How would I modify this?

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C++

I'm using DragonFire SDK to develop my iPad game, but I'm not sure how to modify a section of code to only allow certain directional movement. The section of code that I'm not sure of is this:

void OnTimer()
{
    int BallX, BallY;
	int AccX, AccY, AccZ;
    // Get tilt (accelerometer) data for each axis.
	AccX = TiltGetx();
	AccY = TiltGety();
	AccZ = TiltGetz();    
    // Update the ball's next X and Y values based on G-Force and the strength of gravity:
    BallX = ViewGetx(BallView) + (AccX / 100);
	BallY = ViewGety(BallView) + (AccY / 100);
    // Don't let the ball go off the screen:
    // Stops for Landscape:
    if (BallY >= 731)
        BallY = 731;
    if (BallY <= 0)
        BallY = 0;
    if (BallX >= 987)
        BallX = 987;
    if (BallX <= 0)
        BallX = 0;   
/*
    // Stops for Portrait:
    if (BallX >= 731)
        BallX = 731;
    if (BallX <= 0)
        BallX = 0;
    if (BallY >= 987)
        BallY = 987;
    if (BallY <= 0)
        BallY = 0;
*/
	
    // Update the ball's position:
    ViewSetxy(BallView, BallX, BallY);
}

The problem is I'm not sure how I would update my logic to only allow Left, Right, Up, Down only movement. The code was taking from the DragonFireSDK API Docs (http://www.dragonfiresdk.net/help/DragonFireSDKHelp.html[^]), so it will move any direction that the device is tilted.



Here's the entire code:

//====================================================
// App.cpp
//====================================================
#include "DragonFireSDK.h"

int BallImage;
int BallView;

void AppMain()
{
    // Set up an image of a ball to move around the screen:
    BallImage = ImageAdd("Images/Ball.png");
    BallView = ViewAdd(BallImage, 0, 0);

    LandscapeMode();
}

void AppExit()
{
	// Code to be called on Application Close.

	printf("AppExit");
}

void OnTimer()
{
    int BallX, BallY;
	int AccX, AccY, AccZ;
    // Get tilt (accelerometer) data for each axis.
	AccX = TiltGetx();
	AccY = TiltGety();
	AccZ = TiltGetz();    
    // Update the ball's next X and Y values based on G-Force and the strength of gravity:
    BallX = ViewGetx(BallView) + (AccX / 100);
	BallY = ViewGety(BallView) + (AccY / 100);
    // Don't let the ball go off the screen:
    // Stops for Landscape:
    if (BallY >= 731)
        BallY = 731;
    if (BallY <= 0)
        BallY = 0;
    if (BallX >= 987)
        BallX = 987;
    if (BallX <= 0)
        BallX = 0;   
/*
    // Stops for Portrait:
    if (BallX >= 731)
        BallX = 731;
    if (BallX <= 0)
        BallX = 0;
    if (BallY >= 987)
        BallY = 987;
    if (BallY <= 0)
        BallY = 0;
*/
	
    // Update the ball's position:
    ViewSetxy(BallView, BallX, BallY);
}