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String Format Specifiers

This article summarizes the format specifiers supported by string formatting methods and functions.

Contents:

Format Specifiers
Platform Dependencies


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 NSLog function.

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, %i

Signed 32-bit integer (int)

%u, %U

Unsigned 32-bit integer (unsigned int)

%hi

Signed 16-bit integer (short)

%hu

Unsigned 16-bit integer (unsigned short)

%qi

Signed 64-bit integer (long long)

%qu

Unsigned 64-bit integer (unsigned long long)

%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

%qx

Unsigned 64-bit integer (unsigned long long), printed in hexadecimal using the digits 0–9 and lowercase a–f

%qX

Unsigned 64-bit integer (unsigned long long), 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. %s interprets its input in the system encoding rather than, for example, UTF-8.

%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

%L

Length modifier specifying that a following a, A, e, E, f, F, g, or G conversion specifier applies to a long double argument

%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

%z

Length modifier specifying that a following d, i, 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, i, 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, i, o, u, x, or X conversion specifier applies to a intmax_t or uintmax_t argument

Platform Dependencies

Mac 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 and unsigned 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 2. Note that in some cases you may have to cast the value.

Table 2  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 see below warning when scanning

CFIndex

%ld or %lx

The same as NSInteger

pointer

%p

%p adds 0x to the beginning of the output. If you don't want that, use %lx and cast to long.

long long

%lld or %llx

long long is 64-bit on both 32- and 64-bit platforms

unsigned long long

%llu or %llx

unsigned long long is 64-bit on both 32- and 64-bit platforms

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 2, there is one extra case with scanning: you must distinguish the types for float and double. You should use %f for float, %lf for 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.



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© 1997, 2008 Apple Inc. All Rights Reserved. (Last updated: 2008-10-15)

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