characterAtIndex returning bad values

Question

I am getting some weird numbers returned from the characterAtIndex method for Object-C in the iPhone SDK. The code snippet is as follows:

 NSString *new_text;
 new_text = [new_text stringByAppendingFormat:@"%@",UITextView_1.text];//=a,b,...f

 for(int i = 0; i <= 6; i++)
 {
   char_num   = [new_text characterAtIndex:i];   
 }

I am trying to get the decimal number for the individual character. I am either getting the wrong value each time. Sometimes the value is greater than 8,000! I should be getting back the number 65 through 71.

Answer 1

new_text is nil when you send the stringByAppendingFormat to it. It's legal to send a message to a nil object-- it simply does nothing. Thus new_text is never initialized, hence the weird values.

if new_text was an initialized NSMutableString your code would be ok:

// stringByAppendingFormat is an instance method and must be called on a non-nil instance
NSMutableString* text = [NSMutableString string];
text = [text stringByAppendingFormat:@"%@",UITextView_1.text];

// stringWithFormat is a class method and can be used to initialize an instance
NSString* text = [NSString stringWithFormat:@"%@",UITextView_1.text];

Answer 2

new_text is a local variable, it's not initalized, and you send a message to it before assigning a value to it. Unlike newly alloc'd Objective-C objects, local variable values are not zero'd automatically, thus the pointer value of new_text is whatever happened to be that memory location prior to the method's execution. In other words, your new_text pointer is pointing at a non-deterministic memory location, thus the behavior here is undefined and unpredictable. Usually you'd end up crashing, but by sheer chance you're pointing at something that looks like a string, but the contents of that string are also non-deterministic. Hence weird character values.

n.b. This is a great example of why it's good practice to always initialize local variables when they're declared.