String Format Specifiers

Format Specifiers

The format specifiers supported by the NSString formatting methods and CFString formatting functions follow the IEEE printf specification; the specifiers are summarized in Table 1. Note that you can also use the “n$” positional specifiers such as %1$@ %2$s. For more details, see the IEEE printf specification. You can also use these format specifiers with the NSLogfunction.

Table 1  Format specifiers supported by the NSString formatting methods and CFString formatting functions

Specifier	Description
%@	Objective-C object, printed as the string returned by descriptionWithLocale: if available, or description otherwise. Also works with CFTypeRef objects, returning the result of the CFCopyDescription function.
%%	'%' character.
%d, %D	Signed 32-bit integer (int).
%u, %U	Unsigned 32-bit integer (unsigned int).
%x	Unsigned 32-bit integer (unsigned int), printed in hexadecimal using the digits 0–9 and lowercase a–f.
%X	Unsigned 32-bit integer (unsigned int), printed in hexadecimal using the digits 0–9 and uppercase A–F.
%o, %O	Unsigned 32-bit integer (unsigned int), printed in octal.
%f	64-bit floating-point number (double).
%e	64-bit floating-point number (double), printed in scientific notation using a lowercase e to introduce the exponent.
%E	64-bit floating-point number (double), printed in scientific notation using an uppercase E to introduce the exponent.
%g	64-bit floating-point number (double), printed in the style of %e if the exponent is less than –4 or greater than or equal to the precision, in the style of %f otherwise.
%G	64-bit floating-point number (double), printed in the style of %E if the exponent is less than –4 or greater than or equal to the precision, in the style of %f otherwise.
%c	8-bit unsigned character (unsigned char), printed by NSLog() as an ASCII character, or, if not an ASCII character, in the octal format \\ddd or the Unicode hexadecimal format \\udddd, where d is a digit.
%C	16-bit Unicode character (unichar), printed by NSLog() as an ASCII character, or, if not an ASCII character, in the octal format \\ddd or the Unicode hexadecimal format \\udddd, where d is a digit.
%s	Null-terminated array of 8-bit unsigned characters. Because the %s specifier causes the characters to be interpreted in the system default encoding, the results can be variable, especially with right-to-left languages. For example, with RTL, %s inserts direction markers when the characters are not strongly directional. For this reason, it’s best to avoid %s and specify encodings explicitly.
%S	Null-terminated array of 16-bit Unicode characters.
%p	Void pointer (void *), printed in hexadecimal with the digits 0–9 and lowercase a–f, with a leading 0x.
%a	64-bit floating-point number (double), printed in scientific notation with a leading 0x and one hexadecimal digit before the decimal point using a lowercase p to introduce the exponent.
%A	64-bit floating-point number (double), printed in scientific notation with a leading 0X and one hexadecimal digit before the decimal point using a uppercase P to introduce the exponent.
%F	64-bit floating-point number (double), printed in decimal notation.

Table 2  Length modifiers supported by the NSString formatting methods and CFString formatting functions

Length modifier	Description
h	Length modifier specifying that a following d, o, u, x, or X conversion specifier applies to a short or unsigned short argument.
hh	Length modifier specifying that a following d, o, u, x, or X conversion specifier applies to a signed char or unsigned char argument.
l	Length modifier specifying that a following d, o, u, x, or X conversion specifier applies to a long or unsigned long argument.
ll, q	Length modifiers specifying that a following d, o, u, x, or X conversion specifier applies to a long long or unsigned long long argument.
L	Length modifier specifying that a following a, A, e, E, f, F, g, or G conversion specifier applies to a long double argument.
z	Length modifier specifying that a following d, o, u, x, or X conversion specifier applies to a size_t or the corresponding signed integer type argument.
t	Length modifier specifying that a following d, o, u, x, or X conversion specifier applies to a ptrdiff_t or the corresponding unsigned integer type argument.
j	Length modifier specifying that a following d, o, u, x, or X conversion specifier applies to a intmax_t or uintmax_t argument.

Platform Dependencies

OS X uses several data types—NSInteger, NSUInteger,CGFloat, and CFIndex—to provide a consistent means of representing values in 32- and 64-bit environments. In a 32-bit environment, NSInteger and NSUInteger are defined as int and unsigned int, respectively. In 64-bit environments, NSInteger and NSUInteger are defined as long andunsigned long, respectively. To avoid the need to use different printf-style type specifiers depending on the platform, you can use the specifiers shown in Table 3. Note that in some cases you may have to cast the value.

Table 3  Format specifiers for data types

Type	Format specifier	Considerations
NSInteger	%ld or %lx	Cast the value to long.
NSUInteger	%lu or %lx	Cast the value to unsigned long.
CGFloat	%f or %g	%f works for floats and doubles when formatting; but note the technique described below for scanning.
CFIndex	%ld or %lx	The same as NSInteger.
pointer	%p or %zx	%p adds 0x to the beginning of the output. If you don't want that, use %zx and no typecast.

The following example illustrates the use of %ld to format an NSInteger and the use of a cast.

NSInteger i = 42;

printf("%ld\n", (long)i);

In addition to the considerations mentioned in Table 3, there is one extra case with scanning: you must distinguish the types for float and double. You should use %f for float, %lffor double. If you need to use scanf (or a variant thereof) with CGFloat, switch to double instead, and copy the double to CGFloat.

CGFloat imageWidth;

double tmp;

sscanf (str, "%lf", &tmp);

imageWidth = tmp;

It is important to remember that %lf does not represent CGFloat correctly on either 32- or 64-bit platforms. This is unlike %ld, which works for long in all cases.