Linq Select Array values at specific indexes

Question

I have this method:

public void Foo(double[] values1, double[] values2, int[] indexes)
{
    var values1AtIndexes = new List<double>();
    var values1NotAtIndexes = new List<double>();
    var values2AtIndexes = new List<double>();
    var values2NotAtIndexes = new List<double>();

    for (int i = 0; i < values1.Length; i++)
    {
        if (indexes.Contains(i))
        {
            values1AtIndexes.Add(values1[i]);
            values2AtIndexes.Add(values2[i]);
        }
        else
        {
            values1NotAtIndexes.Add(values1[i]);
            values2NotAtIndexes.Add(values2[i]);
        }
    }
}

And I was wondering if there is a LINQ-way to this?

Raphaël Althaus · Accepted Answer · 2013-12-17 14:57:44Z

7

It could look like that (not looking at perf).

So I'm not sure that it's shorter, or easier to understand.

values1AtIndexes = values1.Where((m, index) => indexes.Contains(index)).ToList();
values2AtIndexes = values2.Where((m, index) => indexes.Contains(index)).ToList();
//alternative
values1AtIndexes = indexes.Select(m => values1[m]).ToList();
values1AtIndexes = indexes.Select(m => values2[m]).ToList();


//might be wrong if your lists contain duplicated values
values1NotAtIndexes = values1.Except(values1AtIndexes).ToList();
values2NotAtIndexes = values2.Except(values2AtIndexes).ToList();

As you pointed, Except might give you wrong results if you have duplicates.

So if you know you can have duplicates, you should rather do

values1NotAtIndexes = values1.Where((m, index) => !indexes.Contains(index)).ToList();

edited Dec 17, 2013 at 14:57

answered Dec 17, 2013 at 14:31

Raphaël Althaus

60.6k6 gold badges106 silver badges127 bronze badges

Sign up to request clarification or add additional context in comments.

3 Comments

metacircle Over a year ago

I think there might be a problem with the Except since it's not certain that there are no duplicate doubles in the source arrays.

metacircle Over a year ago

I will mark this as the accepted answer, but as you stated, in terms of performance my approach will probably be better, because i am only iterating 1 time instead of 4 times.

Raphaël Althaus Over a year ago

@metacircle Indeed (well, you need "big" lists to see any difference, but yes). And the goal of the method is also clearer !

Collectives™ on Stack Overflow

Linq Select Array values at specific indexes

1 Answer 1

3 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

3 Comments

Your Answer

Sign up or log in

Post as a guest

Related