Method CountDown

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CountDown<TSource>(IEnumerable<TSource>, int)

Provides a countdown counter for a given count of elements at the tail of the sequence where zero always represents the last element, one represents the second-last element, two represents the third-last element and so on.

Declaration
public static IEnumerable<(TSource item, int? count)> CountDown<TSource>(this IEnumerable<TSource> source, int count)
Parameters
Type Name Description
IEnumerable<TSource> source

The source sequence.
int count

Count of tail elements of source to count down.
Returns
Type Description
IEnumerable<(TSource item, int? count)>

A sequence of tuples containing the element and it's count from the end of the sequence, or null.
Type Parameters
Name Description
TSource

The type of elements of source
Remarks

At most, count elements of the source sequence may be buffered at any one time unless source is a collection or a list.

This method uses deferred execution semantics and streams its results.

Examples

The following code example demonstrates how to get a count down timer for a sequence, using the CountDown operator:

var sequence = Enumerable.Range(1, 5);
            
// Get a countdown counter for the the sequence
var result = sequence.CountDown(2);

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

// This code produces the following output:
// [(1, ), (2, ), (3, ), (4, 1), (5, 0)]
Exceptions
Type Condition
ArgumentNullException

source is null.
ArgumentOutOfRangeException

count is 0 or negative.
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CountDown<TSource, TResult>(IEnumerable<TSource>, int, Func<TSource, int?, TResult>)

Provides a countdown counter for a given count of elements at the tail of the sequence where zero always represents the last element, one represents the second-last element, two represents the third-last element and so on.

Declaration
public static IEnumerable<TResult> CountDown<TSource, TResult>(this IEnumerable<TSource> source, int count, Func<TSource, int?, TResult> resultSelector)
Parameters
Type Name Description
IEnumerable<TSource> source

The source sequence.
int count

Count of tail elements of source to count down.
Func<TSource, int?, TResult> resultSelector

A function that receives the element and the current countdown value for the element and which returns those mapped to a result returned in the resulting sequence. For elements before the last count, the countdown value is null.
Returns
Type Description
IEnumerable<TResult>

A sequence of results returned by resultSelector.
Type Parameters
Name Description
TSource

The type of elements of source
TResult

The type of elements of the resulting sequence.
Remarks

At most, count elements of the source sequence may be buffered at any one time unless source is a collection or a list.

This method uses deferred execution semantics and streams its results.

Examples

The following code example demonstrates how to get a count down timer for a sequence, using the CountDown operator:

var sequence = Enumerable.Range(1, 5);

// Get a countdown counter for the the sequence
var result = sequence.CountDown(2, (item, cd) => new { Item = item, CountDown = cd?.ToString() ?? "null", });

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

// This code produces the following output:
// [{ Item = 1, CountDown = null }, { Item = 2, CountDown = null }, { Item = 3, CountDown = null }, { Item = 4, CountDown = 1 }, { Item = 5, CountDown = 0 }]
Exceptions
Type Condition
ArgumentNullException

source or resultSelector is null.
ArgumentOutOfRangeException

count is 0 or negative.