Method PartialSortBy

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PartialSortBy<TSource, TKey>(IEnumerable<TSource>, int, Func<TSource, TKey>)

Executes a partial sort of the top count elements of a sequence according to the key for each element. If count is less than the total number of elements in source, then this method will improve performance.

Declaration
public static IEnumerable<TSource> PartialSortBy<TSource, TKey>(this IEnumerable<TSource> source, int count, Func<TSource, TKey> keySelector)
Parameters
Type Name Description
IEnumerable<TSource> source

The source sequence.
int count

Number of (maximum) elements to return.
Func<TSource, TKey> keySelector

A function to extract a key from an element.
Returns
Type Description
IEnumerable<TSource>

A sequence containing at most top count elements from source, in ascending order of their keys.
Type Parameters
Name Description
TSource

Type of elements in the sequence.
TKey

Type of keys.
Remarks

This operation is an O(n * log(K)) where K is count.

This operator uses deferred execution and streams it results.

Examples

The following code example demonstrates how to get the top N items of a sequence using PartialSort.

var sequence = new[]
{
    (key: 5, text: "1"),
    (key: 5, text: "2"),
    (key: 4, text: "3"),
    (key: 4, text: "4"),
    (key: 3, text: "5"),
    (key: 3, text: "6"),
    (key: 2, text: "7"),
    (key: 2, text: "8"),
    (key: 1, text: "9"),
    (key: 1, text: "10"),
};

// Get the top N items
var result = sequence
    .PartialSortBy(
        3,
        x => x.key);

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

// This code produces the following output:
// [(1, 9), (1, 10), (2, 7)]
Exceptions
Type Condition
ArgumentNullException

source or keySelector is null.
ArgumentOutOfRangeException

count is less than 1.
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PartialSortBy<TSource, TKey>(IEnumerable<TSource>, int, Func<TSource, TKey>, OrderByDirection)

Executes a direction partial sort of the top count elements of a sequence according to the key for each element. If count is less than the total number of elements in source, then this method will improve performance.

Declaration
public static IEnumerable<TSource> PartialSortBy<TSource, TKey>(this IEnumerable<TSource> source, int count, Func<TSource, TKey> keySelector, OrderByDirection direction)
Parameters
Type Name Description
IEnumerable<TSource> source

The source sequence.
int count

Number of (maximum) elements to return.
Func<TSource, TKey> keySelector

A function to extract a key from an element.
OrderByDirection direction

The direction in which to sort the elements
Returns
Type Description
IEnumerable<TSource>

A sequence containing at most top count elements from source, in the specified order of their keys.
Type Parameters
Name Description
TSource

Type of elements in the sequence.
TKey

Type of keys.
Remarks

This operation is an O(n * log(K)) where K is count.

This operator uses deferred execution and streams it results.

Examples

The following code example demonstrates how to get the top N items of a sequence using PartialSort.

var sequence = new[]
{
    (key: 5, text: "1"),
    (key: 5, text: "2"),
    (key: 4, text: "3"),
    (key: 4, text: "4"),
    (key: 3, text: "5"),
    (key: 3, text: "6"),
    (key: 2, text: "7"),
    (key: 2, text: "8"),
    (key: 1, text: "9"),
    (key: 1, text: "10"),
};

// Get the top N items
var result = sequence
    .PartialSortBy(
        3,
        x => x.key,
        OrderByDirection.Descending);

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

// This code produces the following output:
// [(5, 1), (5, 2), (4, 3)]
Exceptions
Type Condition
ArgumentNullException

source or keySelector is null.
ArgumentOutOfRangeException

count is less than 1.
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PartialSortBy<TSource, TKey>(IEnumerable<TSource>, int, Func<TSource, TKey>, IComparer<TKey>?)

Executes a partial sort of the top count elements of a sequence according to the key for each element, using comparer to compare the keys. If count is less than the total number of elements in source, then this method will improve performance.

Declaration
public static IEnumerable<TSource> PartialSortBy<TSource, TKey>(this IEnumerable<TSource> source, int count, Func<TSource, TKey> keySelector, IComparer<TKey>? comparer)
Parameters
Type Name Description
IEnumerable<TSource> source

The source sequence.
int count

Number of (maximum) elements to return.
Func<TSource, TKey> keySelector

A function to extract a key from an element.
IComparer<TKey> comparer

A IComparer<T> to compare elements.
Returns
Type Description
IEnumerable<TSource>

A sequence containing at most top count elements from source, in ascending order of their keys.
Type Parameters
Name Description
TSource

Type of elements in the sequence.
TKey

Type of keys.
Remarks

This operation is an O(n * log(K)) where K is count.

This operator uses deferred execution and streams it results.

Examples

The following code example demonstrates how to get the top N items of a sequence using PartialSort.

var sequence = new[]
{
    (key: 5, text: "1"),
    (key: 5, text: "2"),
    (key: 4, text: "3"),
    (key: 4, text: "4"),
    (key: 3, text: "5"),
    (key: 3, text: "6"),
    (key: 2, text: "7"),
    (key: 2, text: "8"),
    (key: 1, text: "9"),
    (key: 1, text: "10"),
};

// Get the top N items
var result = sequence
    .PartialSortBy(
        1,
        x => x.key,
        Comparer<int>.Create((x, y) => (x % 2).CompareTo(y % 2)));

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

// This code produces the following output:
// [(4, 3)]
Exceptions
Type Condition
ArgumentNullException

source or keySelector is null.
ArgumentOutOfRangeException

count is less than 1.
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PartialSortBy<TSource, TKey>(IEnumerable<TSource>, int, Func<TSource, TKey>, IComparer<TKey>?, OrderByDirection)

Executes a direction partial sort of the top count elements of a sequence according to the key for each element, using comparer to compare the keys. If count is less than the total number of elements in source, then this method will improve performance.

Declaration
public static IEnumerable<TSource> PartialSortBy<TSource, TKey>(this IEnumerable<TSource> source, int count, Func<TSource, TKey> keySelector, IComparer<TKey>? comparer, OrderByDirection direction)
Parameters
Type Name Description
IEnumerable<TSource> source

The source sequence.
int count

Number of (maximum) elements to return.
Func<TSource, TKey> keySelector

A function to extract a key from an element.
IComparer<TKey> comparer

A IComparer<T> to compare elements.
OrderByDirection direction

The direction in which to sort the elements
Returns
Type Description
IEnumerable<TSource>

A sequence containing at most top count elements from source, in the specified order of their keys.
Type Parameters
Name Description
TSource

Type of elements in the sequence.
TKey

Type of keys.
Remarks

This operation is an O(n * log(K)) where K is count.

This operator uses deferred execution and streams it results.

Examples

The following code example demonstrates how to get the top N items of a sequence using PartialSort.

var sequence = new[]
{
    (key: 5, text: "1"),
    (key: 5, text: "2"),
    (key: 4, text: "3"),
    (key: 4, text: "4"),
    (key: 3, text: "5"),
    (key: 3, text: "6"),
    (key: 2, text: "7"),
    (key: 2, text: "8"),
    (key: 1, text: "9"),
    (key: 1, text: "10"),
};

// Get the top N items
var result = sequence
    .PartialSortBy(
        1,
        x => x.key,
        Comparer<int>.Create((x, y) => (x % 2).CompareTo(y % 2)),
        OrderByDirection.Descending);

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

// This code produces the following output:
// [(5, 1), (5, 2), (3, 5), (3, 6), (1, 9), (1, 10)]
Exceptions
Type Condition
ArgumentNullException

source or keySelector is null.
ArgumentOutOfRangeException

count is less than 1.