1.创建字符串
（1）常量字符串
NSString *string = @"i am an iOSDevTip!";
（2）常用创建方法
NSString *string = [[NSString alloc] init];
string = @"i am an iOSDevTip too!";
（3）用initWithString创建字符串
NSString *string = [[NSString alloc] initWithString:@"iOSDevTip is here!"];
2.格式化创建字符串
（1）int格式化字符串
int age = 20;
NSString *personAge = [NSString stringWithFormat:@"this person age is %d",age];
既然int格式化字符串，那么float、double等，也可以格式化字符串。
（2）NSString格式化字符串
NSString *name = @"iOSDevTip";
NSString *personName = [NSString stringWithFormat:@"this person name is %@",name];
3.字符串比较
（1）isEqualToString方法比较
NSString *stingOne = @"This is an iOSDevTip!";
NSString *stringTwo = @"This is an iOSDevTip!";
BOOL result = [stingOne isEqualToString:stringTwo];
（2）compare方法比较
BOOL result = [stingOne compare:stringTwo] == NSOrderedSame;
compare:方法返回值类型为NSComparisonResult。而NSComparisonResult有下面几个枚举值。
typedef NS_ENUM(NSInteger, NSComparisonResult) {NSOrderedAscending = -1L, NSOrderedSame, NSOrderedDescending};
4.字符串大小写转换
（1）小写转大写
NSString *string = @"This is an iOSDevTip!";
[string lowercaseString];
（2）大写转小写
NSString *string = @"This is an iOSDevTip!";
[string uppercaseString];
（3）首字母大写
NSString *string = @"ligang";
NSLog(@"string: %@",[string capitalizedString]);
打印：
2015-07-16 23:06:11.652 iOSStrongDemo[10279:3062010] string: Ligang
5.截取字符串
（1）substringToIndex截取字符串
NSString *string = @"This is a operation string!";
NSString *subToString = [string substringToIndex:6];
（2）截取的subToString为This i
substringFromIndex截取字符串
NSString *subFromString = [string substringFromIndex:6];
（3）截取的subFromString为s a operation string!
substringWithRange截取字符串
NSString *rangeString = [string substringWithRange:NSMakeRange(6, 3)];
截取的rangeString为s a!
6.判断字符串是否包含另一个字符串
（1）rangeOfString判断
NSString *string1 = @"This is a iOSDevTip";
NSString *string2 = @"iOSDevTip";
NSRange range = [string1 rangeOfString:string2];
NSInteger location = range.location;
NSInteger leight = range.length;
NSString *logString = [[NSString alloc] initWithString:[NSString stringWithFormat:@"Location:%ld,Leight:%ld",location,leight]];
NSLog(@"logString:%@",logString);
打印出来：
iOSStrongDemo[8837:2221170] logString:Location:10,Leight:9
（2）判断是否包含前后缀
NSString *string = @"I love China";
BOOL isHasI = [string hasPrefix:@"I"];
BOOL isHasChina = [string hasSuffix:@"China"];
7.分割字符串
NSString *string = @"This is a iOSDevTip";
NSArray *array = [string componentsSeparatedByString:@"a"];
NSString *string1 = [array objectAtIndex:0];
NSString *string2 = [array objectAtIndex:1];
NSLog(@"string1:%@ string2:%@",string1,string2);
打印：
2015-07-16 22:40:39.559 iOSStrongDemo[10165:3055448] string1:This is string2: iOSDevTip
8.插入字符串
NSMutableString *string = [[NSMutableString alloc] initWithString:@"I China"];
[string insertString:@"Love " atIndex:2];
NSLog(@"string: %@",string);
打印：
2015-07-16 22:44:10.706 iOSStrongDemo[10206:3057014] string: I Love China
（1）追加字符串
NSMutableString *string = [[NSMutableString alloc] initWithString:@"I Love "];
[string appendString:@"China"];
NSLog(@"string:%@",string);
打印：
2015-07-16 22:42:32.305 iOSStrongDemo[10189:3056410] string:I Love China
9.删除字符串
NSMutableString *string = [[NSMutableString alloc] initWithString:@"I love China"];
[string deleteCharactersInRange:NSMakeRange(2, 4)];
NSLog(@"String1: %@",string);
打印：
2015-07-16 22:46:58.437 iOSStrongDemo[10219:3057749] String1: I China
10.替换字符串
NSString *string = @"I love China";
NSString *replaceString = [string stringByReplacingOccurrencesOfString:@"love" withString:@"like"];
NSLog(@"replaceString: %@",replaceString);
打印：
2015-07-16 22:56:07.405 iOSStrongDemo[10236:3059503] replaceString: I like China
11.去除字符串首尾的空格和换行符
NSString *string = @" I love China ";
NSString *text = [string stringByTrimmingCharactersInSet:[NSCharacterSet whitespaceAndNewlineCharacterSet]];
NSLog(@"text:%@",text);
打印：
2015-07-16 23:00:47.845 iOSStrongDemo[10265:3061013] text:I love China
