Method FindIndex

FindIndex<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 first occurrence within the entire IEnumerable<T>.

Declaration

public static int FindIndex<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 first 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.

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 first element that satisfies a condition, using FindIndex.

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.FindIndex(x => x.name.StartsWith("J")));

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

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

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

Exceptions

Type	Condition
ArgumentNullException	source or predicate is null.

FindIndex<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 first occurrence within the range of elements in the IEnumerable<T> that extends from the specified index to the last element.

Declaration

public static int FindIndex<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 starting element within the sequence.

Returns

Type	Description
int	The zero-based index of the first occurrence of an element that matches the conditions defined by predicate within the the range of elements in the IEnumerable<T> that extends from index to the last 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 index and ending at the last element.

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 first element that satisfies a condition, using FindIndex.

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}, after index 4",
    sequence.FindIndex(
        x => x.name.StartsWith("J"),
        4));

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

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

Exceptions

Type	Condition
ArgumentNullException	source or predicate is null.

FindIndex<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 first occurrence within the range of elements in the IEnumerable<T> that starts at the specified index to the last element and contains the specified number of elements.

Declaration

public static int FindIndex<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 starting 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 first occurrence of an element that matches the conditions defined by predicate within the the range of elements in the IEnumerable<T> that that starts 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 index and ending at index plus count minus 1, if count is greater than 0.

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 first element that satisfies a condition, using FindIndex.

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}, after index 2-4",
    sequence.FindIndex(
        x => x.name.StartsWith("J"),
        2,
        3));

Console.WriteLine(
    "'J' starts at index {0}, after index 2-2",
    sequence.FindIndex(
        x => x.name.StartsWith("J"),
        2,
        1));

// This code produces the following output:
// 'J' starts at index 3, after index 2-4
// 'J' starts at index -1, after index 2-2

Exceptions

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