Objective-C class for working with fractions -

i'm trying learn programming, , language chose start objective c! i'm studying : programing in objective-c fourth edition, , bit stuck @ classes chapter.

we have following code :

#import <foundation/foundation.h>  @interface fraction: nsobject;  -(void) print; -(void) setnumerator: (int) n; -(void) setdenominator: (int) d;  @end  @implementation fraction {     int numerator;     int denominator;             }  -(void) print     {     nslog(@"%i/%i" , numerator, denominator); }  -(void) setnumerator:(int)n {     numerator = n;  }  -(void) setdenominator:(int)d {     denominator = d; } @end  int main (int argc, char * argv[]) {     @autoreleasepool {         fraction *myfraction;          myfraction = [fraction alloc];         myfraction = [fraction init];          [myfraction setdenominator:1];         [myfraction setnumerator:3];          nslog(@"the value of myfraction :");         [myfraction print];     }     return 0;  } 

i copied code book, in hopes me understand better, reason when try run following message :

2014-04-25 22:23:28.374 cocoterminal[1751:303] *** terminating app due uncaught exception 'nsinvalidargumentexception', reason: '*** +[fraction<0x1000011d8> init]: cannot init class object.' *** first throw call stack: (     0   corefoundation                      0x00007fff8f5a825c __exceptionpreprocess + 172     1   libobjc.a.dylib                     0x00007fff86403e75 objc_exception_throw + 43     2   corefoundation                      0x00007fff8f5ab490 +[nsobject(nsobject) dealloc] + 0     3   cocoterminal                        0x0000000100000dde main + 110     4   libdyld.dylib                       0x00007fff8d43a5fd start + 1 ) libc++abi.dylib: terminating uncaught exception of type nsexception (lldb)  

if kind explain , why causing error, complete noob forever grateful !

this line:

myfraction = [fraction init]; 

is giving rise cannot init class object exception. should be:

myfraction = [myfraction init]; 

you send alloc messages classes, init messages instances of classes. can't send init class object, error states. sending alloc class gives instance of class, , instance should sending init to.

as others have said, combining 2 lines fraction * myfraction = [[fraction alloc] init]; more usual way of doing it, way you're doing not wrong, once send init message correct thing.