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c#arrayslinq
initialZero
initialZero

Reputation: 6367

Find index of a value in an array

Can linq somehow be used to find the index of a value in an array?

For instance, this loop locates the key index within an array.

for (int i = 0; i < words.Length; i++)
{
    if (words[i].IsKey)
    {
        keyIndex = i;
    }
}

Upvotes: 127

Views: 176692

Answers (8)

Paolo Moretti
Paolo Moretti

Reputation: 55946

For arrays you can use: Array.FindIndex<T>:

int keyIndex = Array.FindIndex(words, w => w.IsKey);

For lists you can use List<T>.FindIndex:

int keyIndex = words.FindIndex(w => w.IsKey);

You can also write a generic extension method that works for any Enumerable<T>:

///<summary>Finds the index of the first item matching an expression in an enumerable.</summary>
///<param name="items">The enumerable to search.</param>
///<param name="predicate">The expression to test the items against.</param>
///<returns>The index of the first matching item, or -1 if no items match.</returns>
public static int FindIndex<T>(this IEnumerable<T> items, Func<T, bool> predicate) {
    if (items == null) throw new ArgumentNullException("items");
    if (predicate == null) throw new ArgumentNullException("predicate");

    int retVal = 0;
    foreach (var item in items) {
        if (predicate(item)) return retVal;
        retVal++;
    }
    return -1;
}

And you can use LINQ as well:

int keyIndex = words
    .Select((v, i) => new {Word = v, Index = i})
    .FirstOrDefault(x => x.Word.IsKey)?.Index ?? -1;

Upvotes: 73

Roshna Omer
Roshna Omer

Reputation: 721

This solution helped me more, from msdn microsoft:

var result =  query.AsEnumerable().Select((x, index) =>
              new { index,x.Id,x.FirstName});

query is your toList() query.

Upvotes: 1

sidney.andrews
sidney.andrews

Reputation: 5256

int keyIndex = Array.FindIndex(words, w => w.IsKey);

That actually gets you the integer index and not the object, regardless of what custom class you have created

Upvotes: 207

Jonas B&#246;tel
Jonas B&#246;tel

Reputation: 4482

int keyIndex = words.TakeWhile(w => !w.IsKey).Count();

Upvotes: 10

Marcel Valdez Orozco
Marcel Valdez Orozco

Reputation: 3015

int index = -1;
index = words.Any (word => { index++; return word.IsKey; }) ? index : -1;

Upvotes: 0

joelsand
joelsand

Reputation: 2275

Just posted my implementation of IndexWhere() extension method (with unit tests):

http://snipplr.com/view/53625/linq-index-of-item--indexwhere/

Example usage:

int index = myList.IndexWhere(item => item.Something == someOtherThing);

Upvotes: 2

Grizzly
Grizzly

Reputation: 20191

If you want to find the word you can use

var word = words.Where(item => item.IsKey).First();

This gives you the first item for which IsKey is true (if there might be non you might want to use .FirstOrDefault()

To get both the item and the index you can use

KeyValuePair<WordType, int> word = words.Select((item, index) => new KeyValuePair<WordType, int>(item, index)).Where(item => item.Key.IsKey).First();

Upvotes: 7

user110714
user110714

Reputation:

Try this...

var key = words.Where(x => x.IsKey == true);

Upvotes: 4

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