C# Program to filter array elements based on a predicate

In C#, filtering array elements based on a predicate allows you to select elements that meet specific conditions. A predicate is a function that returns true or false for each element, determining whether it should be included in the result.

The most common approach is using LINQ's Where method, which applies a predicate function to filter elements. You can also use traditional loops or the Array.FindAll method for filtering.

Syntax

Following is the syntax for filtering with LINQ's Where method −

IEnumerable<T> result = array.Where(element => condition);

Following is the syntax for filtering with Array.FindAll −

T[] result = Array.FindAll(array, element => condition);

Using LINQ Where Method

The Where method is the most flexible approach for filtering arrays. It accepts a lambda expression as a predicate −

using System;
using System.Linq;
using System.Collections.Generic;

public class Demo {
   public static void Main() {
      int[] arr = { 40, 42, 12, 83, 75, 40, 95 };
      Console.WriteLine("Original Array:");
      foreach (int a in arr) {
         Console.WriteLine(a);
      }
      
      // Filter elements above 50
      IEnumerable<int> filtered = arr.Where(a => a >= 50);
      Console.WriteLine("\nElements above or equal to 50:");
      foreach (int res in filtered) {
         Console.WriteLine(res);
      }
   }
}

The output of the above code is −

Original Array:
40
42
12
83
75
40
95

Elements above or equal to 50:
83
75
95

Using Array.FindAll Method

The Array.FindAll method returns a new array containing elements that match the predicate −

using System;

public class Demo {
   public static void Main() {
      string[] names = { "Alice", "Bob", "Charlie", "David", "Eve" };
      Console.WriteLine("Original Names:");
      foreach (string name in names) {
         Console.WriteLine(name);
      }
      
      // Filter names with length greater than 4
      string[] longNames = Array.FindAll(names, name => name.Length > 4);
      Console.WriteLine("\nNames with more than 4 characters:");
      foreach (string name in longNames) {
         Console.WriteLine(name);
      }
   }
}

The output of the above code is −

Original Names:
Alice
Bob
Charlie
David
Eve

Names with more than 4 characters:
Alice
Charlie
David

Multiple Conditions and Complex Predicates

You can combine multiple conditions using logical operators in the predicate −

using System;
using System.Linq;

public class Demo {
   public static void Main() {
      int[] numbers = { 15, 23, 8, 42, 67, 19, 88, 5, 34 };
      Console.WriteLine("Original Numbers:");
      Console.WriteLine(string.Join(", ", numbers));
      
      // Filter even numbers greater than 20
      var filtered = numbers.Where(n => n % 2 == 0 && n > 20);
      Console.WriteLine("\nEven numbers greater than 20:");
      Console.WriteLine(string.Join(", ", filtered));
      
      // Filter numbers between 10 and 50
      var rangeFiltered = numbers.Where(n => n >= 10 && n <= 50);
      Console.WriteLine("\nNumbers between 10 and 50:");
      Console.WriteLine(string.Join(", ", rangeFiltered));
   }
}

The output of the above code is −

Original Numbers:
15, 23, 8, 42, 67, 19, 88, 5, 34

Even numbers greater than 20:
42, 88, 34

Numbers between 10 and 50:
15, 23, 42, 19, 34

Comparison of Filtering Methods

Conclusion

Filtering arrays in C# using predicates provides powerful ways to select elements based on conditions. LINQ's Where method offers flexibility and lazy evaluation, while Array.FindAll provides immediate results as arrays. Choose the method that best fits your performance needs and result type requirements.