Array.IndexOf

This .NET method searches an array from start to end. It acts upon an array of any type, and returns the position of the value. LastIndexOf goes from end to start.

IndexOf, in both its generic form and its LastIndexOf form, is useful in many programs. This is declarative (method-based) programming.

IndexOf example

The Array type is an abstract base type, which means it can be used with instances of arrays in your program. We call the Array.IndexOf static method.

using System;

// Example integer array is declared.
int[] array = new int[6];
array[0] = 1;
array[1] = 3;
array[2] = 5;
array[3] = 7;
array[4] = 8;
array[5] = 5;

// Find index of element with value 5.
int index1 = Array.IndexOf(array, 5);

// Find index of value 3.
int index2 = Array.IndexOf<int>(array, 3);

// Find last index of 5.
int index3 = Array.LastIndexOf(array, 5);

// Write the results.
Console.WriteLine(index1);
Console.WriteLine(index2);
Console.WriteLine(index3);
2
1
5

String example

Here we use a string array and Array.IndexOf. The result of IndexOf is -1 when the value is not found. The IndexOf method uses the default IEqualityComparer for the type.

using System;

// Example string array is declared.
string[] array = new string[6];
array[0] = null;
array[1] = "carrot";
array[2] = "rat";
array[3] = "";
array[4] = "carrot";
array[5] = "apple";

// Find string with this value starting at offset 2.
int index1 = Array.IndexOf(array, "carrot", 2, 3);

// Find a nonexistent string.
int index2 = Array.IndexOf(array, "banana");

// Write the result.
Console.WriteLine(index1);
Console.WriteLine(index2);
4
-1

LastIndexOf

Array.LastIndexOf finds the last matching element. It searches from the end of an array. It returns the index of the element that contains the specified value.

using System;

int[] array = { 2, 4, 6, 8, 6, 2 };

int result1 = Array.IndexOf(array, 6);
Console.WriteLine(result1);

int result2 = Array.LastIndexOf(array, 6);
Console.WriteLine(result2);

int result3 = Array.LastIndexOf(array, 100);
Console.WriteLine(result3);
2
4
-1

Benchmark

Here we test IndexOf performance. Internally, the generic method has lots of error checking for invalid arguments, and calls into a virtual method depending on the type.

using System;
using System.Diagnostics;

class Program
{
    static int[] _array = { 3, 5, 7, 9, 11, 13 };

    const int _max = 1000000;
    static void Main()
    {
        // Version 1: use Array.IndexOf.
        var s1 = Stopwatch.StartNew();
        for (int i = 0; i < _max; i++)
        {
            int index = Array.IndexOf<int>(_array, 7);
            if (index == -1)
            {
                return;
            }
        }
        s1.Stop();
        // Version 2: use for-loop.
        var s2 = Stopwatch.StartNew();
        for (int i = 0; i < _max; i++)
        {
            int index = IndexOfInt(_array, 7);
            if (index == -1)
            {
                return;
            }
        }
        s2.Stop();
        Console.WriteLine(((double)(s1.Elapsed.TotalMilliseconds * 1000000) / _max).ToString("0.00 ns"));
        Console.WriteLine(((double)(s2.Elapsed.TotalMilliseconds * 1000000) / _max).ToString("0.00 ns"));
    }

    static int IndexOfInt(int[] arr, int value)
    {
        for (int i = 0; i < arr.Length; i++)
        {
            if (arr[i] == value)
            {
                return i;
            }
        }
        return -1;
    }
}
11.14 ns    Array.IndexOf
4.77 ns     for-loop

Notes, not found condition

Array.LastIndexOf, like Array.IndexOf, uses an error code of negative one to indicate a not-found condition. This can be at first confusing.

Discussion

Arrays are lower-level types than other types such as the List in .NET. When you use arrays you should expect to have to do some looping on your own.

We tested the Array.IndexOf method and found it to be much slower than searching with a custom method. We tested the generic form of Array.IndexOf as well.