Coding in Chapter 2

chrisoflaherty · June 25th, 2012, 11:31 AM

My code does't match the code in the book. I don't know why. Can you please help!

thepianoguy · June 25th, 2012, 02:44 PM

Some specifics will be needed if you would like some help. i.e. with what page or pages in the book are you finding discrepancies? How is your code different? Are you referring to template generated code not matching the book's code?

Bob

chrisoflaherty · June 25th, 2012, 02:46 PM

Yes that's what I mean

Stefano123 · September 9th, 2012, 12:55 PM

In the book beginning ios game development..
at page 120 in MindPlayer.m I dont know why does the author write Peg* peg=nil;
the author doesnt explain the command nil .
Does anybody can explain this?

thepianoguy · November 7th, 2012, 09:25 AM

Actually it is not necessary to set the ivar to nil, since this is done automatically. For example if you did the following

Code:

    Peg *peg1 = nil;
    Peg *peg2;
    
    NSLog(@"peg1 = %@ at address %p",peg1,peg1);
    NSLog(@"peg2 = %@ at address %p",peg2,peg2);

The output would be
2012-11-07 08:08:21.977 TestNil[1133:c07] peg1 = (null) at address 0x0
2012-11-07 08:08:21.980 TestNil[1133:c07] peg2 = (null) at address 0x0

nil is the address 0x0. Messages can be safely sent to nil since they are just ignored.

Only when you alloc and init it do you get an address other than nil

Code:

    peg1 = [[Peg alloc] init];
    peg1.color = 'r';
    NSLog(@"peg1 = %@ at address %p",peg1,peg1);

2012-11-07 08:08:21.981 TestNil[1133:c07] peg1 = <Peg: 0x743ff30> at address 0x743ff30

If a variable is a type, not an ivar, it is not safe to fail to initialize it. For example

Code:

    int i;
    int j = 0;

    NSLog(@"i is %d at address %p",i,&i);
    NSLog(@"j is %d at address %p",j,&j);

Both i and j are given an address in memory, and what ever was in that memory address before will provide a garbage value.

2012-11-07 08:08:21.982 TestNil[1133:c07] i is 6045295 at address 0xbfffdb64
2012-11-07 08:08:21.983 TestNil[1133:c07] j is 0 at address 0xbfffdb60

The value of i in this case is unpredictable.

Bob