Method CountBy

CountBy<TSource, TKey>(IEnumerable<TSource>, Func<TSource, TKey>)

Applies a key-generating function to each element of a sequence and returns a sequence of unique keys and their number of occurrences in the original sequence.

Declaration

public static IEnumerable<KeyValuePair<TKey, int>> CountBy<TSource, TKey>(this IEnumerable<TSource> source, Func<TSource, TKey> keySelector)

Parameters

Type	Name	Description
IEnumerable<TSource>	source	Source sequence.
Func<TSource, TKey>	keySelector	Function that transforms each item of source sequence into a key to be compared against the others.

Returns

Type	Description
IEnumerable<KeyValuePair<TKey, int>>	A sequence of unique keys and their number of occurrences in the original sequence.

Type Parameters

Name	Description
TSource	Type of the elements of the source sequence.
TKey	Type of the projected element.

Remarks

This method is implemented by using deferred execution. However, source will be consumed in it's entirety immediately when first element of the returned sequence is consumed.

Examples

The following code example demonstrates how to get the count by key in a sequence, using the CountBy operator:

var sequence = Enumerable.Range(1, 19);
            
// Count elements in a sequence grouped by key
var result = sequence.CountBy(x => x % 3);

Console.WriteLine(
    "[" +
    string.Join(", ", result) +
    "]");

// This code produces the following output:
// [(1, 7), (2, 6), (0, 6)]

Exceptions

Type	Condition
ArgumentNullException	source or keySelector is null.

CountBy<TSource, TKey>(IEnumerable<TSource>, Func<TSource, TKey>, IEqualityComparer<TKey>?)

Applies a key-generating function to each element of a sequence and returns a sequence of unique keys and their number of occurrences in the original sequence. An additional argument specifies a comparer to use for testing equivalence of keys.

Declaration

public static IEnumerable<KeyValuePair<TKey, int>> CountBy<TSource, TKey>(this IEnumerable<TSource> source, Func<TSource, TKey> keySelector, IEqualityComparer<TKey>? comparer)

Parameters

Type	Name	Description
IEnumerable<TSource>	source	Source sequence.
Func<TSource, TKey>	keySelector	Function that transforms each item of source sequence into a key to be compared against the others.
IEqualityComparer<TKey>	comparer	The equality comparer to use to determine whether or not keys are equal. If null, the default equality comparer for TSource is used.

Returns

Type	Description
IEnumerable<KeyValuePair<TKey, int>>	A sequence of unique keys and their number of occurrences in the original sequence.

Type Parameters

Name	Description
TSource	Type of the elements of the source sequence.
TKey	Type of the projected element.

Remarks

This method is implemented by using deferred execution. However, source will be consumed in it's entirety immediately when first element of the returned sequence is consumed.

Examples

The following code example demonstrates how to get the count by key in a sequence, using the CountBy operator:

var sequence = new[] { "a", "B", "c", "A", "b", "A", };

// Count elements in a sequence grouped by key
var result = sequence.CountBy(SuperEnumerable.Identity, StringComparer.OrdinalIgnoreCase);

Console.WriteLine(
    "[" +
    string.Join(", ", result) +
    "]");

// This code produces the following output:
// [(a, 3), (B, 2), (c, 1)]

Exceptions

Type	Condition
ArgumentNullException	source or keySelector is null.