by custom substring equality

3 Answers 3

2 Comments

5 Comments

1 Comment

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

3 Answers 3

2 Comments

5 Comments

1 Comment

Your Answer

Sign up or log in

Post as a guest

Linked

Related