NSString NSMutableString

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// NSString
        //代开API文档
        //Xcode -> help - Documentation and API Reference
    
        //快速打开API
        //alt(option) + 鼠标左键
    
        //快速进入头文件
        //command + 鼠标左键
    
//        NSString是oc中的不可变字符串类,被创建后,不能修改
    
        //常用方法
        //1.创建字符串
    NSString *str1 = @"iPhone";
    NSLog(@"%@", str1);
    NSString *str2 = [[NSString alloc] init];
    NSLog(@"%@", str2);
    NSString *str3 = [[NSString alloc] initWithString:@"iPhone"];
    NSLog(@"%@", str3);
//    NSString *str4 = [[NSString alloc] initWithString:nil];
//    NSLog(@"%@", str4);  error
    NSString *str5 = [[NSString alloc] initWithFormat:@"%@5s", str1];
    NSLog(@"%@", str5);
    
//    NSString *str6 = [NSString stringWithString:@"iPhone"];
//    NSLog(@"%@", str6);
    NSString *str7 = [NSString stringWithFormat:@"123"];
    NSLog(@"%@", str7);
        //2.获取字符串长度
    NSUInteger length = [str7 length];
    NSLog(@"%lu", length);
    
        //3.判断字符串是否以指定字符串开始或结束
    NSString *str8 = @"123ABC";
    
   BOOL result = [str8 hasPrefix:@"223"];
    NSLog(@"%d", result);
    BOOL result1 = [str8 hasSuffix:@"C"];
    NSLog(@"%d", result1);
    
        //4.搜索字符串的范围
    
    NSRange range = [str8 rangeOfString:@"3A"];
    
    NSLog(@"%lu, %lu", range.length, range.location);
    
        //5.字符串截取
    NSRange rs = {3, 3};
    NSString *str9 = [str8 substringWithRange:rs];
    NSLog(@"%@", str9);
        //6.拼接字符串
   NSString *str10 = [str8 stringByAppendingString:@"123"];
    NSLog(@"%@", str8);
    NSLog(@"%@", str10);

        //7.替换字符串
   NSString *str11 = [str8 stringByReplacingOccurrencesOfString:@"3A" withString:@"A"];
    NSLog(@"%@", str11);
    
        //8.字符串比较"123" "123" "321"
    
    NSInteger flag = [@"123" compare:@"123"];
    NSLog(@"%ld", flag);
    
        //9.字符串和数值类型的转换
    NSString *str12 = @"123";
    int a = [str12 intValue];
    NSLog(@"%d", a);
    double b = [str12 doubleValue];
    NSLog(@"%f", b);
    float c = [str12 floatValue];
    NSLog(@"%f", c);
    BOOL e = [str12 boolValue];
    NSLog(@"%d", e);

//10.大小写转换操作
    NSString *aaa = @"aBcE";
    NSLog(@"%@", [aaa capitalizedString]);
    NSLog(@"%@", [aaa uppercaseString]);
    NSLog(@"%@", [aaa lowercaseString]);
    NSLog(@"%@", aaa);
    
    
        //NSMutableString
        //capctiy 是一个预估的值,可以改变
        //由于内存比较紧张,一般把capctiy设置成0,让字符串自己去判断大小,扩充容器大小
        //创建一个新的字符串
    NSMutableString *mString = [[NSMutableString alloc] initWithCapacity:0];
        //拼接字符串
    [mString appendString:@"taiyang"];
    NSLog(@"%@", mString);
    
         //插入字符
    [mString insertString:@"aaa" atIndex:1];
    NSLog(@"%@", mString);
    
        //删除字符串
//    NSRange r = {2, 3};
//    [mString deleteCharactersInRange:r];
//             ||  等价
    [mString deleteCharactersInRange

练习 :   

//    1.截取字符串“20|http://www.baidu.com”中 “|” 前面和后面的字符串，并输出。

//    NSString *str1 = @"20|http://www.baidu.com";
//         1
//    NSString *str2 = [str1 substringWithRange:NSMakeRange(0, 2)];
//    NSString *str3 = [str1 substringWithRange:NSMakeRange(3, 20)];
//    NSLog(@"str2 = %@ str3 = %@", str2, str3);

        //2
//    NSString *str2 = [str1 substringFromIndex:3];
//    NSLog(@"str2 = ‘%@‘", str2);
//    NSString *str3 = [str1 substringToIndex:2];
//    NSLog(@"str3 = ‘%@‘", str3);

//    2.将“文艺青年”改成“213青年”。
    
//    NSString *str1 = @"文艺青年";
//    NSString *str2 = [str1 stringByReplacingOccurrencesOfString:@"文艺" withString:@"213"];
//    NSLog(@"%@", str2);

        //    3.给定一个图片文件名，判断字符串中是否有后缀,如果有(如:以“.png”结尾)，就替换成“jpg”;如果没有，就拼接”.jpg”
    
    

//    NSString *str1 = @"taiyang";
//    NSUInteger length = [str1 length];
//    NSString *str2 = [str1 substringFromIndex:length - 4];
//    if ([@".jpg" compare:str2] == 0) {
//        NSLog(@"该字符串是以.jpg开头");
//    }else if ([@".png" compare:str2] == 0){
//        NSString *str3 = [str1 stringByReplacingOccurrencesOfString:@".png" withString:@".jpg"];
//        NSLog(@"str3 = ‘%@‘", str3);
//    }else{
//        NSString *str4 = [str1 stringByAppendingString:@".jpg"];
//        NSLog(@"str4 = ‘%@‘", str4);
//    }
    

NSString NSMutableString

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原文地址：http://www.cnblogs.com/tian-sun/p/4309428.html