Objective-C Fundamentals

42 CHAPTER 2 Data types, variables, and constants
Portability is never as simple as it first seems
If you followed our previous advice and made use of data types such as NSInteger
and NSUInteger, you must be extra careful when using functions, such as NSLog,
which accept format specifiers. On 32-bit targets, such as the iPhone, it’s common
to use the %d specifier to format integer values, as demonstrated in the following
code sample:
NSInteger i = 45;
NSLog(@"My value is %d", i);
If you reuse this code in a 64-bit-based desktop application, however, it could result
in incorrect behavior. In a 64-bit environment, a variable declared of type int stays
at 32 bits in size, whereas a variable declared of type NSInteger is redefined to be
equivalent to long, which makes it now 64 bits in size. Hence, the correct format
specifier for NSInteger in a 64-bit environment is %ld:
NSLog(@"My value is %ld", i);
To avoid altering source code like this, Apple’s 64-bit Transition Guide for Cocoa recommends
always casting such values to long or unsigned long (as appropriate).
For example:
NSLog(@"My value is %ld", (long)i);
The typecast to long ensures that NSLog is always provided a 64-bit argument even
if the current platform is 32 bit. This means %ld will always be the correct specifier.
To further control the presentation of values, you can place a number between the %
and the field data type. This acts as a minimum field width and will right-align the value
by padding out the string with spaces if it’s not already long enough. For example:
int a = 92;
int b = 145;
NSLog(@"Numbers:\nA: %6d\nB: %6d", a, b);
This should result in the console window displaying output similar to the following:
Numbers:
A: 92
B: 145
Placing a negative sign in front of the number causes NSLog to instead left-align the
field, whereas prefixing the number with a zero pads with zeros instead of spaces.
When formatting floating-point numbers, it’s also possible to specify the desired number
of decimal places by preceding the minimum field-width specifier with a decimal
point and the desired number of decimal places. As an example, the following code
snippet emits the string "Current temperature is 40.54 Fahrenheit":
float temp = 40.53914;
NSLog(@"Current temperature is %0.2f Fahrenheit", temp);