break／continue、goto（使用を避ける）、スコープ、例外とガードチェック

レッスン概要

break の動作

using System;

public class Program
{
  public static void Main(string[] args)
  {
    for (int i = 1; i <= 10; i = i + 1)
    {
      if (i == 4)
      {
        Console.WriteLine("Stop at 4");
        break; // exit loop completely
      }
      Console.WriteLine("i = " + i);
    }
  }
}

continue の動作

using System;

public class Program
{
  public static void Main(string[] args)
  {
    for (int i = 1; i <= 5; i = i + 1)
    {
      if (i % 2 == 0)
      {
        continue; // skip even numbers
      }
      Console.WriteLine("Odd: " + i);
    }
  }
}

goto を避ける

using System;

public class Program
{
  public static void Main(string[] args)
  {
    // Goto works but harms readability; prefer structured loops.
    int i = 0;
start:
    i = i + 1;
    Console.WriteLine("i = " + i);
    if (i < 3)
    {
      goto start; // avoid using goto in real code
    }

    // Prefer: while loop (clear intent)
    int j = 0;
    while (j < 3)
    {
      j = j + 1;
      Console.WriteLine("j = " + j);
    }
  }
}

スコープの基礎

using System;

public class Program
{
  public static void Main(string[] args)
  {
    int x = 10; // outer scope
    if (x > 0)
    {
      int y = 5; // inner scope
      Console.WriteLine("x + y = " + (x + y));
    }
    // y is not visible here; this would not compile:
    // Console.WriteLine(y);

    // Shadowing (avoid reusing names in nested scopes)
    int value = 1;
    {
      int valueInner = 2; // use a new name instead of shadowing
      Console.WriteLine("value=" + value + ", inner=" + valueInner);
    }
  }
}

診断の基礎

using System;

public class Program
{
  // Guard check: return a bool and an out parameter (no exception on normal invalid input).
  public static bool TryDivide(int x, int y, out int result)
  {
    if (y == 0)
    {
      result = 0;
      return false; // guard fail
    }
    result = x / y;
    return true;
  }

  // Exceptional state: throw when something truly exceptional happens.
  public static int PriceWithTax(int price, int ratePercent)
  {
    if (price < 0)
    {
      throw new ArgumentOutOfRangeException("price");
    }
    if (ratePercent < 0 || ratePercent > 100)
    {
      throw new ArgumentOutOfRangeException("ratePercent");
    }
    return price + (price * ratePercent / 100);
  }

  public static void Main(string[] args)
  {
    int r;
    bool ok = TryDivide(10, 0, out r);
    Console.WriteLine("TryDivide ok=" + ok + " result=" + r);

    try
    {
      int total = PriceWithTax(100, 18);
      Console.WriteLine("Total=" + total);
    }
    catch (Exception ex)
    {
      Console.WriteLine("Error: " + ex.Message);
    }
  }
}

break と continue

まとめ