Visual C# object array changes all elements in array instead of just one

Question

I'm new to C# so I'm making a Snake Game to learn. I created an object of a snake which contains an array of objects called pieces. So the pieces make up the snake. I'm trying to change the first piece's x value by 1 in order move the first piece one unit to the right. The problem is that when I change the first piece's x value by 1, the second piece's x value also changes by 1. So myPiece[1].x = 6 and myPiece[2].x = 6 even though I only increased the first one's value. How can I make it so I can change the value of each individual piece without it changing the rest of the piece's values?

class theSnake
{
    private int size;
    private thePiece[] myPieces=new thePiece[50];
    private int xDim;
    private int yDim;


    public theSnake(int pxDim,int pyDim)
    {
        size = 1;
        xDim = pxDim;
        yDim = pyDim;
        for (int i = 1; i <= 49; i++)
        {
           myPieces[i] = new thePiece();
        }

        myPieces[1].setX(xDim/2);  //sets the starting of the snake in the center of the field
        myPieces[1].setY(yDim/2);
        myPieces[1].setDir(3);
    }

    public void move()
    {

         //the pieces trade positions with the positions in front of it
        //the pieces travel backwards instead of forwards so there is no gap created
        for (int i = size; i >= 2; i--)
        {
            myPieces[i] = myPieces[i-1];
        }

        //moves leading snake based on direction given
        switch (myPieces[1].getDir())
        {
            case 1:
                myPieces[1].setY(myPieces[1].getY() + 1);
                break;
            case 2:
                myPieces[1].setY(myPieces[1].getY() - 1);
                break;
            case 3:
                myPieces[1].setX(myPieces[1].getX() + 1);
                break;
            case 4:
                myPieces[1].setX(myPieces[1].getX() - 1);
                break;
        }

    }

class thePiece
{
    private int x;
    private int y;
    private int direction;
    //north = 1 south = 2 east = 3 west =4
    public thePiece()
    {
        x = 0;
        y = 0;
        direction = 1;
    }

}

A couple things: You've posted a lot of code that isn't even used, what haven't you posted? Arrays in C# are zero-relative unlike VB.NET. C# has syntax for auto-generating getters and setters, you should really use it. — Robert Allan Hennigan Leahy
– Robert Allan Hennigan Leahy, Commented Dec 31, 2011 at 6:39
thePiece is a poor choice for a class name. Same with theSnake. — user47589
– user47589, Commented Dec 31, 2011 at 7:00

谢圣鑫 · Accepted Answer · 2012-01-01 05:28:42Z

2

for (int i = size; i >= 2; i--) 
{ 
    myPieces[i] = myPieces[i-1]; 
}

because myPieces[2] point to myPieces[1], in an other word, myPieces[2] and myPieces[1] are the same object. the orignal myPieces[2] object now is referenced by myPieces[3].

if you want to assign values, you can use following code

 for (int i = size; i >= 2; i--) 
    { 
        myPieces[i].x = myPieces[i-1].x; 
        myPieces[i].y = myPieces[i-1].y; 
        myPieces[i].direction = myPieces[i-1].direction; 
    }

edited Jan 1, 2012 at 5:28

answered Dec 31, 2011 at 7:01

谢圣鑫

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1 Comment

user1124016 Over a year ago

So on this line of code: "myPieces[i] = myPieces[i-1];" I'm actually assigning a pointer to myPieces[i] instead of assigning the values within it?

competent_tech · Accepted Answer · 2011-12-31 19:25:36Z

0

Your issue is that you are not properly swapping the objects in the array:

Change the following:

   for (int i = size; i >= 2; i--)
    {
        myPieces[i] = myPieces[i-1];
    }

To something like the following (this may need minor tweaks if I didn't understand your logic correctly):

   for (int i = size; i >= 2; i--)
    {
        // Save a reference to the piece that will be written over
        var savePiece = myPieces[i];
        // Move the previous piece
        myPieces[i] = myPieces[i-1];
        // Now move the saved piece to the previous position
        myPieces[i-1] = savePiece;
    }

answered Dec 31, 2011 at 19:25

competent_tech

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