Changing the incremental value of a C# Parallel.For loop

Question

I want to convert a for loop which increments the iterator by 2 every pass into a Parallel For loop using the TPL. The data is not order dependent or constrained in any way, but I only want to process the data in every other element of my source array (which is _Datalist in the code below), hence the need to increment by 2.

My For Loop:

for (int i = 1; i < _DataList.Length - 1; i += 2)
{
     // Do work for _DataList[i]
}

Is it possible to tell the parallel loop that I want to increment i by two instead of one?

Here's the Parallel Loop, but obviously i is only incrementing by 1 each iteration:

        Task.Factory.StartNew(() =>
            Parallel.For(1, _DataList.Length, i =>
            {
                // do work for _DataList[i]                    
            })
        );

I could tell the inner loop body to ignore odd values of i, but that seems a litle messy - is there a way of doing it in the loop initialisation somehow?

Answer 1

You can halve the number of steps and double the indices:

Parallel.For(0, _DataList.Length / 2, i =>
{
    // do work for _DataList[2 * i]                    
});

Answer 2

or alternativley just create a new array that references to every other item and use the parrallel for on that?

Answer 3

How about:

var odds = Enumerable.Range(1, _DataList.Length).Where(i => i % 2 != 0);

Task.Factory.StartNew(() =>
    Parallel.ForEach(odds, i =>
    {
        // do work for _DataList[i]                    
    })
);

Answer 4

Darin Dimitrov's answer shows an easy way to accomplish this.

However, this wasn't added, as it's typically a sign that the loop bodies aren't truly distinct. In most cases, the need to use a different increment value typically only comes with the need for processing in a specific order or other concerns that will cause parallelization to create race conditions.