Hexlet:: OS X #3, 7 http://hexlet.org/lesson/osx_3_7/

Cell.h

//
//  DNA.h
//  Hexlet DNA
//
//  Created by Roman Timashev on 12/11/13.
//  Copyright (c) 2013 Roman Timashev's Company. All rights reserved.
//

#import <Foundation/Foundation.h>

@interface Cell : NSObject

@property NSMutableArray *dna;

-(int) hammingDistance: (Cell*) cell;

@end

Cell.m

//
//  DNA.m
//  Hexlet DNA
//
//  Created by Roman Timashev on 12/11/13.
//  Copyright (c) 2013 Roman Timashev's Company. All rights reserved.
//

#import "Cell.h"

@implementation Cell{

}

-(id) init{
    self = [super init];
    _dna = [[NSMutableArray alloc] init];
    
    NSString *dnaSymbols = @"ATGC";
    for (int i = 0; i<100; i++) {
        NSUInteger index = (int) (rand() % 4);
        unichar c = [dnaSymbols characterAtIndex:index];
        NSString *s = [NSString stringWithCharacters:&c length:1];
        
        [_dna setObject:s atIndexedSubscript:i];
    }
    return self;
}

-(int) hammingDistance:(Cell*) cell{
    int dist = 0;
    for(int i=0; i<[_dna count]; i++){
        NSString *dna1 = [self.dna objectAtIndex:i];
        NSString *dna2 = [cell.dna objectAtIndex:i];
        if([dna1 isEqualToString:dna2]) dist ++;
    }
    return dist;
}

-(NSString*) description{
    NSString *out = @"";
    for(int i=0; i<[_dna count]; i++){
        NSString *dnaSymbol = [_dna objectAtIndex:i];
        out = [out stringByAppendingString:dnaSymbol];
    }

    return out;
}


@end

main.m

//
//  main.m
//  Hexlet DNA
//
//  Created by Roman Timashev on 12/11/13.
//  Copyright (c) 2013 Roman Timashev's Company. All rights reserved.
//

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

int main(int argc, const char * argv[])
{

    @autoreleasepool {
        Cell *c1 = [[Cell alloc] init];
        Cell *c2 = [[Cell alloc] init];
        
        // insert code here...
        NSLog(@"%@", c1);
        NSLog(@"%@", c2);
        NSLog(@"%i", [c1 hammingDistance:c2]);
        
    }
    return 0;
}