How To Find An Element’s Index In An Array In C#

The index of an element in an array in C# will be covered in this article.

If the requested element is absent from the array, the solution should return -1 instead of the index of the element’s first occurrence.

1. Making use of an array. IndexOf() method –
The method Array.IndexOf(), which returns the index of the first instance of the supplied element in this array, is the one that is advised as a fix.

using System;
 
public static class Extensions
{
    public static int findIndex<T>(this T[] array, T item) {
        return Array.IndexOf(array, item);
    }
}
 
public class Example
{
    public static void Main()
    {
        int[] array = { 1, 2, 3, 4, 5 };
        int item = 4;
 
        int index = array.findIndex(item);
        if (index != -1) {
            Console.WriteLine(String.Format("Element {0} is found at index {1}", item, index));
        }
        else {
            Console.WriteLine("Element not found in the given array.");
        }
    }
}
Output: Element 4 is found at index 3

2. Making use of an array. the FindIndex() function –
The function FindIndex() provides the index of the first element that meets the given criteria, or -1 if no such element exists.

using System;
 
public static class Extensions
{
    public static int findIndex<T>(this T[] array, T item) {
        return Array.FindIndex(array, val => val.Equals(item));
    }
}
 
public class Example
{
    public static void Main()
    {
        int[] array = { 1, 2, 3, 4, 5 };
        int item = 4;
 
        int index = array.findIndex(item);
        if (index != -1) {
            Console.WriteLine(String.Format("Element {0} is found at index {1}", item, index));
        }
        else {
            Console.WriteLine("Element not found in the given array.");
        }
    }
}
 
/*
    Output: Element 4 is found at index 3
*/

3. Making use of Enumerable. the Select() function –
The System.Linq.Enumerable. The sequence’s individual elements are projected into new forms via the Select() function. The code snippet below shows how to use Select() to iterate over a list of values and discover the first instance of a given element in an array by using both the value and the index of each element.

using System;
using System.Linq;
using System.Collections.Generic;
 
public static class Extensions
{
    public static int findIndex<T>(this T[] array, T item)
    {
        try {
            return array
                .Select((element, index) => new KeyValuePair<T, int>(element, index))
                .First(x => x.Key.Equals(item)).Value;
        }
        catch (InvalidOperationException) {
            return -1;
        }
    }
}
 
public class Example
{
    public static void Main()
    {
        int[] array = { 1, 2, 3, 4, 5 };
        int item = 4;
 
        int index = array.findIndex(item);
        if (index != -1) {
            Console.WriteLine(String.Format("Element {0} is found at index {1}", item, index));
        }
        else {
            Console.WriteLine("Element not found in the given array.");
        }
    }
}
 
/*
    Output: Element 4 is found at index 3
*/

4.Using FirstOrDefault() instead of First() will allow us to bypass the try-catch block:

using System;
using System.Linq;
 
public static class Extensions
{
    public static int findIndex<T>(this T[] array, T item)
    {
        return array
            .Select((element, index) => new { element, index })
            .FirstOrDefault(x => x.element.Equals(item)) ?. index ?? -1;
    }
}
 
public class Example
{
    public static void Main()
    {
        int[] array = { 1, 2, 3, 4, 5 };
        int item = 4;
 
        int index = array.findIndex(item);
        if (index != -1) {
            Console.WriteLine(String.Format("Element {0} is found at index {1}", item, index));
        }
        else {
            Console.WriteLine("Element not found in the given array.");
        }
    }
}
 
/*
    Output: Element 4 is found at index 3
*/

5.Implementing a linear search –
To determine whether the target element is present in the array, a basic way is to run a linear search on the supplied array.

using System;
using System.Collections.Generic;
 
public static class Extensions
{
    public static int findIndex<T>(this T[] array, T item)
    {
        EqualityComparer<T> comparer = EqualityComparer<T>.Default;
        for (int i = 0; i < array.Length; i++)
        {
            if (comparer.Equals(array[i], item)) {
                return i;
            }
        }
 
        return -1;
    }
}
 
public class Example
{
    public static void Main()
    {
        int[] array = { 1, 2, 3, 4, 5 };
        int item = 4;
 
        int index = array.findIndex(item);
        if (index != -1) {
            Console.WriteLine(String.Format("Element {0} is found at index {1}", item, index));
        }
        else {
            Console.WriteLine("Element not found in the given array.");
        }
    }
}
 
/*
    Output: Element 4 is found at index 3
*/

That’s all there is to locating an element’s index in an array in C#.