What is System.Numerics.Vector.ConditionalSelect used for?

Question

Can anyone explain preferably with an example what/when/how System.Numerics.Vector.ConditionalSelect can be used?

I cannot understand much from the docs.

Answer 1

The condition parameter is actually Vector<int> when using 4 byte overloads of the ConditionalSelect such as the ones with Vector<int> or Vector<float> arguments. condition is Vector<long> when using 8 byte versions, etc.

To expand on @Hans' comment in your question, condition means something like: double c = cond == -1 ? a : b;. That is, when cond == -1, it selects the left values. When cond == 0, it selects the right values.

When cond is something else, I saw some results I don't particularly understand yet and didn't actually research.

class Program
{
    static void Main(string[] args)
    {
        //Length depends on your Vector<int>.Count. In my computer it is 4
        Vector<int> vector1 = new Vector<int>(4); //vector1 == {<4, 4, 4, 4>}
        Vector<int> vector2 = new Vector<int>(5); //vector2 == {<5, 5, 5, 5>}
        Vector<int> mask = Vector.GreaterThan(vector1, vector2); //mask == {<0, 0, 0, 0>}
        Vector<int> selected = Vector.ConditionalSelect(mask, vector1, vector2); //selected == {<5, 5, 5, 5>}

        vector1 = new Vector<int>(4); //vector1 == {<4, 4, 4, 4>}
        vector2 = new Vector<int>(3); //vector2 == {<3, 3, 3, 3>}
        mask = Vector.GreaterThan(vector1, vector2); //mask == {<-1, -1, -1, -1>}
        selected = Vector.ConditionalSelect(mask, vector1, vector2); //selected == {<4, 4, 4, 4>}

        mask = new Vector<int>(123); //mask == {<123, 123, 123, 123>}
        selected = Vector.ConditionalSelect(mask, vector1, vector2); //selected == {<0, 0, 0, 0>}

        mask = new Vector<int>(4);
        selected = Vector.ConditionalSelect(mask, vector1, vector2); //selected == {<7, 7, 7, 7>}
    }
}