global variable in objective C

Question

I'm new in Objective C and stuck on this problem already 5 days)) What i have to do is write implementation for simple task about city and metropolis. I have class City with properties and class metropolis that has an global array which adds city object through createCity method. I have implemented this task but this arrays returns nothing. Can anybody help me?

Here is part of the task:

1. Write a “City” class, which inherits from NSObject. Your class should contain the following:
Variables:
name, age, population.
Instance methods:
setName:age:population (single method) which set city’s name, age and population. getName, getAge, getPopulation which return city’s name, age and population, respectfully.
nextDay which adds a random number to city’s population, then subtracts a random number from city’s population. Figure out a way to generate random numbers yourself.
2. Create an instance of City class, set its name, age and population as you want.
3. Write a for-­‐loop (if in doubt how to do it – google or use Xcode’s help system) for 10 steps. Each step send ‘nextDay’ message to your object and print out the population.

    4. Write a “Metropolis” class. It should contain the following:
Variable:
array of 10 cities.
Instance method:
createCity:atIndex:withPopulation: (single method) which creates a city with first parameter being a name at index (from the second parameter) and sets its population to that of third parameter. So, you should be able to do this:
[myMetropolis createCity: @”Almaty” atIndex: 2 withPopulation: 1500000]
5. Create an instance of Metropolis class and create all 10 cities.

Here is my implementation:

City.h

#import <Foundation/Foundation.h>

@interface City : NSObject
{
    NSString* name;
    int age;
    int population;
}

-(void)setName: (NSString*)n age: (int)a population: (int)p;
-(NSString*)getName;
-(int)getAge;
-(int)getPopulation;
-(void)nextDay;


@end

City.m

#import "City.h"

@implementation City

-(void)setName:(NSString*)n age:(int)a population:(int)p
{
    name = n;
    age = a;
    population = p;
}

-(NSString*)getName
{
    return name;
}

-(int)getAge
{
    return age;
}

-(int)getPopulation
{
    return population;
}

-(void)nextDay
{
    int r = arc4random() % 100;
    int r2 = arc4random() % 100;
    population = population + r;
    population = population - r2;

}


@end

Metropolis.h

#import <Foundation/Foundation.h>
    #import "City.h"

@interface Metropolis : NSObject{
        NSMutableArray* myArray;
    }



-(void)createCity: (NSString*)n atIndex: (int)a withPopulation: (int)p;

-(NSMutableArray*) getArray;

@end

Metropolis.m

#import "Metropolis.h"
#import "City.h"
@implementation Metropolis

NSMutableArray* myArray = nil;
- (void)initialize {
        myArray = [[NSMutableArray alloc]initWithCapacity:10];
}

-(void)createCity:(NSString*)n atIndex:(int)a withPopulation:(int)p
{
    City* newCity = [[City alloc]init];
    [newCity setName:n age:0 population:p];

    [myArray insertObject:newCity atIndex:a];

}
-(NSMutableArray*)getArray
{
    return myArray;
}




@end

main.m

#import <Foundation/Foundation.h>
#import "City.h"
#import "Metropolis.h"
int main(int argc, const char * argv[])
{

    @autoreleasepool {
        Metropolis* myMetropolis = [[Metropolis alloc]init];
        [myMetropolis createCity:@"Aktobe" atIndex:0 withPopulation:15];
        [Metropolis initialize];
        NSMutableArray* c = [[NSMutableArray alloc]init];
        c = [myMetropolis getArray];


        NSLog(@"%@", [[c objectAtIndex:0] getName]);

    }
    return 0;
}

Answer 1

I've rewritten my answer to make it more complete, and to incorporate some of the other ideas generated in the other answers, especially @Hannes Sverrisson

The easy way to fix your issue is to call initialize BEFORE createCity (otherwise your trying to add objects to a nil array) and to also make sure you're not calling initialize from a static context. i.e. change [Metropolis initialize]; to [myMetropolis initialize];

The better way, and by better I mean more consistent with typical objective-c design, you should override the instance method init. This is done in the Metropolis implementation and replaces your initialize method.

-(id) init { 
    self = [super init]; 
    if (self) { 
        myArray = [[NSMutableArray alloc]initWithCapacity:10]; 
    } 
    return self; 
} 

or to make it more fun, create a new init method that takes the number of cities as a parameter.

    -(id) initWithNumberOfCities:(NSInteger)numCities { 
    self = [super init]; 
    if (self) { 
        myArray = [[NSMutableArray alloc]initWithCapacity:numCities]; 
    } 
    return self; 
} 

Then in your main method, remove the call to [Metropolis initialize]. The reason for this is when you say:

Metropolis* myMetropolis = [[Metropolis alloc]init];

or

Metropolis* myMetropolis = [[Metropolis alloc]initWithNumberOfCities:10];

the init method is being called inline after the allocation takes place.

Answer 2

The method for initialization is -(void)init; this method should be overwritten in your implementation of Metropolis. You are calling - (void)initialize; which is wrong in this case.

So, simply change - (void)initialize { to -(void)init { in your implementation of Metropolis and delete the line: [Metropolis initialize]; in main.

After the comment below the proper init method should be:

-(id) init { 
self = [super init]; 
if (self) { 
    myArray = [[NSMutableArray alloc]initWithCapacity:10]; 
    } 
    return self; 
} 

Answer 3

You don't need to write getters or create backing instance variables. You can use Objective-C 2.0's @property syntax.

@property (strong) NSString *name;
@property (assign) NSInteger age;
@property (assign) NSInteger population;

- (void)setName:(NSString*)name age:(NSInteger)age population:(NSInteger)population;
- (void)nextDay;

Then you access the properties using self.name, self.age, self.population or if you need to access the backing variable itself, _name, _age, _population.