Method FindLastIndex

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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.
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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.
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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.