How can i check for duplicate items in a List?

Question

I have this code:

private void test(List<string> a)
{

}

I want to use for in for loop's and to check that if there are duplicated items in the List a Then mark it with null.

You might as well not have posted any code at all, as what you did post is meaningless. Voting to close this as "not a real question", as it shows zero effort to solve the problem yourself, and no research at all before posting it. — Ken White, Sep 14 '12 at 00:07

score 1 · Accepted Answer · answered Sep 14 '12 at 00:01

1

Not sure exactly what you want to do here but if you wanted to just get a collection of the distinct values in the list you could use LINQ to do this:

var distinctOnes = a.Distinct();

answered Sep 14 '12 at 00:01

itsmatt

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score 0 · Answer 2 · answered Sep 14 '12 at 00:02

Loop through the strings and put new strings in a HashSet so that you can efficiently check for repeats:

var set = new HashSet<string>();
for (int i = 0; i < a.Count; i++) {
  if (set.Contains(a[i])) {
    // duplicate
    a[i] = null;
  } else {
    set.Add(a[i]);
  }
}

score 0 · Answer 3 · edited May 23 '17 at 12:33

0

You should really avoid methods with side effects Side Effects Answer. You can achieve what you want like this:

private List<String> test(List<String> originalList)
{
    List<String> returnList = new ArrayList<String>();
    for (String value : originalList)
    {
        if (!returnList.contains(value))
        {
            returnList.add(value);
        }
    }
    return returnList;
}

You can easily amend this method to add null as well if you really wanted to (instead of not adding anything if its already been added)...

edited May 23 '17 at 12:33

Community

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answered Sep 14 '12 at 00:03

ramsinb

1,985
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Rather than the (particularly inefficient) method, you could just use `return originalList.Distinct()`; – Servy Sep 14 '12 at 01:31

score 0 · Answer 4 · answered Sep 14 '12 at 00:03

0

private bool HasDuplicates(List<string> a)
{
    return a.Distinct().Count() != a.Count();
}

answered Sep 14 '12 at 00:03

armen.shimoon

6,303
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score 0 · Answer 5 · answered Sep 14 '12 at 00:04

0

you can use LINQ

var list = (from n in a select n).Distinct().ToList();

so know you have a unique list that you can check against "a"

answered Sep 14 '12 at 00:04

Ehsan Zargar Ershadi

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why do you have an identity select at the start of the query? Just call `list.Distinct()`. – Servy Sep 14 '12 at 01:32

How can i check for duplicate items in a List?

5 Answers5