Check a List contains the same value multiple times

Question

Problem

Check collection contains the same number multiple times.

If it does contain the same numbers more than once, then I want to keep the first number and give a new value to the rest of the numbers, which are the same as the first one.

list newFitnessList does contain These numbers:

0. 4054.230995  --> after code= 4054.230995
1. 4041.416004  --> after code= 4041.416004
2. 3926.227397  --> after code= 3926.227397
3. 4722.250903  --> after code= 4722.250903
4. 4722.250903  --> after code= 0
5. 4226.636776  --> after code= 4226.636776
6. 4061.499026  --> after code= 4061.499026
7. 3876.278254  --> after code= 3876.278254
8. 4041.416004  --> after code= 0
9. 4779.468077  --> after code= 4779.468077
10. 4226.636776  --> after code= 0
11. 3876.278254  --> after code= 0
12. 4779.468077  --> after code= 0
13. 3926.227397  --> after code= 0

To achieve the solution explained above I tried the following code, but nothing is happening. The Output of the list is the same as before:

public List<double> sortDoppelganger(List<double> inputFitnessList)
{
  List<double> newFitnessList = inputFitnessList.ToList();
  for(int i = 0; i < newFitnessList.Count; i++)
  {
    double Nothing=0;
    double actual = newFitnessList[i];
    for(int j = newFitnessList.Count-1; j >= 0; j--)
    {
      double next = newFitnessList[j];
      if(actual == next)
      {
        next = Nothing;
      }
    }
  }
  return newFitnessList;
}

I would really appreciate it if someone has any idea what's wrong with my code. And maybe it would be better not to hide, that I'm a newbie with programming.

After reading the explanations: I tried out two idea's explained. First one was the idea of @Tim Schmelter and the second idea was from @user3185569.

And here you can take a look at what i was tyring to achieve:

Here the suggestion of Tim:

Here the Suggestion of user3185569:

Answer 1

You could use a HashSet<double> to find out if there's a duplicate:

public List<double> SortDoppelganger(List<double> inputFitnessList, double replacementValue = 0)
{
    HashSet<double> doppelgangerFinder = new HashSet<double>();
    for (int i = 0; i < inputFitnessList.Count; i++)
    {
        double value = inputFitnessList[i];
        bool istDoppelganger = !doppelgangerFinder.Add(value);
        if (istDoppelganger)
            inputFitnessList[i] = replacementValue;
    }
    return inputFitnessList;
}

This solution modifies the original list. If that's not desired create a copy at the beginning by using var newList = new List<double>(inputFitnessList).

For what it's worth, here is a generic extension method that works with any type:

public static List<T> ReplaceDuplicates<T>(this IEnumerable<T> sequence, T replacementValue)
{
    HashSet<T> duplicateFinder = new HashSet<T>();
    List<T> returnList = new List<T>();
    foreach (T item in sequence)
    {
        bool isDuplicate = !duplicateFinder.Add(item);
        returnList.Add(isDuplicate ? replacementValue : item);
    }
    return returnList;
}

Explanation: user3185569 is right, i have forgotten to mention what you've done wrong. The main problem is that you try to assign the replacement value to the local variable next:

double next = newFitnessList[j];
if(actual == next)
{
  next = Nothing;
}

It has nothing to do with value- or reference-types differences in this case. The only reason why this doesn't work is that you modify only the value of the variable. Not the value the variable was referencing before(newFitnessList[j]). The variable does not even know that it was linked to the list. It just knows the value to store. If it was a reference type the problem would be the same. By replacing it with another value the list won't be modifed at all.

To cut a long story short, this would fix the main problem:

double next = newFitnessList[j];
if(actual == next)
{
  newFitnessList[j] = Nothing;
}