Getting index of duplicate items in a list in c#

Question

I am looking for a way to get the index of all elements in a list from a keyword search within the list. So for example my list has:

Hello World
Programming          


        

Answer 1

searchInList.Select((value, index) => new {value, index})
    .Where(a => string.Equals(a.value, "Hello World"))
    .Select(a => a.index)

If you're trying to search for more than just "Hello World", you could do

searchInList.Select((value, index) => new {value, index})
    .Where(a => stringsToSearchFor.Any(s => string.Equals(a.value, s)))
    .Select(a => a.index)

Answer 2

A little ugly but will work:

    var searchInList = new List<string>();

//Populate your list

    string stringToLookUp= "Hello world";
    var foundHelloWorldIndexes = new List<int>();

    for (int i = 0; i < searchInList.Count; i++)
        if (searchInList[i].Equals(stringToLookUp))
            foundHelloWorldIndexes.Add(i);

Answer 3

The FindAll method for lists is here. The list extension method Where is here.

Both of these will do pretty much exactly what you want and they are pretty easy to use. There are abundant examples of each on the internet but if you need help using them just let me know.

Answer 4

Since you know you're looking for ALL occurrences and therefore you must traverse the entire list anyway, you'll gain a lot of readability over using IndexOf by simply examining each element yourself:

var i=0;
foreach(var value in searchInList)
{
   if(value == "Hello World")
      foundHelloWorld.Add(i); //foundHelloWorld must be an IList

   i++;
}

You can also use an overload of the Linq Select method that incorporate's the element's index in the source collection; this should be highly readable (and thus maintainable) to Linq-experienced programmers:

foundHelloWorld = searchInList
                     .Select((v,i)=>new {Index = i, Value = v})
                     .Where(x=>x.Value == "Hello World")
                     .Select(x=>x.Index)
                     .ToList();

The above code takes the list and transforms the string into a simple anonymous type incorporating each item's place in the original list. Then, it filters down to only matching elements, and then it projects out the Index (which didn't change through the filtering) into a new List object. However, all this transformation will make this solution perform slower, because this statement will traverse the entire list multiple times.