Objective C : Array

Question

I am new to objective C and trying to learn it. I am trying to write calculator program which performs simple mathematical calculation(addition, subtraction and so forth).

I want to create an array which stores for numbers(double value) and operands. Now, my pushOperand method takes ID as below:

-(void) pushOperand:(id)operand
{
    [self.inputStack addObject:operand];

}

when I try to push double value as below:

- (IBAction)enterPressed:(UIButton *)sender
{

    [self.brain pushOperand:[self.displayResult.text doubleValue]];
}

It gives my following error: "Sending 'double' to parameter of incompatible type 'id'"

I would appreciate if you guys can answer my following questions:

'id' is a generic type so I would assume it will work with any type without giving error above. Can you please help me understand the real reason behind the error?
How can I resolve this error?

user3386109 · Accepted Answer · 2014-07-15 03:19:39Z

1

id is a pointer to any class. Hence, it does not work with primitive types such as double or int which are neither pointers, nor objects. To store a primitive type in an NSArray, one must first wrap the primitive in an NSNumber object. This can be done in using alloc/init or with the new style object creation, as shown in the two snippets below.

old style

NSNumber *number = [[NSNumber alloc] initWithDouble:[self.displayResult.text doubleValue]];
[self.brain pushOperand:number];

new style

NSNumber *number = @( [self.displayResult.text doubleValue] );
[self.brain pushOperand:number];

answered Jul 15, 2014 at 3:19

user3386109

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Comments

klcjr89 · Accepted Answer · 2014-07-15 03:22:19Z

1

I suggest using it with an NSNumber: Try not to abuse using id where you don't need to; lots of issues can arise if not.

- (void)pushOperand:(NSNumber *)operand
{
    [self.inputStack addObject:operand];
}

- (IBAction)enterPressed:(UIButton *)sender
{
    [self.brain pushOperand:@([self.displayResult.text doubleValue])];
}

edited Jul 15, 2014 at 3:22

answered Jul 15, 2014 at 3:17

klcjr89

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1 Comment

Mike Over a year ago

You should also be able to use the short-hand @([self.displayResult.text doubleValue]) rather than [NSNumber numberWithDouble:[self.displayResult.text doubleValue]] to make it a bit more concise, but yes you can't push primitives into an objective-C array or pass as a generic objective-c object (id) because it's not an object.

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