LINQ to find array indexes of a value

Question

Assuming I have the following string array:

string[] str = new string[] {"max", "min", "avg", "max", "avg", "min"} 

Is it possbile to use LINQ to get a list of indexes that match one string?

As an example, I would like to search for the string "avg" and get a list containing

2, 4

meaning that "avg" can be found at str[2] and str[4].

Answer 1

.Select has a seldom-used overload that produces an index. You can use it like this:

str.Select((s, i) => new {i, s})     .Where(t => t.s == "avg")     .Select(t => t.i)     .ToList() 

The result will be a list containing 2 and 4.

Documentation here

Answer 2

You can do it like this:

str.Select((v,i) => new {Index = i, Value = v}) // Pair up values and indexes    .Where(p => p.Value == "avg") // Do the filtering    .Select(p => p.Index); // Keep the index and drop the value 

The key step is using the overload of Select that supplies the current index to your functor.