Objective-C exercises undeclared identifier error - ios

Question

I'm currently teaching myself Objective-C as a first language. I understand the difficulty involved, but I'm quiet a persevering individual. I've began to do the exercises on the Apple Objective-C documentation. My goal is to have my program log out my first and last name instead of a generic Hello World greeting.
I keep receiving a Use of Undeclared identifier error. I'm trying to figure out what is causing the error.
Here is the introClass.h
#import <UIKit/UIKit.h>
#interface XYZperson : NSObject
#property NSString *firstName;
#property NSString *lastName;
#property NSDate *dateOfBirth;
- (void)sayHello;
- (void)saySomething:(NSString *)greeting;
+ (instancetype)person;
-(int)xYZPointer;
-(NSString *)fullName;
#end
Here is IntroClass.m
#import "IntroClass.h"
#implementation XYZperson
-(NSString *)fullName
{
return[NSString stringWithFormat:#" %# %#", self.firstName, self.lastName];
}
-(void)sayHello
{
[self saySomething:#"Hello %#", fullName]; //use of undeclared identifier "fullName"
};
-(void)saySomething:(NSString *)greeting
{
NSLog(#"%#", greeting);
}
+(instancetype)person{
return [[self alloc] init];
};
- (int)xYZPointer {
int someInteger;
if (someInteger != nil){
NSLog(#"its alive");
}
return someInteger;
};
#end

Answer 1

The problem is that fullName is the name of a method. It should be invoked on self with square brackets.
Since saySomething: expects a single parameter, you need to either (1) remove the #"Hello %#" portion of the call, like this:
-(void)sayHello {
[self saySomething:[self fullName]];
};
or to make a single string from #"Hello %#" and [self fullName], like this:
-(void)sayHello {
[self saySomething:[NSString stringWithFormat:#"Hello %#", [self fullName]]];
};

Answer 2

You are passing back a string of first and last name but I don't see anywhere that you had set a value for them. As others noted try
-(void)sayHello
{
_firstName = [NSString stringWithFormat:#"John"];
_lastName = [NSString stringWithFormat:#"Doe"];
//if you want to see what's happening through out your code, NSLog it like
NSLog(#"_firstName: %# ...", _firstName);
NSLog(#"_lastName: %# ...", _lastName);
NSString *strReturned = [self fullName];
NSString *concatStr = [NSString stringWithFormat:#"Hello %#", strReturned];
NSLog(#"strReturned: %# ...", strReturned);
NSLog(#"concatStr: %# ...", concatStr);
[self saySomething:concatStr];
};
-(NSString *)fullName
{
return[NSString stringWithFormat:#" %# %#", self.firstName, self.lastName];
}

Answer 3

use
[self saySomething:#"Hello %#", self.fullName]];
or
[self saySomething:#"Hello %#", [self fullName]];