Linq/lambda question about .Select (newby learning 3.0)

Question

I am playing with the new stuff of C#3.0 and I have this code (mostly taken from MSDN) but I can only get true,false,true... and not the real value :

        int[] numbers = { 5, 4, 1, 3, 9, 8, 6, 7, 2, 0 };

        var oddNumbers = numbers.Select(n => n % 2 == 1);

        Console.WriteLine("Numbers < 5:");
        foreach (var x in oddNumbers)
        {
            Console.WriteLine(x);
        }

How can I fix that to show the list of integer?

Solution 1

Change your "Select" to a "Where"

    int[] numbers = { 5, 4, 1, 3, 9, 8, 6, 7, 2, 0 };

    var oddNumbers = numbers.Where(n => n % 2 == 1);

    Console.WriteLine("Odd Number:");
    foreach (var x in oddNumbers)
    {
        Console.WriteLine(x);
    }

The "Select" method is creating a new list of the lambda result for each element (true/false). The "Where" method is filtering based on the lambda.

In C#, you could also use this syntax, which you may find clearer:

        var oddNumbers = from n in numbers
                         where n % 2 == 1
                         select n;

which the compiler translates to:

var oddNumbers = numbers.Where(n => n % 2 == 1).Select(n => n);

Solution 2

numbers.Select(n => n % 2 == 1);

Change this to

numbers.Where(n => n % 2 == 1);

What select does is "convert" one thing to another. So in this case, it's "Converting" n to "n % 2 == 1" (which is a boolean) - hence you get all the true and falses.

It's usually used for getting properties on things. For example if you had a list of Person objects, and you wanted to get their names, you'd do

var listOfNames = listOfPeople.Select( p => p.Name );

You can think of this like so:

• Convert the list of people into a list of strings, using the following method: ( p => p.Name)

To "select" (in the "filtering" sense of the word) a subset of a collection, you need to use Where.

Thanks Microsoft for the terrible naming

Comments

[Lei Yang]

Seems n % 2 == 1 has nothing to do with Numbers < 5?

[TheSoftwareJedi]

It does select data. It's selecting the result of your lambda - which is a boolean value.

[Mitkins]

Yeah, same as SQL, but different to every other programming language (ruby, lisp, python, etc, etc) which use map for 'select' and select for 'where'

[Patrick Desjardins]

Just to let you know that var oddNumbers = numbers.Where(n => n % 2 == 1).Select(n); doesn't work.

[TheSoftwareJedi]

I see what you mean, you were referring to my attempted compiler translation example. I fixed that - but I didn't mean for you to use that. Just the first answer that I gave (which compiled). var oddNumbers = numbers.Where(n => n % 2 == 1);

[Martin R-L]

... and in F#, "map" is "Select", and "filter" is "Where". Very nice IMHO.