Array.Find and FindAll

C# course · Beginner · Lesson 63/116

Learn to find elements matching a condition with Array.Find and FindAll.

Array.Find and FindAll

Array.Find returns the first element in an array that matches a condition. The condition is given as a predicate: a lambda that takes one parameter and returns a bool. The call Array.Find(nums, n => n > 10) returns the first element greater than 10; if none is found, the default value of the type is returned.

Array.FindAll returns all matching elements as a new array. Find gives a single value, FindAll gives an array.

Lambda and predicate

A predicate is a function that takes one parameter and returns a bool. In C# it is most often written as a lambda expression: n => n > 10. The n on the left of the arrow is the parameter, the n > 10 on the right is the returned bool value.

This lambda is passed as the second parameter to methods like Array.Find, Array.FindAll and Array.FindIndex. The condition is evaluated for each element separately.

Exercise Find the large numbers

On the given int[] nums = { 3, 8, 15, 22, 7 }; array, find all elements greater than 10 with Array.FindAll(nums, n => n > 10) and print big.Length with Console.WriteLine. You must use a lambda.

Show example solution

Get the array with int[] big = Array.FindAll(nums, n => n > 10);, then print the count with Console.WriteLine(big.Length);.
Example solution:
using System;

class Program
{
    static void Main()
    {
        int[] nums = { 3, 8, 15, 22, 7 };
        // Find elements greater than 10 with FindAll and print the count
        int[] big = Array.FindAll(nums, n => n > 10);
        Console.WriteLine(big.Length);
    }
}

Test yourself

Match each method to its meaning

Array.Find

First match
Array.FindAll

All matches
Array.FindIndex

First index
n => n > 10

Predicate

What does Array.Find print?

int[] nums = { 5, 12, 8, 20 };
int first = Array.Find(nums, n => n > 10);
Console.WriteLine(first);

Show answer

12 — Correct. The first element greater than 10 is 12.

FindAll call

int[] nums = { 5, 12, 8, 20 };
int[] big = ____;
Console.WriteLine(big.Length);

Show answer

int[] nums = { 5, 12, 8, 20 };
int[] big = Array.FindAll(nums, n => n > 10);
Console.WriteLine(big.Length);

Array.Find and FindAll