incompatible pointer type? - objective-c

Question

I am a newbie in programming, currently across some difficulties in my first school work. Can anyone tell me am I doing on the right track? The aim of this code is to do add two numbers around 70 digits which cannot be done using the int or long int type in Objective-C. The following code keeps getting warning: incompatible pointer types returning 'NSString *__strong' from a result type 'MPInteger *' Help please, I have already been figuring out for ages and got nothing.
MPInteger.h
#import <Foundation/Foundation.h>
#interface MPInteger : NSObject
#property (copy, nonatomic) NSString * description;
-(id) initWithString: (NSString *) x;
-(NSString *) description;
-(MPInteger *) add: (MPInteger *) x;
MPInteger.m
#import "MPInteger.h"
#implementation MPInteger
#synthesize description;
-(id) initWithString: (NSString *) x {
self = [super init];
if (self) {
description = [NSString stringWithString: x];
}
return self;
}
-(NSString *) description {
return description;
}
-(MPInteger *) add: (MPInteger *) x
{
int carry = 0;
int index = 0;
int index2 = 0;
int i;
int num1;
int num2;
int result;
index = [description length];
index2 = [x.description length];
while (index2 < index) {
x.description = [x.description stringByPaddingToLength:index withString:#"0" startingAtIndex:0];
}
for (i = index; i <= index || carry != 0; i--) {
num1 = [description characterAtIndex:index];
num2 = [x.description characterAtIndex:index];
result = num1 + num2 + carry;
// if the sum is less than 10
if (result < 10) {
NSString *intString = [NSString stringWithFormat:#"%i", result];
[description replaceValueAtIndex:index inPropertyWithKey:intString withValue:#"%#"];
// replace the description index value with the sum
} else { //otherwise
NSString *intString = [NSString stringWithFormat:#"%i", result];
//getting the index'1' is the value replace the description index value
[description replaceValueAtIndex:index inPropertyWithKey:[intString substringFromIndex:1] withValue:#"%#"];
carry = 1;
}
index--; // decrement index
}
return description;
}

Answer 1

You declare your method to return an MPInteger:
- (MPInteger *)add:(MPInteger *)other;
but you finally return description, which is an NSString instance.
You perhaps meant to make an MPInteger instance out of the string before returning:
return [[MPInteger alloc] initWithString:description];
(add autorelease if you don't use ARC).

Answer 2

It's pretty much exactly what the error says:
#property (copy, nonatomic) NSString * description;
[...]
-(MPInteger *) add: (MPInteger *) x
{
[...]
return description;
}
You say that your add method will return a reference to a MPInteger object but your code returns a reference to a NSString instead. You need to make those match, either by declaring a string type of return value for the method or by returning an instance of MPInteger.

Answer 3

The warning is beacuse you're returning description from your add: method but description is an NSString. You need description to be an MPInteger.
Unfortunately, NSObject already has a method calleddescriptionwhich returns anNSString. It's this method that's called each time you use%#` in a format string i.e.
NSLog(#"I am %#", self);
will actually call description on self and put it into the string in place of the %#.
At the end of your method, you need to return an MPInteger instead of description. Try replacing
return description;
with
MPInteger newInteger = [MPInteger new];
newInteger.description = description;
return newInteger;
to return a new integer.