学ぶ Filtering Arrays

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Filtering an array means selecting only those elements that meet specific conditions, such as being above a certain value or matching a pattern. This process is called filtering because you "filter out" the elements that do not meet your criteria, leaving you with only the elements you want to keep. Filtering is a common task when you want to work with a subset of data from a larger collection.

Program.cs


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using System;

namespace ConsoleApp
{
    public class Program
    {
        public static void Main(string[] args)
        {
            int[] numbers = { 10, 55, 23, 67, 89, 34, 51, 42 };
            Console.WriteLine("Numbers greater than 50:");
            for (int i = 0; i < numbers.Length; i++)
            {
                if (numbers[i] > 50)
                {
                    Console.WriteLine(numbers[i]);
                }
            }
        }
    }
}

To understand how filtering works in practice, look at each step in the code above. First, you have an array of integers called numbers. You want to display only the numbers that are greater than 50. To do this, you loop through the array using a for loop. Inside the loop, you use an if statement to check if the current number is greater than 50. If the condition is true, you print that number. This way, only the numbers that meet your condition—being greater than 50—are printed to the console.

Program.cs


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using System;
using System.Collections.Generic;

namespace ConsoleApp
{
    public class Program
    {
        public static void Main(string[] args)
        {
            int[] numbers = { 10, 55, 23, 67, 89, 34, 51, 42 };
            List<int> filteredList = new List<int>();

            for (int i = 0; i < numbers.Length; i++)
            {
                if (numbers[i] > 50)
                {
                    filteredList.Add(numbers[i]);
                }
            }

            int[] filteredArray = filteredList.ToArray();
            Console.WriteLine("Filtered array:");
            for (int i = 0; i < filteredArray.Length; i++)
            {
                Console.WriteLine(filteredArray[i]);
            }
        }
    }
}

Note

Arrays in C# have a fixed size. When you filter elements, you often do not know in advance how many will meet your condition. A common approach is to use a temporary List<T> to collect the filtered elements, then convert the list to an array if needed.

1. Why can't you simply remove elements from a C# array?

2. What is a common approach to store filtered results from an array?

3. How do you check if an element meets a filtering condition?

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セクション 1. 章 10

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