title: can't add objects to NSMutableArray
body: <p>I'm a newbie working on an RPN calculator, and I'm trying to set up an array of two-member arrays to store string/value pairs so that I can use them later to show the complete sequence of digits/operators pressed to get to wherever the user is now.  The thing is, whatever I've gotten wrong means the arrays won't populate.  Any thoughts about how to fix this?</p>
  
  <p>Here's my code (<code>oneStringValuePair</code> and <code>arrayOfStringValuePairs</code> are NSMutableArrays declared in .h as <code>(nonatomic, strong)</code> and synthesized in .m):</p>
  
  <pre><code>NSLog(@"result is float: %f",result);
  
  self.formattedNumber = [NSString stringWithFormat:@"( %@%@%@ )",firstNumber,operation,secondNumber];
  
  NSLog(@"String is %@",self.formattedNumber);
  
  NSMutableArray *pair = self.oneStringValuePair;
  NSMutableArray *array = self.arrayOfStringValuePairs;
  
  pair = [pair init];
  array = [array init];
  
  if(pair==nil)
  {
      NSLog(@"pair is nil.");
  }
  
  if (array==nil)
  {
      NSLog(@"array is nil.");
  }
  
  [pair addObject:self.formattedNumber];
  
  NSLog(@"first array element description: %@", [[pair lastObject] description]);
  [pair addObject:[NSNumber numberWithFloat:result]];
  
  NSLog(@"second array element description: %@",[[pair lastObject] description]);
  NSLog(@"pair count: %@",self.oneStringValuePair.count);
  
  if (self.arrayOfStringValuePairs)
  {
      [array addObject:pair];
  }
  
  NSLog(@"count of pairs:  %@",array.count);
  </code></pre>
  
  <p>Here's the NSLog for <code>5 3 + 6 /</code> :</p>
  
  <pre><code>2012-08-04 08:02:54.550 Calculator[83891:f803] result is float: 8.000000
  2012-08-04 08:02:54.552 Calculator[83891:f803] String is ( 3+5 )
  2012-08-04 08:02:54.552 Calculator[83891:f803] pair is nil.
  2012-08-04 08:02:54.553 Calculator[83891:f803] array is nil.
  2012-08-04 08:02:54.554 Calculator[83891:f803] first array element description: (null)
  2012-08-04 08:02:54.555 Calculator[83891:f803] second array element description: (null)
  2012-08-04 08:02:54.555 Calculator[83891:f803] pair count: (null)
  2012-08-04 08:02:54.556 Calculator[83891:f803] count of pairs:  (null)
  2012-08-04 08:02:55.987 Calculator[83891:f803] result is float: 0.750000
  2012-08-04 08:02:55.988 Calculator[83891:f803] String is ( 6/8 )
  2012-08-04 08:02:55.989 Calculator[83891:f803] pair is nil.
  2012-08-04 08:02:55.990 Calculator[83891:f803] array is nil.
  2012-08-04 08:02:55.991 Calculator[83891:f803] first array element description: (null)
  2012-08-04 08:02:55.992 Calculator[83891:f803] second array element description: (null)
  2012-08-04 08:02:55.993 Calculator[83891:f803] pair count: (null)
  2012-08-04 08:02:55.994 Calculator[83891:f803] count of pairs:  (null)
  </code></pre>