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copied!<p>As a starting point, you should set up your <code>.h</code> to something more like this:</p> <pre><code>@interface Circle : NSObject @property double radius; @property (readonly) double area; @property (readonly) double diameter; @property (readonly) double circumference; @property (readonly) double pi; -(id)initWithRadius:(double)r; +(instancetype)circleWithRadius:(double)r; @end </code></pre> <p>This will set up a <code>setter</code> and <code>getter</code> for <code>radius</code> as well as <code>getters</code> for <code>area</code>, <code>diameter</code>, and <code>circumference</code>. It also sets up an <code>init</code> and <code>factory</code> method for your <code>circle</code> which takes a double for the radius.</p> <p>I will come back and edit in some modifications you need to make to your <code>.m</code> as well as your <code>main</code> file in order to make this work. As a note, at a minimum we'll override the <code>getters</code> for the 3 <code>readonly</code> properties. This will prevent the compiler from creating <code>ivars</code> (instance variables) for these properties (because we can just calculate and return the number we calculation when we call it).</p> <p>In your <code>.m</code>:</p> <pre><code>#import Circle.h @implementation Circle -(id)initWithRadius:(double)r { self = [super init]; if(self) { self.radius = r; } return self; } +(instancetype)circleWithRadius:(double)r { return [[Circle alloc] initWithRadius:r]; } -(void)setRadius:(double)r //This method is automatically created by @property { //include any verification logic (make sure r>0 etc), then... self.radius = r; } //we don't really need to override the radius getter -(double)pi { return 3.14159; //or however much accuracy you want } -(double)area { return (self.pi * self.radius * self.radius); } -(double)diameter { return (2.0 * self.radius); } -(double)circumference { return (self.diameter * self.pi); } </code></pre> <p>In <code>main</code>, you use this <code>Circle</code> class in just the same way you use any other object in Objective-C (think about <code>NSString</code>, <code>NSArray</code>, etc).</p> <pre><code>int main(int argc, const char * argv[]) { @autoreleasepool { double radius; NSLog(@"Enter the circles radius:"); scanf ("%lf", &radius); Circle *myCircle = [Circle circleWithRadius:radius]; //the factory method we set up NSLog(@"myCircle radius: %lf", myCircle.radius); NSLog(@"myCircle area: %lf", myCircle.area); NSLog(@"myCircle diameter: %lf", myCircle.diameter); NSLog(@"myCircle circumference: %lf", myCircle.circumference); } return 0; } </code></pre>

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