C# Array.Findallindexof Which Findall Indexof

c# Array.FindAllIndexOf which FindAll IndexOf

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");

Get the list of index for a string in an array?

You can try the following,

string[] cars = {"Volvo", "BMW", "Volvo", "Mazda","BMW","BMW"};
//returns **{1,4,5}** as a list
var res = Enumerable.Range(0, cars.Length).Where(i => cars[i] == "BMW").ToList();
//joined items of list as 1,4,5
Label1.Text = string.Join(",", res);

In PowerShell is there a way to return the index of an array with substring of a string

You can use LINQ (adapted from this C# answer):

$myarray = 'herp', 'dederp', 'dedoo', 'flerp'
$substring = 'erp'

[int[]] $indices = [Linq.Enumerable]::Range(0, $myarray.Count).
                     Where({ param($i) $myarray[$i] -match $substring })

$indices receives 0, 1, 3.

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As for what you tried:

$thisiswrong = @($myarray.IndexOf($substring))

System.Array.IndexOf only ever finds one index and matches entire elements, literally and case-sensitively in the case of strings.

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There's a more PowerShell-like, but much slower alternative, as hinted at by js2010's answer; you can take advantage of the fact that the match-information objects that Select-String outputs have a .LineNumber property, reflecting the 1-based index of the position in the input collection - even if the input doesn't come from a file:

$myarray = 'herp', 'dederp', 'dedoo', 'flerp'
$substring = 'erp'

[int[]] $indices = 
  ($myarray | Select-String $substring).ForEach({ $_.LineNumber - 1 })

Note the need to subtract 1 from each .LineNumber value to get the 0-based array indices, and the use of the .ForEach() array method, which performs better than the ForEach-Object cmdlet.

Get corners of a 2d array

From that, I want to get both the first and last non-null elements from the first and last rows each.

2D arrays are a pain in the ass, but they behave like one long array if you iterate them

var w = array.GetLength(1);
var h = array.GetLength(0);
var a = array.Cast<int?>();

var ff = a.First(e => e.HasValue);
var fl = a.Take(w).Last(e => e.HasValue);
var lf = a.Skip((h-1)*w).First(e => e.HasValue);
var ll = a.Last(e => e.HasValue);

Width is 9, Height is 3, Casting turns it into an enumerable of int? which means your corners are the first non null, the last non null in the initial 9, the first non null after skipping 18 and the last non null

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