Question

I’m having hard time with switching to LINQ with the following example:

int[] inputs = { 5, 3, 5, 66, 4, 5 };
int[] indexes; // I want to have the indexes of '5's in 
               // the inputs array, which is { 0, 2, 5 }

// My way (traditional for loop)

List<int> indexesList = new List<int>();
for (int i = 0; i < inputs.Length; i++)
    if (inputs[i] == 5)
        indexesList.Add(i);

indexes = indexesList.ToArray();

// LINQ way

var indexes = inputs.Select((s, i) => new { i, s })
                    .Where(t => t.s == 5)
                    .Select(t => t.i).ToArray();

Question 1. In terms of efficiency (speed, memory usage), am I going to have any advantage if I convert my code to LINQ?

Question 2. If true, is there a more elegant way of doing it with LINQ?

PS: Note that, this method is called very frequently in my real project. So, having a little improvement in speed or memory usage will help the overall process a lot.

Answer 1

You are certainly not going to have a performance advantage if your use the LINQ code. This is evident simply by reading it: to associate each value with its index, a new object is created:

(s, i) => new { i, s }

These anonymous objects serve no other purpose than being a vessel to glue index together with value; therefore all the associated memory management is pure overhead compared to straight up keeping a counter.

One might say that LINQ provides a readability advantage because most of the time it highlights the intent instead of the mechanism, but in this particular case I don’t think it’s any better than the prosaic solution.