在iOS系统，NSString可能是最常用的对象，很多用法跟其他语言不一样。

字符串对象NSString

使用格式创建字符串

+ (id)stringWithFormat:(NSString *)format...
- (id)initWithFormat:(NSString *)format...

NSString *str = "hello";
NSString *string = [NSString stringWithFormat:@"%@ world",str];
NSLog(string); 结果：hello world

常用的替换符

%@ NSString实例
%d,%D,%i 整数
%4d,%4D,%4i 格式化整数
%ld,%lD,%li 长整数
%u,%U 无符号整数
%x 将无符号整数以十六进制小写字母显示
%X 将无符号整数以十六进制大写字母显示
%f 小数
%c 字符
%s C语言字符串
%% 显示％字符本身

范围集合NSRange

定义

typedef struct _NSRange
{
     unsigned int location;
     unsigned int length;
}NSRange;

NSMakeRange函数

   这个函数比较特殊 返回一个NSRange的对象。

NSMakeRanger(unsigned int location,unsigned int length);

   例如：

NSRange range = NSMakeRanger(0,5);
NSLog(@"location is %d,length is %d",range.location,range.length);

查找

   如果找到就返回范围，否则NSRange的location项被设置为NSNotFound

 - (NSRange)rangeOfString:(NSString *)subString;
 - (NSRange)rangeOfString:(NSString *)subString option:(unsigned)mask;
 - (NSRange)rangeOfString:(NSString *)subString option:(unsigned)mask range:(NSRange)range;

//mask常用选项列表
//NSCaseInsensitiveSearch   不区分字母大小写
//NSLiteralSearch           对字符串进行字节单位的比较，一般可提高检索速度
//NSBackwardsSearch         从范围的末尾开始检索
//NSAnchoredSearch          仅检索制定范围的前部。忽略字符串中间的检索字符

NSString *string = @"hello world";
NSRange range = [string rangeOfString:@"he"];
if(range.location != NSNotFound)
{
        NSLog(@" location=%d,length=%d",range.location,range.length);
}

截取字符串

 NSString  
 //返回字符串开头至index位的字符串 不包含索引位
 - (NSString *)substringToIndex:(unsigned)index;

 //返回字符串第index位至末尾的字符串 包含索引位
 - (NSString *)substringFromIndex:(unsigned)index;

  //返回字符串中范围range内的字符串
 - (NSString *)substringWithRange:(NSRange)range;

  //包含索引位
 NSString *string = [string substringWithRange:NSMakeRange(5,2)];

比较字符串

NSString *String1 = @"NSStringInformation.txt";

//hasPrefix 前缀比较
[String1 hasPrefix:@"NSString"] = = 1 ?  NSLog(@"YES") : NSLog(@"NO");

//hasSuffix 后缀比较
[String1 hasSuffix:@".txt"] = = 1 ?  NSLog(@"YES") : NSLog(@"NO");

//isEqualToString 完全比较
if([string1 isEqualToString:@""])
{
    NSLog(@"string1 is blank");
}

替换字符串

 NSString *newString  = [oldString stringByReplacingOccurrencesOfString:@"x" withString:@"y"];

分离字符串成数组

NSString *string = @"A|B|C|D";
NSArray *array = [string componentsSeparatedByString:@"|"];

读取文本文件

NSString
 + (id)stringWithContentsOfFile:(NSString *)path usedEncoding:(NSStringEncoding *)enc error:(NSError **)error     //自动释放内存

 - (id)initWithContentsOfFile:(NSString *)path encoding:(NSStringEncoding)enc error:(NSError **)error

 NSString *string = [NSString stringWithContentsOfFile:@"/user/test/yw.txt" encoding:NSUTF8StringEncoding error:&error];
 if(string){}

输出文本文件

NSString
 - (BOOL)writeToFile:(NSString *)path atomically:(BOOL)useAuxiliaryFile encoding:(NSStringEncoding)enc error:(NSError **)error
 //参数 atomically 暂时将文件保存到辅助文件中
 //path
 The file to which to write the receiver. If path contains a tilde (~) character, you must expand it withstringByExpandingTildeInPath before invoking this method.


//扩展路径
NSString *Path = @"~/NSData.txt";
NSString *absolutePath = [Path stringByExpandingTildeInPath];
NSLog(@"absolutePath:%@",absolutePath);
NSLog(@"Path:%@",[absolutePath stringByAbbreviatingWithTildeInPath]);

//文件扩展名
NSString *Path = @"~/NSData.txt";
NSLog(@"Extension:%@",[Path pathExtension]);