Method FindLastIndex

FindLastIndex<TSource>(IEnumerable<TSource>, Func<TSource, bool>)

Searches for an element that matches the conditions defined by the specified predicate and returns the zero-based index of the last occurrence within the entire IEnumerable<T>.

Declaration

public static int FindLastIndex<TSource>(this IEnumerable<TSource> source, Func<TSource, bool> predicate)

Parameters

Type	Name	Description
IEnumerable<TSource>	source	The source sequence.
Func<TSource, bool>	predicate	The predicate that defines the conditions of the element to search for.

Returns

Type	Description
int	The zero-based index of the last occurrence of an element that matches the conditions defined by predicate within the entire IEnumerable<T>, if found; otherwise, -1.

Type Parameters

Name	Description
TSource	The type of elements of source

Remarks

The IEnumerable<T> is searched forward starting at the first element and ending at the last element, and the index of the last instance of an element that matches the conditions defined by predicate is returned.

The predicate is a delegate to a method that returns true if the object passed to it matches the conditions defined in the delegate. The elements of the current IEnumerable<T> are individually passed to the predicate delegate.

This operator executes immediately.

Examples

The following code example demonstrates how to find the index of the last element that satisfies a condition, using FindLastIndex.

var sequence = new[]
{
    (key: 2, name: "Frank"),
    (key: 3, name: "Jill"),
    (key: 5, name: "Dave"),
    (key: 8, name: "Jack"),
    (key: 12, name: "Judith"),
    (key: 14, name: "Robert"),
    (key: 1, name: "Adam"),
};
            
// Find the first index that matches a condition
Console.WriteLine(
    "'J' starts at index {0}",
    sequence.FindLastIndex(
        x => x.name.StartsWith("J")));

Console.WriteLine(
    "'Ji' starts at index {0}",
    sequence.FindLastIndex(
        x => x.name.StartsWith("Ji")));

Console.WriteLine(
    "'K' starts at index {0}",
    sequence.FindLastIndex(
        x => x.name.StartsWith("K")));

// This code produces the following output:
// 'J' starts at index 4
// 'Ji' starts at index 1
// 'K' starts at index -1

Exceptions

Type	Condition
ArgumentNullException	source or predicate is null.

FindLastIndex<TSource>(IEnumerable<TSource>, Func<TSource, bool>, Index)

Searches for an element that matches the conditions defined by the specified predicate and returns the zero-based index of the last occurrence within the range of elements in the IEnumerable<T> that extends backwards from the specified index to the first element.

Declaration

public static int FindLastIndex<TSource>(this IEnumerable<TSource> source, Func<TSource, bool> predicate, Index index)

Parameters

Type	Name	Description
IEnumerable<TSource>	source	The source sequence.
Func<TSource, bool>	predicate	The predicate that defines the conditions of the element to search for.
Index	index	The Index of the ending element within the sequence.

Returns

Type	Description
int	The zero-based index of the last occurrence of an element that matches the conditions defined by predicate within the the range of elements in the IEnumerable<T> that extends backwards from index to the first element, if found; otherwise, -1.

Type Parameters

Name	Description
TSource	The type of elements of source

Remarks

The IEnumerable<T> is searched forward starting at the first element and ending at index, and the index of the last instance of an element that matches the conditions defined by predicate is returned.

The predicate is a delegate to a method that returns true if the object passed to it matches the conditions defined in the delegate. The elements of the current IEnumerable<T> are individually passed to the predicate delegate.

This operator executes immediately.

Examples

The following code example demonstrates how to find the index of the last element that satisfies a condition, using FindLastIndex.

var sequence = new[]
{
    (key: 2, name: "Frank"),
    (key: 3, name: "Jill"),
    (key: 5, name: "Dave"),
    (key: 8, name: "Jack"),
    (key: 12, name: "Judith"),
    (key: 14, name: "Robert"),
    (key: 1, name: "Adam"),
};
            
// Find the first index that matches a condition
Console.WriteLine(
    "'J' starts at index {0}, before index 0",
    sequence.FindLastIndex(
        x => x.name.StartsWith("J"),
        0));

Console.WriteLine(
    "'J' starts at index {0}, before index ^2",
    sequence.FindLastIndex(
        x => x.name.StartsWith("J"),
        ^2));

// This code produces the following output:
// 'J' starts at index -1, before index 0
// 'J' starts at index 4, before index ^2

Exceptions

Type	Condition
ArgumentNullException	source or predicate is null.

FindLastIndex<TSource>(IEnumerable<TSource>, Func<TSource, bool>, Index, int)

Searches for an element that matches the conditions defined by the specified predicate and returns the zero-based index of the last occurrence within the range of elements in the IEnumerable<T> that ends at the specified index to the last element and contains the specified number of elements.

Declaration

public static int FindLastIndex<TSource>(this IEnumerable<TSource> source, Func<TSource, bool> predicate, Index index, int count)

Parameters

Type	Name	Description
IEnumerable<TSource>	source	The source sequence.
Func<TSource, bool>	predicate	The predicate that defines the conditions of the element to search for.
Index	index	The Index of the ending element within the sequence.
int	count	The number of elements in the section to search.

Returns

Type	Description
int	The zero-based index of the last occurrence of an element that matches the conditions defined by predicate within the the range of elements in the IEnumerable<T> that that ends at index and contains count number of elements, if found; otherwise, -1.

Type Parameters

Name	Description
TSource	The type of elements of source

Remarks

The IEnumerable<T> is searched forward starting at the first element and ending at index, and the index of the last instance of an element that matches the conditions defined by predicate no earlier in the sequence than count items before index is returned.

The predicate is a delegate to a method that returns true if the object passed to it matches the conditions defined in the delegate. The elements of the current IEnumerable<T> are individually passed to the predicate delegate.

This operator executes immediately.

Examples

The following code example demonstrates how to find the index of the last element that satisfies a condition, using FindLastIndex.

var sequence = new[]
{
    (key: 2, name: "Frank"),
    (key: 3, name: "Jill"),
    (key: 5, name: "Dave"),
    (key: 8, name: "Jack"),
    (key: 12, name: "Judith"),
    (key: 14, name: "Robert"),
    (key: 1, name: "Adam"),
};

// Find the first index that matches a condition
Console.WriteLine(
    "'J' starts at index {0}, before index 0",
    sequence.FindLastIndex(
        x => x.name.StartsWith("J"),
        1,
        1));

Console.WriteLine(
    "'J' starts at index {0}, before index ^2",
    sequence.FindLastIndex(
        x => x.name.StartsWith("J"),
        ^2));

// This code produces the following output:
// 'J' starts at index -1, before index 0
// 'J' starts at index 4, before index ^2

Exceptions

Type	Condition
ArgumentNullException	source or predicate is null.
ArgumentOutOfRangeException	count is less than 0.