initWithFormat 是实例方法，用法如下：
NSString* str = [[NSString alloc] initWithFormat:@"%@", @"Hello, world!"];
self.label.text = str;
[str release];
如果是运行在在 iOS 5.0 之前或者没有使用 ARC 的情况下， 需要手工调用 release 方法进行回收。
stringWithFormat 是类方法， 用法如下：
NSString* str = [NSString stringWithFormat:"%@", @"Hello, world!"];
sef.label.text = str;
stringWithFormat 不需要手工调用 release 方法进行回收。
NSString* str = @"123";
NSInteger intVal = [str integerValue];
int val = [str intValue];
NSString* str = @"123";
NSNumberFormatter* f = [[NSNumberFormatter alloc] init];
[f setNumberStyle:NSNumberFormatterDecimalStyle];
NSNumber* num = [f numberFromString:str];
%@
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
更多格式化文档， 参考苹果文档 String Format Specifiers 。