Question

I have an array of strings, f.e.

string [] letters = { "a", "a", "b", "c" };

I need to find a way to determine if any string in the array appears more than once.
I thought the best way is to make a new string-array without the string in question and to use Contains,

foreach (string letter in letters)
{
    string [] otherLetters = //?
    if (otherLetters.Contains(letter))
    {
        //etc.     
    }
}

but I cannot figure out how.
If anyone has a solution for this or a better approach, please answer.

Answer 1

The easiest way is to use GroupBy:

var lettersWithMultipleOccurences = letters.GroupBy(x => x)
                                           .Where(g => g.Count() > 1)
                                           .Select(g => g.Key);

This will first group your array using the letters as keys. It then returns only those groups with multiple entries and returns the key of these groups. As a result, you will have an IEnumerable<string> containing all letters that occur more than once in the original array. In your sample, this is only “a”.

Beware: Because LINQ is implemented using deferred execution, enumerating lettersWithMultipleOccurences multiple times, will perform the grouping and filtering multiple times. To avoid this, call ToList() on the result:

var lettersWithMultipleOccurences = letters.GroupBy(x => x)
                                           .Where(g => g.Count() > 1)
                                           .Select(g => g.Key).
                                           .ToList();

lettersWithMultipleOccurences will now be of type List<string>.