Difference between the returned values of two selector functions in this code

Question

var Students = new Dictionary<string, int>();
Students.Add( "Bob" , 60);
Students.Add( "Jim" , 62);
Students.Add( "Jack", 75);
Students.Add( "John", 81);
Students.Add( "Matt", 60);
Students.Add( "Jill", 72);
Students.Add( "Eric", 83);
Students.Add( "Adam", 60);
Students.Add( "Gary", 75);

var MyVar  = Students.GroupBy(r=> r.Value)
                     .ToDictionary(t=> t.Key, t=> t.Select(x=> x.Key));

The Students object has Name and Weight key-value pairs.

In the ToDictionary method, t variable is of type IEnumerable<IGrouping<K, T>>. That is, IEnumerable<IGrouping<int, Students>>

Why are the Key values returned by t=>t.Key and t=> t.Select(**x=> x.Key**) different? They both use the same t variable. Key should be of type int.

The image was taken after the GroupBy method had been executed.(It is not the full image) One of the Key has the value of 60 and the other one has the values of Bob, Matt and Adam.

enter image description here

Eli Arbel · Accepted Answer · 2014-03-28 06:23:35Z

2

Because it's not a

IEnumerable<IGrouping<int, Student>>

It's a

IEnumerable<IGrouping<int, KeyValuePair<int, Student>>>

In t => t.Key you're referring to the grouping key, and in Select(x => x.Key) you're referring to the KeyValuePair.

You can hover over the variables in Visual Studio to see the types and the type parameters.

In more detail:

// return an IEnumerable<IGrouping<int, KeyValuePair<int, Student>>>
Students.GroupBy(r=> r.Value)
// iterate over each IGrouping<int, KeyValuePair<int, Student>>
    .ToDictionary(
        group => group.Key,
// iterate over each KeyValuePair<int, Student> in the group
// because IGrouping<TKey, TElement> inherits from IEnumerable<TElement>
// so it's a Select over an IEnumerable<KeyValuePair<int, Student>>
        group => group.Select(pair => pair.Key));

edited Mar 28, 2014 at 6:23

answered Mar 27, 2014 at 14:51

Eli Arbel

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1 Comment

user3467437 Over a year ago

same question: why does Select(x => x.Key) refer to the KeyValuePairreturned by GroupBy?

Habib · Accepted Answer · 2014-03-27 16:11:17Z

2

It is creating a new dictionary with number as Key and all the student names from original dictionary related to particular key.

Here is how it works.

Why are the Key values returned by t=>t.Key

That is operating over the GroupBy, since GroupBy is applied on Value, which is int in the original dictionary. Thus the new dictionary would have the distinct keys based on original dictionary's values.

and t=> t.Select(x=> x.Key) different?

Now this Select is operating on each GroupBy item from original dictionary. Where Key is the student name. Thus getting all the student names based on the Key selected previously in GroupBy

edited Mar 27, 2014 at 16:11

answered Mar 27, 2014 at 14:51

Habib

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2 Comments

user3467437 Over a year ago

why does Select operate on original dictionary? How does Select decide this?

Habib Over a year ago

@user3467437, Select operates on each item of GroupBy, Which has Key as Value from original dictionary and Values as collection of related keys from Original dictionary

Heinzi · Accepted Answer · 2014-03-27 14:55:45Z

1

This is what the result of Students.GroupBy(r => r.Value) looks like. It's a bit confusing, since both GroupBy as well as Dictionary use a Key.

    { (Key = 60, {(Key = "Bob", Value = 60), (Key = "Matt", Value = 60), ...}),
      (Key = 62, {(Key = "Jim", Value = 62)}),
      (Key = 72. ...),
      ...
    }

t is one row of this grouping.

The first selector t.Key yields 60.

The second selector t.Select(x => x.Key) operates on the list and yields {"Bob", "Matt", "Adam"}.

edited Mar 27, 2014 at 14:55

answered Mar 27, 2014 at 14:50

Heinzi

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Comments

Servy · Accepted Answer · 2014-03-27 14:48:45Z

0

The Key of the IGrouping (t.Key) is the key that you grouped on in GroupBy. In this case this is the Value of the KeyValuePair.

The Key within the Select (x.Key) is getting the key of the KeyValuePair for each pair within the group.

It's somewhat less obvious here due to the inferred types, but you can hover over both x and t in visual studio to see that t is an IGrouping and x is a KeyValuePair. You could also improve the clarity of this query (quite a bit, in my mind) by using more meaningful variable names.

var MyVar = Students.GroupBy(pair => pair.Value)
                        .ToDictionary(group => group.Key,
                        group => group.Select(pair => pair.Key));

answered Mar 27, 2014 at 14:48

Servy

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