Objective-C中的字符串格式化输出

转自 http://blog.kungfusoft.com/programming/iphone/topic-281.html
在使用诸如NSLog, [NSString stringWithFormat:]之类的函数时,都是基于c/c++风格的字符串格式化工作的.

本来c/c++就没怎么用过,到iphone开发开发时掺合上NS系的对象,格式化输出更是一头的乱.
看了一下Programming Guide for Cocoa的文档,还是有比较详尽的说明的,整理出来备查.

格式定义
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


定义
说明


%@
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



平台依赖
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


类型
定义
建议


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.



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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.



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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.