List, Find

A C# List can be searched imperatively (with a for-loop). It can be searched instead with the Find method: this often uses a lambda expression.

Other methods

Exists() is just like Find on List except it returns true or false, not a new List of matches. It can make some code simpler. Find() and Exists() makes some C# code clearer.

Find example

Here we consider the Find() method on List. Find accepts a Predicate, which we can specify as a lambda expression. It returns the first match.

using System;
using System.Collections.Generic;

// Step 1: create the List.
List<int> list = new List<int>(new int[] { 19, 23, 29 });

// Step 2: find first element greater than 20.
int result = list.Find(item => item > 20);

// Step 3: print result.
Console.WriteLine(result);
23

FindLast

To search backwards, use the FindLast method. FindLast will scan the List from the last element to the first. Here we use FindLast instead of Find.

using System;
using System.Collections.Generic;

var list = new List<int>(new int[] { 19, 23, 29 });

// Find last element greater than 20.
int result = list.FindLast(item => item > 20);
Console.WriteLine("FINDLAST: {0}", result);
FINDLAST: 29

FindIndex

There are also FindIndex and FindLastIndex methods. FindIndex returns the index of the first matching element, starting its search from the first element.

using System;
using System.Collections.Generic;

var list = new List<int>() { 0, 1, 700, 0 };

// Find index of element 100 or greater.
int result = list.FindIndex(element => element >= 100);
Console.WriteLine("FINDINDEX: {0}", result);
FINDINDEX: 2

FindAll

We can also call the FindAll method. FindAll() returns a modified List of the same element type as the original List. It also receives a Predicate.

using System;
using System.Collections.Generic;

var list = new List<string>() { "bird", "frog", "ball", "leaf" };

// Invoke FindAll in foreach.
// ... FindAll is only invoked once for the entire loop.
foreach (string value in list.FindAll(element => element.StartsWith("b")))
{
    Console.WriteLine("FINDALL, STARTSWITH B: {0}", value);
}
FINDALL, STARTSWITH B: bird
FINDALL, STARTSWITH B: ball

Exists

We examine the Exists method on List. Exists returns whether a List element is present. We invoke this method with a lambda expression.

using System;
using System.Collections.Generic;

List<int> list = new List<int>();
list.Add(7);
list.Add(11);
list.Add(13);

// See if any elements with values greater than 10 exist.
bool exists = list.Exists(element => element > 10);
Console.WriteLine(exists);

// Check for numbers less than 7.
exists = list.Exists(element => element < 7);
Console.WriteLine(exists);
True
False

Loops

When the iteration logic is important, we can use for and foreach loops with List. Using loops is usually faster than invoking delegates (like the Predicate arguments to Find).

A summary

A List can be searched with built-in methods and lambda expressions. These methods are convenient and fast. They can help us place the emphasis on other logic in the program.

Summary, continued

Exists() is a quick way to determine if a matching element is present. Exists() and find() may not be the fastest searching methods, but they are clear and often useful.