//NSString转换成NSData类型NSString * str = @"hello, world!";NSData * data = [str dataUsingEncoding:NSUTF8StringEncoding]; NSLog(@"%s", data.bytes);//NSData转换成NSString类型NSString * newStr = [[NSString alloc] initWithData:data encoding:NSUTF8StringEncoding];NSLog(@"%@", newStr);

 //获取图片路径NSString *imgPath = [[NSBundle mainBundle] pathForResource:@"01loading.png" ofType:nil];NSData *imgData = [NSData dataWithContentsOfFile:imgPath];
//直接使用，也可写入文件
self.imageView.image = [UIImage imageWithData:imgData];1>.数据库里存储图片 存储图片路径 /images/car@2x.png；2>.如果图片是较小的 可以将UIImage转换成NSData存数据库。

NsMutableData* md = [NSMutableData data];
NSMutableData* md=[[NSMutableData alloc]initWithLength:100];
//追加data内容:
[md appendData: dt];
//设置Data内容：
[md setData:dt];
//变更size内容：
[md setLength:50];

//1、创建常量字符串NSString *astring = @"This is a String!";//2、创建空字符串，给予赋值。NSString *astring = [[NSString alloc] init]; astring = @"This is a String!"; [astring release]; NSLog(@"astring:%@",astring);//3、在以上方法中，提升速度:initWithString方法NSString *astring = [[NSString alloc] initWithString:@"This is a String!"]; NSLog(@"astring:%@",astring); [astring release];//4、用标准c创建字符串:initWithCString方法char *Cstring = "This is a String!"; NSString *astring = [[NSString alloc] initWithCString:Cstring]; NSLog(@"astring:%@",astring); [astring release];//5、创建格式化字符串:占位符（由一个%加一个字符组成）int i = 1; int j = 2; NSString *astring = [[NSString alloc] initWithString:[NSString stringWithFormat:@"%d.This is %i string!",i,j]]; NSLog(@"astring:%@",astring); [astring release];//6、创建临时字符串NSString *astring; astring = [NSString stringWithCString:"This is a temporary string"]; NSLog(@"astring:%@",astring);//7、从文件创建字符串NSString *path = [[NSBundlemainBundle] pathForResource:@"astring.text"ofType:nil];NSString *astring = [[NSString alloc] initWithContentsOfFile:path];NSLog(@"astring:%@",astring);[astring release];//8、用字符串创建字符串，并写入到文件NSString *astring = [[NSString alloc] initWithString:@"This is a String!"]; NSLog(@"astring:%@",astring); NSString *path = @"astring.text";     [astring writeToFile: path atomically: YES]; [astring release];
注：此路径path只只是示意，真实路径并非如此//9、用C比较:strcmp函数char string1[] = "string!"; char string2[] = "string!"; if(strcmp(string1, string2) == 0){ NSLog(@"1"); }//10、isEqualToString方法    NSString *astring01 = @"This is a String!"; NSString *astring02 = @"This is a String!"; BOOL result = [astring01 isEqualToString:astring02]; NSLog(@"result:%d",result);//11、compare方法(comparer返回的三种值)    NSString *astring01 = @"This is a String!"; NSString *astring02 = @"This is a String!";     BOOL result = [astring01 compare:astring02] == NSOrderedSame;    //NSOrderedSame判断两者内容是否相同 NSLog(@"result:%d",result);     NSString *astring01 = @"This is a String!"; NSString *astring02 = @"this is a String!"; BOOL result = [astring01 compare:astring02] == NSOrderedAscending;    //NSOrderedAscending判断两对象值的大小(按字母顺序进行比较，astring02大于astring01为真) NSLog(@"result:%d",result);NSString *astring01 = @"this is a String!"; NSString *astring02 = @"This is a String!"; BOOL result = [astring01 compare:astring02] == NSOrderedDescending;   //NSOrderedDescending判断两对象值的大小(按字母顺序进行比较，astring02小于astring01为真)NSLog(@"result:%d",result);  //12、不考虑大小写比较字符串//1.NSString *astring01 = @"this is a String!"; NSString *astring02 = @"This is a String!"; BOOL result = [astring01 caseInsensitiveCompare:astring02] == NSOrderedSame;        //NSOrderedDescending判断两对象值的大小(按字母顺序进行比较，astring02小于astring01为真)NSLog(@"result:%d",result); //2.NSString *astring01 = @"this is a String!"; NSString *astring02 = @"This is a String!"; BOOL result = [astring01 compare:astring02 options:NSCaseInsensitiveSearch | NSNumericSearch] == NSOrderedSame;    //NSCaseInsensitiveSearch:不区分大小写比较 NSLiteralSearch:进行完全比较，区分大小写 NSNumericSearch:比较字符串的字符个数，而不是字符值。 NSLog(@"result:%d",result); //13、输出大写或者小写字符串NSString *string1 = @"A String";  NSString *string2 = @"String";  NSLog(@"string1:%@",[string1 uppercaseString]);//大写 NSLog(@"string2:%@",[string2 lowercaseString]);//小写 NSLog(@"string2:%@",[string2 capitalizedString]);//首字母大小//14、-rangeOfString: //查找字符串某处是否包含其它字符串NSString *string1 = @"This is a string"; NSString *string2 = @"string"; NSRange range = [string1 rangeOfString:string2]; int location = range.location; int leight = range.length; NSString *astring = [[NSString alloc] initWithString:[NSString stringWithFormat:@"Location:%i,Leight:%i",location,leight]]; NSLog(@"astring:%@",astring); [astring release];//15、-substringToIndex: 从字符串的开头一直截取到指定的位置，但不包括该位置的字符NSString *string1 = @"This is a string"; NSString *string2 = [string1 substringToIndex:3]; NSLog(@"string2:%@",string2);//16、-substringFromIndex: 以指定位置开始（包括指定位置的字符），并包括之后的全部字符NSString *string1 = @"This is a string"; NSString *string2 = [string1 substringFromIndex:3]; NSLog(@"string2:%@",string2);//17、-substringWithRange: //按照所给出的位置，长度，任意地从字符串中截取子串
    NSString *string1 = @"This is a string"; NSString *string2 = [string1 substringWithRange:NSMakeRange(0, 4)]; NSLog(@"string2:%@",string2);//18、-stringWithCapacity: //按照固定长度生成空字符串NSMutableString *String; String = [NSMutableString stringWithCapacity:40];//19、-appendString: and -appendFormat: //把一个字符串接在另一个字符串的末尾NSMutableString *String1 = [[NSMutableString alloc] initWithString:@"This is a NSMutableString"]; [String1 appendString:@", I will be adding some character"]; [String1 appendFormat:[NSString stringWithFormat:@", I will be adding some character"]]; NSLog(@"String1:%@",String1);//20、-insertString: atIndex: //在指定位置插入字符串NSMutableString *String1 = [[NSMutableString alloc] initWithString:@"This is a NSMutableString"]; [String1 insertString:@"Hi! " atIndex:0]; NSLog(@"String1:%@",String1);//21、-setString: NSMutableString *String1 = [[NSMutableString alloc] initWithString:@"This is a NSMutableString"]; [String1 setString:@"Hello Word!"]; NSLog(@"String1:%@",String1);//22、-replaceCharactersInRange: withString: //用指定字符串替换字符串中某指定位置、长度的字符串NSMutableString *String1 = [[NSMutableString alloc] initWithString:@"This is a NSMutableString"]; [String1 replaceCharactersInRange:NSMakeRange(0, 4) withString:@"That"]; NSLog(@"String1:%@",String1);//23、-hasPrefix: //检查字符串是否以另一个字符串开头NSString *String1 = @"NSStringInformation.txt"; [String1 hasPrefix:@"NSString"] = = 1 ?  NSLog(@"YES") : NSLog(@"NO"); [String1 hasSuffix:@".txt"] = = 1 ?  NSLog(@"YES") : NSLog(@"NO");//24、扩展路径NSString *Path = @"~/NSData.txt"; NSString *absolutePath = [Path stringByExpandingTildeInPath]; NSLog(@"absolutePath:%@",absolutePath); NSLog(@"Path:%@",[absolutePath stringByAbbreviatingWithTildeInPath]);//25、文件扩展名NSString *Path = @"~/NSData.txt"; NSLog(@"Extension:%@",[Path pathExtension]);