ตัวดำเนินการและนิพจน์: เลขคณิต การเปรียบเทียบ ตรรกะ

แผนผังตัวดำเนินการ

พื้นฐานเลขคณิต

using System;

public class Program
{
  public static void Main(string[] args)
  {
    int a = 9;
    int b = 4;
    Console.WriteLine("a + b = " + (a + b));
    Console.WriteLine("a - b = " + (a - b));
    Console.WriteLine("a * b = " + (a * b));
    Console.WriteLine("a / b = " + (a / b)); // integer division
    Console.WriteLine("a % b = " + (a % b)); // remainder
  }
}

พื้นฐานการเปรียบเทียบ

using System;

public class Program
{
  public static void Main(string[] args)
  {
    int x = 5;
    int y = 7;
    Console.WriteLine("x == y: " + (x == y));
    Console.WriteLine("x != y: " + (x != y));
    Console.WriteLine("x < y:  " + (x < y));
    Console.WriteLine("x <= y: " + (x <= y));
    Console.WriteLine("x > y:  " + (x > y));
    Console.WriteLine("x >= y: " + (x >= y));
  }
}

ตัวดำเนินการทางตรรกะ

using System;

public class Program
{
  static bool ExpensiveCheck()
  {
    Console.WriteLine("ExpensiveCheck ran");
    return true;
  }

  public static void Main(string[] args)
  {
    bool ok = false;
    // && short-circuits when the left is false
    if (ok && ExpensiveCheck())
    {
      Console.WriteLine("Both true");
    }
    else
    {
      Console.WriteLine("Left was false, skipped right");
    }

    bool isAdmin = true;
    bool hasTicket = false;

    // || short-circuits when the left is true
    if (isAdmin || hasTicket)
    {
      Console.WriteLine("Allowed");
    }

    // Negation
    Console.WriteLine("!hasTicket = " + (!hasTicket));
  }
}

ลำดับความสำคัญและ ()

using System;

public class Program
{
  public static void Main(string[] args)
  {
    int n = 10;
    int m = 3;
    bool r1 = n - m * 2 == 4;        // * before -
    bool r2 = (n - m) * 2 == 14;     // grouped
    Console.WriteLine("r1 = " + r1);
    Console.WriteLine("r2 = " + r2);

    // Logical precedence: && before ||, but use () for clarity
    bool a = true, b = false, c = true;
    bool r3 = a || b && c;           // b&&c first → true
    bool r4 = (a || b) && c;         // grouped → true
    Console.WriteLine("r3 = " + r3);
    Console.WriteLine("r4 = " + r4);
  }
}

ข้อควรระวัง

using System;

public class Program
{
  static int Counter = 0;

  static int Next()
  {
    Counter = Counter + 1;
    return Counter;
  }

  public static void Main(string[] args)
  {
    // Integer division pitfall
    int a = 1, b = 2;
    Console.WriteLine("1/2 int = " + (a / b)); // 0
    Console.WriteLine("1/2 double = " + (a / 2.0)); // 0.5

    // Side effects: function called only if needed
    bool cond = false;
    if (cond && Next() > 0)
    {
      Console.WriteLine("Will not run");
    }
    Console.WriteLine("Counter = " + Counter); // 0 because Next() skipped
  }
}

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