Setting Objective-C instance variable values

Question

I have the following code:

@implementation Fraction   
{  
      int   numerator;  
      int   denominator;  
}  

    -(void) setNumerator: (int) n   
{  
    numerator = n;   
}  
    -(void) setDenominator: (int) d   
{  
    denominator = d;   
}
@end

I was wondering why you have to have both "n" and "numerator" for the numerator? For example why can't you just set

-(void) setNumerator: (int) numerator

as apposed to:

-(void) setNumerator: (int) n
     numerator = n;
}

Sorry if this is such a basic question but I'm starting from the beginning with no programming experience.

Ernest Friedman-Hill · Accepted Answer · 2012-02-02 14:11:54Z

2

The setNumerator method is used to tell a Fraction object what value it should hold in the variable named numerator. As in "Here, Mr. Fraction, please use the value n for your numerator." In the implementation of this method, the code has to deal with two different concepts: the parameter variable n containing the new value, and the instance variable numerator which must be changed to the value in n; hence the need for two different names. The line

numerator = n;

literally means "copy the number in n into the variable numerator."

Remember, there are two halves to the transaction. Say I'm a Fraction object, and you want me to set my numerator to 4. You say "Set your numerator to 4, or setNumerator(4)", which is fine, because you're human, and you get to choose whatever number you want.

But as a lowly Objective-C object, all my code was written some time in the past; it was written before the value 4 was even a twinkle in your eye. So the code for setNumerator() has to be generic; it has to be written to say "set numerator to whatever value the human wants -- call it n". Remember, my actual variable numerator is hidden from you; all you can do is call my method, and it's up to me to set the variable.

That's why the method must be written to use an abstract name -- n -- for the value, since when the method is written, the value is unknown.

edited Feb 2, 2012 at 14:11

answered Feb 2, 2012 at 13:57

Ernest Friedman-Hill

81.9k11 gold badges158 silver badges191 bronze badges

Sign up to request clarification or add additional context in comments.

6 Comments

CloudDweller Over a year ago

Wow that was fast thank you. But I'm still unclear as to why there needs to be a paramenter variable and an instance variable. Because numerator has been declared as an (int) and when executing the method you give (int) a value like 4, why do we need to first give n the value of 4 and then pass this to the numerator?

Ernest Friedman-Hill Over a year ago

I added quite a bit to the answer; hope it helps.

CloudDweller Over a year ago

I kinda get what you mean and I know its me being stupid but I'm still not 100% as to why there needs to be two. We declare both numerator as an (int) and n as an (int) so the program knows that an int is going to be used. Why is the numerator ivar hidden as don't we show it in the interface?

Ernest Friedman-Hill Over a year ago

In a word: encapsulation. See en.wikipedia.org/wiki/… . It's an important concept in object-oriented programming.

CloudDweller Over a year ago

Are I see okay I think I get it now. So ivar's are the outlet of the object (data being passed out of the object). So by separating the value derived from the method (in this case “n” or “d”) and then passing this to the instance variable (“numerator” or “denominator”) we create a buffer or separation between the internal workings of the object (method and arguments) and the output (ivars) Have I got this right?

|

Paul Sasik · Accepted Answer · 2012-02-02 14:04:49Z

0

I think the concept you may be looking is called properties. Take a look here and here and here. Properties will allow you to set member values without having to name a parameter or call a method. You simply assign a value to the property.

With a function call you have to name the parameter that you are passing into the function so that you can assign it to the class member.

edited Feb 2, 2012 at 14:04

answered Feb 2, 2012 at 13:57

Paul Sasik

81.9k23 gold badges152 silver badges192 bronze badges

1 Comment

CloudDweller Over a year ago

Thanks for taking the time to answer my question. I've now grasped the concept.

Collectives™ on Stack Overflow

Setting Objective-C instance variable values

2 Answers 2

6 Comments

1 Comment

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

6 Comments

1 Comment

Your Answer

Sign up or log in

Post as a guest

Related