I'm having trouble understanding a finer point of Writing and calling Classes. It's probably easlier to grasp in Swift but it bothers me to start that study without getting it right in obj_c first. Currently I do everything in the ViewControllers with iVars and Globals. With two apps 18 months in the App store its overdue to put them right.

I've formed a notion that properties are the Object's State, and any methods within determine the Objects Behaviour but so far no-one is able to tell me.

here be a typical Class header:

@interface Math : NSObject

@property (nonatomic, assign) int a;
@property (nonatomic, assign) int b;
@property (nonatomic, assign) int c;

-(int)mathemagic:(int)a adding:(int)b;

@end

and the corresponding Class implementation:

@implementation Math

@synthesize a = _a;
@synthesize b = _b;
@synthesize c = _c;

- (instancetype)init {
    self = [super init];
    if (self) {
       _a = 0;
       _b = 0;
       _c = 0;
    }
    return self;
}

-(int)mathemagic:(int)a adding:(int)b {
    _c = (a + b);
    return _c;
}

@end

and finally in the appropriate places in my ViewController

#import "Math"

- (void)viewDidLoad {

    [super viewDidLoad];

    Math *theMath = [Math alloc]; // makes no difference if I init[]

    theMath.a = 10;
    theMath.b = 20;
    NSLog (@" answer is %i",theMath.c);
    // but still outputs to:
    // answer is 0
 }

Now I know can make an iVar and do it this way,

int d = [self.theMath mathemagic:theMath.a adding:theMath.b];
NSLog (@" sum: %i",d);

But i shouldn't have to. Stanford CS193P seems to always make the Class a property of the ViewController, but then everything is again expressed as self.theMath.whatever and the Data Model is no longer encapsulated away from the VC ? Maybe Stanford leaves advanced distractions to Java graduates till later.

Well for this person who's read David Flanagan's "Java in A Nutshell" , and Niemeyer-Knudsen's "Learning Java", It's later Now.

I shouldn't have to touch theMath.c, just by assigning values to [ theMath.a ] and [ theMath.b ] should be enough.

Where am I wrong?

I think that is because you are setting a and b = 0 in alloc init . and you are not calling [self mathemagic:a adding:b] anywhere.

I think im Math.m you should change -(instancetype)init to

    - (instancetype)initWith:(int)a andb:(int)b {
self = [super init];
if (self) {
    _c = [self mathemagic:a adding:b];
}
return self;

and in viewDidLoad use

     Math *theMath = [[Math alloc]initWith:10 andb:20];

Hope this helps :)