Please help me with some simple code

I just want to make a simple program that displays a message on my iPhone screen. I've been following the guide at http://www.cocoalab.com/BecomeAnXcoder

and I put some of their example code into the main.m file in my Xcode software, and when I run the simulator all I get is either a blank screen or the app just closes and doesn't run.

How can I get this code to work?


#import <Foundation/Foundation.h>
float circleArea(float theRadius);
float rectangleArea(float width, float height);
int main()
{
float pictureWidth, pictureHeight, pictureSurfaceArea,
circleRadius, circleSurfaceArea;
pictureWidth = 8.0;
pictureHeight = 4.5;
circleRadius = 5.0;
pictureSurfaceArea = rectangleArea(pictureWidth, pictureHeight);
circleSurfaceArea = circleArea(circleRadius);
NSLog(@"Area of circle: %10.2f.", circleSurfaceArea);
NSLog(@"Area of picture: %f. ", pictureSurfaceArea);
return 0;
}
float circleArea(float theRadius) // first custom function
{
float theArea;
theArea = 3.1416 * theRadius * theRadius;
return theArea;
}
float rectangleArea(float width, float height) // second custom function
{
return width*height;
}

I just want to find some code that produces a result so I can build from that knowledge.

If I put that into the main.m the app just closes when I run it.