Question

I’ve been struggling to understand what’s going on with a method I’ve got to understand for my test, I’m having a hard time figuring out the reason I’m getting the results I get, any explanation on how the “f” method works it’s greatly appreciated

class Program {
  static void Main(string[] args)
  {
    A b = new A(); b.y = b.x;
    b.f(b.y); b.g();
    Console.WriteLine(b.x[0] + " " + b.x[1]); // Prints 1 7
    Console.WriteLine(b.y[0] + " " + (b.x[1] + b.y[1])); // 1 14
  }
}

public class A {
  public int[] x = {1, 2};
  public int[] y;
  public void f(int[] z)
  {
      z[1] += 5;
  }

  public void g()
  {
      A a = new A ();
      a.x[0]++;
      a.x[1]--;
  }
}

Let me explain what I did understand, b.y gets created as an array and it gets the values in b.x, now, when we call b.f, we pass that method b.y which is [1, 2], now, and here’s where I get stuck, z seems to be the b.y array, so it has [1, 2] as value, when the method adds 5 to the element in the position 1 (which is 2) I get [1, 7] as result of that, when the method ends and my program goes back to the main, somehow, b.y AND b.x BOTH are now [1, 7], how did that happen?, I thought the method was only modifying b.y since that’s the one that got passed. Also, the g function doesn’t add anything as the “a” value is a local variable that “dies” as the method ends, right?. I hope someone can help me, I’ve got to pass this test!. Thanks ;]

Answer 1

Here we go:

• A new variable named b is initialized with a type of A.
• When b is created, it sets the value of b.x to {1, 2}.
• b.y is then assigned to b.x, however, because they are arrays, they are now referencing the same data.
• b.f is called, and has b.y passed to it (remembering, that b.y and b.x are referencing the same data right now). Essentially, z also points to the same data during the f function.
• b.f adds 5 to the value at index 1 of the shared data, which is 2. So 2 + 5 = 7.
• First Console.WriteLine prints b.x[0] which is still 1. Then it prints b.x[1] which is now 7 (as above).
• Second Console.WriteLine prints b.y[0], which is still 1 (because they share the same data). Then it prints b.x[1] + b.y[1]. They both share the same data.. and the data at index 1 is 7. 7 + 7 = 14.

You are correct about the g method in that the variable is local and doesn’t do anything.

Hope that helps.