因iPhone应用程序的Objective-C中未捕获异常而终止应用程序

我在Objective-C中编写了一个for循环,这就是我的代码的样子

NSString *string = [NSString stringWithContentsOfFile=@"/Users/Home/myFile.doc"];
NSString *seperator=@"\n";
NSArray *mainarray = [string componentsSeparatedByString:seperator];
// Since i want to parse each element of mainarray
for(NSString *s in mainarray)
{
  //again parising the string using a new separator
    NSString newseparator = @"=";
    NSArray *subarray = [s componentsSeparatedByString : newseparator];

  //Copying the elements of array into key and object string variables

    NSString *key = [subarray objectAtIndex:0];
    NSLog(@"%@",key);
    NSString *class_name= [subarray objectAtIndex:1];
    NSLog(@"%@",class_name);

   // create an instance for the class_name
      //dont knw how it ll take the value from file and ???  

 //Putting the key and objects values into hashtable 
    NSMutableDictionary = [NSDictionary dictinaryWithObject:class_name forKey:key];
}

每当我执行此代码时,这会使我的程序崩溃,并说,由于未捕获的异常而终止应用程序nsrangeexception

如何知道数组的范围以及如何在for循环中指定终止条件?你说什么?请允许我知道如何处理这个例外?你说什么?