c# Array.FindAllIndexOf which FindAll IndexOf

Question

I know c# has Array.FindAll and Array.IndexOf.

Is there a Array.FindAllIndexOf which returns int[]?

Answer 1

string[] myarr = new string[] {"s", "f", "s"};

int[] v = myarr.Select((b,i) => b == "s" ? i : -1).Where(i => i != -1).ToArray();

This will return 0, 2

If the value does not exist in the array then it will return a int[0].

make an extension method of it

public static class EM
{
    public static int[] FindAllIndexof<T>(this IEnumerable<T> values, T val)
    {
        return values.Select((b,i) => object.Equals(b, val) ? i : -1).Where(i => i != -1).ToArray();
    }
}

and call it like

string[] myarr = new string[] {"s", "f", "s"};

int[] v = myarr.FindAllIndexof("s");

Answer 2

I know this is an old post, but you can try the following,

string[] cars = {"Volvo", "BMW", "Volvo", "Mazda","BMW","BMW"};
var res = Enumerable.Range(0, cars.Length).Where(i => cars[i] == "BMW").ToList();

returns {1,4,5} as a list

Answer 3

You can solve this problem by creating only 2 integer variables. More power to you!

string[] seasons= { "Fall","Spring", "Summer", "Fall", "Fall", "Winter"};

int i = 0;

int IndexOfFallInArray = 0;

int[] IndexesOfFall= new int[seasons.Length];

foreach (var item in seasons)
{
    if (item == "Fall")
        {
            IndexesOfFall[i] = IndexOfFallInArray;
            i++;
        }
        IndexOfFallInArray++;
}

Answer 4

I've used Nikhil Agrawal's answer to create the following related method, which may be useful.

public static List<int> FindAllIndexOf<T>(List<T> values, List<T> matches)
    {
        // Initialize list
        List<int> index = new List<int>();

        // For each value in matches get the index and add to the list with indexes
        foreach (var match in matches)
        {
            // Find matches 
            index.AddRange(values.Select((b, i) => Equals(b, match) ? i : -1).Where(i => i != -1).ToList());

        }

        return index;
    }

Which takes a list with values and a list with values that are to be matched. It returns a list of integers with the index of the matches.

Answer 5

You can loop with findIndex giving an index

string[] arr = { "abc", "asd", "def", "abc", "lmn", "wer" };
int index = -1;
do
{
    index = Array.IndexOf(arr, "abc", index + 1);
    System.Console.WriteLine(index);
} while (-1 != index);

Answer 6

Searches for an element that matches the conditions defined by the specified predicate, and returns all the zero-based index of the occurrence within the entire System.Array.

public static int[] FindAllIndex<T>(this T[] array, Predicate<T> match)
{
    return array.Select((value, index) => match(value) ? index : -1)
            .Where(index => index != -1).ToArray();
}