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How to remove duplicates from object list based on that object property in c#

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0

I've got a problem with removing duplicates at runtime from my list of object.

I would like to remove duplicates from my list of object and then set counter=counter+1 of base object.

public class MyObject 

 MyObject(string name) 
 
 this.counter = 0;
 this.name = name;
 
 public string name;
 public int counter;


List<MyObject> objects_list = new List<MyObject>();
objects_list.Add(new MyObject("john"));
objects_list.Add(new MyObject("anna"));
objects_list.Add(new MyObject("john"));
foreach (MyObject my_object in objects_list) 

 foreach (MyObject my_second_object in objects_list) 
 
 if (my_object.name == my_second_object.name) 
 
 my_object.counter = my_object.counter + 1;
 objects_list.remove(my_second_object);
 
 

It return an error, because objects_list is modified at runtime. How can I get this working?

c#

share|improve this question

edited Mar 28 at 13:10

equnex

asked Mar 28 at 13:00

equnexequnex

322 bronze badges

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  do a for loop instead of a foreach and go backwards. this way when you remove an element you can still move up the list
  
  – sLw
  Mar 28 at 13:02
  
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  If you want to mofify the elemnts in a list while iterating that list, use for instead of foreach.
  
  – HimBromBeere
  Mar 28 at 13:02
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  You are by the way comparing object to object.name, which makes no sense and cannot compile, I guess just a mistype?
  
  – Ilya Chernomordik
  Mar 28 at 13:09
  
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  I can't just use for, because during delete the iterator will change. Also I can't make two backwards for, because I will get the same error.
  
  – equnex
  Mar 28 at 13:09
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  You can't use foreach either to iterate a collection that you change
  
  – Ilya Chernomordik
  Mar 28 at 13:10
  

  
  
  
  

 | 
show 2 more comments

0

I've got a problem with removing duplicates at runtime from my list of object.

I would like to remove duplicates from my list of object and then set counter=counter+1 of base object.

public class MyObject 

 MyObject(string name) 
 
 this.counter = 0;
 this.name = name;
 
 public string name;
 public int counter;


List<MyObject> objects_list = new List<MyObject>();
objects_list.Add(new MyObject("john"));
objects_list.Add(new MyObject("anna"));
objects_list.Add(new MyObject("john"));
foreach (MyObject my_object in objects_list) 

 foreach (MyObject my_second_object in objects_list) 
 
 if (my_object.name == my_second_object.name) 
 
 my_object.counter = my_object.counter + 1;
 objects_list.remove(my_second_object);
 
 

It return an error, because objects_list is modified at runtime. How can I get this working?

c#

share|improve this question

edited Mar 28 at 13:10

equnex

asked Mar 28 at 13:00

equnexequnex

322 bronze badges

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  do a for loop instead of a foreach and go backwards. this way when you remove an element you can still move up the list
  
  – sLw
  Mar 28 at 13:02
  
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  If you want to mofify the elemnts in a list while iterating that list, use for instead of foreach.
  
  – HimBromBeere
  Mar 28 at 13:02
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  You are by the way comparing object to object.name, which makes no sense and cannot compile, I guess just a mistype?
  
  – Ilya Chernomordik
  Mar 28 at 13:09
  
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  I can't just use for, because during delete the iterator will change. Also I can't make two backwards for, because I will get the same error.
  
  – equnex
  Mar 28 at 13:09
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  You can't use foreach either to iterate a collection that you change
  
  – Ilya Chernomordik
  Mar 28 at 13:10
  

  
  
  
  

 | 
show 2 more comments

I've got a problem with removing duplicates at runtime from my list of object.

I would like to remove duplicates from my list of object and then set counter=counter+1 of base object.

public class MyObject 

 MyObject(string name) 
 
 this.counter = 0;
 this.name = name;
 
 public string name;
 public int counter;


List<MyObject> objects_list = new List<MyObject>();
objects_list.Add(new MyObject("john"));
objects_list.Add(new MyObject("anna"));
objects_list.Add(new MyObject("john"));
foreach (MyObject my_object in objects_list) 

 foreach (MyObject my_second_object in objects_list) 
 
 if (my_object.name == my_second_object.name) 
 
 my_object.counter = my_object.counter + 1;
 objects_list.remove(my_second_object);
 
 

It return an error, because objects_list is modified at runtime. How can I get this working?

c#

share|improve this question

edited Mar 28 at 13:10

equnex

asked Mar 28 at 13:00

equnexequnex

322 bronze badges

public class MyObject 

 MyObject(string name) 
 
 this.counter = 0;
 this.name = name;
 
 public string name;
 public int counter;


List<MyObject> objects_list = new List<MyObject>();
objects_list.Add(new MyObject("john"));
objects_list.Add(new MyObject("anna"));
objects_list.Add(new MyObject("john"));
foreach (MyObject my_object in objects_list) 

 foreach (MyObject my_second_object in objects_list) 
 
 if (my_object.name == my_second_object.name) 
 
 my_object.counter = my_object.counter + 1;
 objects_list.remove(my_second_object);
 
 

share|improve this question

edited Mar 28 at 13:10

equnex

asked Mar 28 at 13:00

equnexequnex

322 bronze badges

share|improve this question

edited Mar 28 at 13:10

equnex

asked Mar 28 at 13:00

equnexequnex

322 bronze badges

asked Mar 28 at 13:00

equnexequnex

322 bronze badges

asked Mar 28 at 13:00

equnexequnex

322 bronze badges

2 Answers
2

active

oldest

votes

0

The other answer seem to be correct, though I think it will do scan of the whole list twice, depending on your requirement this might or might not be good enough. Here is how you can do it in one go:

var dictionary = new Dictionary<string, MyObject>();
foreach(var obj in objects_list) 

 if(!dictionary.ContainsKey(obj.name)
 
 dictionary[obj.name] = obj;
 obj.counter++;
 
 else
 
 dictionary[obj.name].counter++;
 

Then dictionary.Values will contain your collection

share|improve this answer

edited Mar 28 at 13:36

answered Mar 28 at 13:14

Ilya ChernomordikIlya Chernomordik

11.1k88 gold badges5353 silver badges101101 bronze badges

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  That also won´t give number of dupes.
  
  – HimBromBeere
  Mar 28 at 13:15
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Well, the example is simplified, but list can contain 100 000 records :/
  
  – equnex
  Mar 28 at 13:15
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Not sure which number of dupes OP wanted: the one I counted or the number of "uniqeq" dupes
  
  – Ilya Chernomordik
  Mar 28 at 13:16
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Then you'll need to create a dictionary
  
  – Ilya Chernomordik
  Mar 28 at 13:17
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  Just count of the same names. If list contains MyObject with name "anna" counter at the end should be 1, if list contains two MyObject with names "john" counter at the end should be 2. I think it's clear.
  
  – equnex
  Mar 28 at 13:18
  

  
  
  
  

 | 
show 5 more comments

1

With a help of Linq GroupBy we can combine duplicates in a single group and process it (i.e. return an item which represents all the duplicates):

 List<MyObject> objects_list = ...

 objects_list = objects_list
 .GroupBy(item => item.name) 
 .Select(group => // given a group of duplicates we
 var item = group.First(); // - take the 1st item
 item.counter = group.Sum(g => g.counter); // - update its counter
 return item; // - and return it instead of group
 )
 .ToList();

share|improve this answer

edited Mar 28 at 13:27

answered Mar 28 at 13:05

Dmitry BychenkoDmitry Bychenko

123k1515 gold badges115115 silver badges150150 bronze badges

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  With a little bit change it works: item.counter = group.Sum(g => g.counter + 1); but as far I'm not from C# and it looks a little bit confusing for me.
  
  – equnex
  Mar 28 at 13:25
  
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Actually it should work if you just write item.counter = group.Count(). You will get the number of same names in each group (you might want to subtract one if that's what you want)
  
  – Ilya Chernomordik
  Mar 28 at 13:32
  
  

  
  
  
  

add a comment
 | 

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0

The other answer seem to be correct, though I think it will do scan of the whole list twice, depending on your requirement this might or might not be good enough. Here is how you can do it in one go:

var dictionary = new Dictionary<string, MyObject>();
foreach(var obj in objects_list) 

 if(!dictionary.ContainsKey(obj.name)
 
 dictionary[obj.name] = obj;
 obj.counter++;
 
 else
 
 dictionary[obj.name].counter++;
 

Then dictionary.Values will contain your collection

share|improve this answer

edited Mar 28 at 13:36

answered Mar 28 at 13:14

Ilya ChernomordikIlya Chernomordik

11.1k88 gold badges5353 silver badges101101 bronze badges

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  That also won´t give number of dupes.
  
  – HimBromBeere
  Mar 28 at 13:15
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Well, the example is simplified, but list can contain 100 000 records :/
  
  – equnex
  Mar 28 at 13:15
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Not sure which number of dupes OP wanted: the one I counted or the number of "uniqeq" dupes
  
  – Ilya Chernomordik
  Mar 28 at 13:16
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Then you'll need to create a dictionary
  
  – Ilya Chernomordik
  Mar 28 at 13:17
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  Just count of the same names. If list contains MyObject with name "anna" counter at the end should be 1, if list contains two MyObject with names "john" counter at the end should be 2. I think it's clear.
  
  – equnex
  Mar 28 at 13:18
  

  
  
  
  

 | 
show 5 more comments

0

The other answer seem to be correct, though I think it will do scan of the whole list twice, depending on your requirement this might or might not be good enough. Here is how you can do it in one go:

var dictionary = new Dictionary<string, MyObject>();
foreach(var obj in objects_list) 

 if(!dictionary.ContainsKey(obj.name)
 
 dictionary[obj.name] = obj;
 obj.counter++;
 
 else
 
 dictionary[obj.name].counter++;
 

Then dictionary.Values will contain your collection

share|improve this answer

edited Mar 28 at 13:36

answered Mar 28 at 13:14

Ilya ChernomordikIlya Chernomordik

11.1k88 gold badges5353 silver badges101101 bronze badges

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  That also won´t give number of dupes.
  
  – HimBromBeere
  Mar 28 at 13:15
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Well, the example is simplified, but list can contain 100 000 records :/
  
  – equnex
  Mar 28 at 13:15
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Not sure which number of dupes OP wanted: the one I counted or the number of "uniqeq" dupes
  
  – Ilya Chernomordik
  Mar 28 at 13:16
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Then you'll need to create a dictionary
  
  – Ilya Chernomordik
  Mar 28 at 13:17
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  Just count of the same names. If list contains MyObject with name "anna" counter at the end should be 1, if list contains two MyObject with names "john" counter at the end should be 2. I think it's clear.
  
  – equnex
  Mar 28 at 13:18
  

  
  
  
  

 | 
show 5 more comments

The other answer seem to be correct, though I think it will do scan of the whole list twice, depending on your requirement this might or might not be good enough. Here is how you can do it in one go:

var dictionary = new Dictionary<string, MyObject>();
foreach(var obj in objects_list) 

 if(!dictionary.ContainsKey(obj.name)
 
 dictionary[obj.name] = obj;
 obj.counter++;
 
 else
 
 dictionary[obj.name].counter++;
 

Then dictionary.Values will contain your collection

share|improve this answer

edited Mar 28 at 13:36

answered Mar 28 at 13:14

Ilya ChernomordikIlya Chernomordik

11.1k88 gold badges5353 silver badges101101 bronze badges

var dictionary = new Dictionary<string, MyObject>();
foreach(var obj in objects_list) 

 if(!dictionary.ContainsKey(obj.name)
 
 dictionary[obj.name] = obj;
 obj.counter++;
 
 else
 
 dictionary[obj.name].counter++;
 

share|improve this answer

edited Mar 28 at 13:36

answered Mar 28 at 13:14

Ilya ChernomordikIlya Chernomordik

11.1k88 gold badges5353 silver badges101101 bronze badges

answered Mar 28 at 13:14

Ilya ChernomordikIlya Chernomordik

11.1k88 gold badges5353 silver badges101101 bronze badges

answered Mar 28 at 13:14

Ilya ChernomordikIlya Chernomordik

11.1k88 gold badges5353 silver badges101101 bronze badges

1

With a help of Linq GroupBy we can combine duplicates in a single group and process it (i.e. return an item which represents all the duplicates):

 List<MyObject> objects_list = ...

 objects_list = objects_list
 .GroupBy(item => item.name) 
 .Select(group => // given a group of duplicates we
 var item = group.First(); // - take the 1st item
 item.counter = group.Sum(g => g.counter); // - update its counter
 return item; // - and return it instead of group
 )
 .ToList();

share|improve this answer

edited Mar 28 at 13:27

answered Mar 28 at 13:05

Dmitry BychenkoDmitry Bychenko

123k1515 gold badges115115 silver badges150150 bronze badges

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  With a little bit change it works: item.counter = group.Sum(g => g.counter + 1); but as far I'm not from C# and it looks a little bit confusing for me.
  
  – equnex
  Mar 28 at 13:25
  
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Actually it should work if you just write item.counter = group.Count(). You will get the number of same names in each group (you might want to subtract one if that's what you want)
  
  – Ilya Chernomordik
  Mar 28 at 13:32
  
  

  
  
  
  

add a comment
 | 

1

With a help of Linq GroupBy we can combine duplicates in a single group and process it (i.e. return an item which represents all the duplicates):

 List<MyObject> objects_list = ...

 objects_list = objects_list
 .GroupBy(item => item.name) 
 .Select(group => // given a group of duplicates we
 var item = group.First(); // - take the 1st item
 item.counter = group.Sum(g => g.counter); // - update its counter
 return item; // - and return it instead of group
 )
 .ToList();

share|improve this answer

edited Mar 28 at 13:27

answered Mar 28 at 13:05

Dmitry BychenkoDmitry Bychenko

123k1515 gold badges115115 silver badges150150 bronze badges

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  With a little bit change it works: item.counter = group.Sum(g => g.counter + 1); but as far I'm not from C# and it looks a little bit confusing for me.
  
  – equnex
  Mar 28 at 13:25
  
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Actually it should work if you just write item.counter = group.Count(). You will get the number of same names in each group (you might want to subtract one if that's what you want)
  
  – Ilya Chernomordik
  Mar 28 at 13:32
  
  

  
  
  
  

add a comment
 | 

With a help of Linq GroupBy we can combine duplicates in a single group and process it (i.e. return an item which represents all the duplicates):

 List<MyObject> objects_list = ...

 objects_list = objects_list
 .GroupBy(item => item.name) 
 .Select(group => // given a group of duplicates we
 var item = group.First(); // - take the 1st item
 item.counter = group.Sum(g => g.counter); // - update its counter
 return item; // - and return it instead of group
 )
 .ToList();

share|improve this answer

edited Mar 28 at 13:27

answered Mar 28 at 13:05

Dmitry BychenkoDmitry Bychenko

123k1515 gold badges115115 silver badges150150 bronze badges

 List<MyObject> objects_list = ...

 objects_list = objects_list
 .GroupBy(item => item.name) 
 .Select(group => // given a group of duplicates we
 var item = group.First(); // - take the 1st item
 item.counter = group.Sum(g => g.counter); // - update its counter
 return item; // - and return it instead of group
 )
 .ToList();

share|improve this answer

edited Mar 28 at 13:27

answered Mar 28 at 13:05

Dmitry BychenkoDmitry Bychenko

123k1515 gold badges115115 silver badges150150 bronze badges

answered Mar 28 at 13:05

Dmitry BychenkoDmitry Bychenko

123k1515 gold badges115115 silver badges150150 bronze badges

answered Mar 28 at 13:05

Dmitry BychenkoDmitry Bychenko

123k1515 gold badges115115 silver badges150150 bronze badges