OC之NSString

```objective-c
#import <Foundation/Foundation.h>

int main(int argc, const char * argv[]) {

@autoreleasepool {

//c语言字符串

char *s = "hello c";

//oc字符串

NSString *str = @"hello oc";

//字符串转化c->oc

NSString *str1 = [NSString stringWithUTF8String:s];

NSLog(@"oc str %@",str1);

//字符串转化oc->c

NSLog(@"str2 %s",[str UTF8String]);

//创建字符串的两种方法

NSString *str3 = @"IOS str"; //此种方法不需地手动释放内存(系统帮我们做)

NSString *str4 = [[NSString alloc] init]; //此种方法需要手动释放内存

str4 = @"IOS str";

//格式化输出

int a = 3;

int b = 5;

NSString *str5 = [NSString stringWithFormat:@"a=%d b=%d",a,b];

NSLog(@"%@",str5);

//拼接字符串

NSString *str6 = [str3 stringByAppendingString:str1];

NSLog(@"%@",str6); //输出  IOS strhello c

//大小写转换

NSString *str7 = @"abcdefg";

str7 = [str7 uppercaseString];

NSLog(@"%@",str7); //输出ABCDEFG

NSString *str8 = @"GWALER";

str8 = [str8 lowercaseString];

NSLog(@"%@",str8); //输出gwaler

//前缀与后缀的判断

NSString *str9 = @"gwalker.sinaapp.com";

BOOL hasPreFix = [str9 hasPrefix:@"gwalker."];

if(hasPreFix){

NSLog(@"是以gwalker.打头的");

}else{

NSLog(@"不是以gwalker.打头的");

}

BOOL hasSuffix = [str9 hasSuffix:@".com"];

if(hasSuffix){

NSLog(@"是以.com结尾的");

}else{

NSLog(@"不是以.com结尾的");

}

//判断两上字符串是否相同

NSString *str10 = @"abc";

NSString *str11 = @"abc";

if([str10 isEqualToString:str11]){

NSLog(@"str10与str11相等");

}else{

NSLog(@"str10与str11不相等");

}

//按照指定字符串分隔成数组

NSString *str12 = @"b,e,s,t,p,h,p,e,r";

NSArray *array = [str12 componentsSeparatedByString:@","]; //按照逗号进行分隔

//遍历数组

for(NSString *tmp in array){

NSLog(@"%@",tmp);

}

//按照范围截取字符串

NSRange range = NSMakeRange(1, 2); //指定截取范围（NSRange是一个范围）

NSString *str13 = @"a朝阳-群众-很行b";

NSString *str14 = [str13 substringWithRange:range];

NSLog(@"%@",str14); //输出 朝阳

//从某一位截取字符串

NSString *str15 = [str13 substringFromIndex:3];

NSLog(@"%@",str15); //输出 -群众-很行b

//从开头截取到某一位

NSString *str16 = [str13 substringToIndex:2];

NSLog(@"%@",str16); //输出 a朝

//字符串拆分成字符

NSString *str17 = @"ios";

for(int i =0 ;i< [str17 length];i++){ //不支持汉字

NSLog(@"%d-%c",i,[str17 characterAtIndex:i]);

}

//字符串查找

NSString *str18 = @"xixy中国xychihaha";

NSRange range1 = [str18 rangeOfString:@"xy中"];

NSLog(@"%ld,%ld",range1.location,range1.length); //输出2,3

//范围替换

NSString *str19 = @"look,有一个小牛上山了,haha";

NSString *str20 = [str19 stringByReplacingCharactersInRange:NSMakeRange(8, 2) withString:@"小羊"];

NSLog(@"%@",str20); //look,有一个小羊上山了,haha

//查找替换

NSString *str21 = [str20 stringByReplacingOccurrencesOfString:@"look" withString:@"你看"];

NSLog(@"%@",str21); //你看,有一个小羊上山了,haha

//把本地文件读到字符串

NSString *fileStr = [NSString stringWithContentsOfFile:@"/Users/gongwen/Desktop/gwalker.txt" encoding:NSUTF8StringEncoding error:nil];

NSLog(@"%@",fileStr);//把文件内容输出

//把字符串写到文件中(会在桌面新建一个demo.txt文件,并把字符串写在里面)

NSString *str22 = @"我是一个字符串\n,我要写在文件里";

BOOL isok = [str22 writeToFile:@"/Users/gongwen/Desktop/demo.txt" atomically:YES encoding:NSUTF8StringEncoding error:nil];

if(isok){

NSLog(@"文件写入成功");

}else{

NSLog(@"文件写入失败");

}

return 0;

}
```