Selecting elements from array according to indices specified in another array c#

Question

Suppose we have an array with data:

double[] x = new double[N] {x_1, ..., x_N};

And array of size N containing labels corresponding to the elements of x:

int[] ind = new int[N] {i_1, ..., i_N};

What is the fastest way to select all elements from x that have certain label I according to ind?

For example,

x = {3, 2, 6, 2, 5}
ind = {1, 2, 1, 1, 2}
I = ind[0] = 1

Result:

y = {3, 6, 2}

Clearly, it can be easily (but not efficiently and clean) done with loops, but I think there should be the way how to do that using .Where and lambdas..Thanks

EDIT:

The answer provided by MarcinJuraszek is completely correct, thanks. However, I've simplified the question in hope that it would work in my original situation. Could you please take a look what is the problem if we have generic types:

T1[] xn = new T1[N] {x_1, ..., x_N};
T2[] ind = new T2[N] {i_1, ..., i_N};
T2 I = ind[0]

Using the solution provided I get an error "Delegate 'System.Func' does not take 2 arguments":

T1[] y = xn.Where((x, idx) => ind[idx] == I).ToArray();

Thank you very much

Answer 1

How about that:

var xs = new[] { 3, 2, 6, 2, 5 };
var ind = new[] { 1, 2, 1, 1, 2 };
var I = 1;

var results = xs.Where((x, idx) => ind[idx] == I).ToArray();

It uses second, less popular, Where overload:

Enumerable.Where<TSource>(IEnumerable<TSource>, Func<TSource, Int32, Boolean>)

which has item index available as predicate parameter (called idx in my solution).

Generic version

public static T1[] WhereCorresponding<T1, T2>(T1[] xs, T2[] ind) where T2 : IEquatable<T2>
{
    T2 I = ind[0];
    return xs.Where((x, idx) => ind[idx].Equals(I)).ToArray();
}

Usage

static void Main(string[] args)
{
    var xs = new[] { 3, 2, 6, 2, 5 };
    var ind = new[] { 1, 2, 1, 1, 2 };

    var results = WhereCorresponding(xs, ind);
}

Generic + double version

public static T[] Test<T>(T[] xs, double[] ind)
{
    double I = ind[0];

    return xs.Where((x, idx) => ind[idx] == I).ToArray();
}