Objective-C字符串初识（上）

1.创建字符串

        // 1 直接创建字符串
        NSString * str = @"abc";
        
//       NSString * stra = [[NSString alloc] initWithString:@"def"];
        // 2 用C语言字符串创建OC字符串
 6　　　　　 char * p = "你好";

 7         NSString * strc = [[NSString alloc] initWithCString:p encoding:NSUTF8StringEncoding];

        NSLog(@"cstr %@",strc);
        // 3 格式化的方式创建字符串
        // format
        int age = 19;
        
        NSString * strf = [[NSString alloc] initWithFormat:@"%d岁上的%@",age,@"大二"];
        NSLog(@"%@",strf);

2. 字符串的拼接

    /*
     //1.已知有两个字符串NSString * str1 = @“hello” NSString * str2 =  @“world”，将两个字符串拼接到一起，生成新字符串str，中间用空格分开。
     
     @"hello world"
     */
        
        // 用printf函数打印 "hello world"
        printf("%s %s",a,b);
        NSString * str1 = @"hello";
        NSString * str2 = @"world";
        
        NSLog(@"%@ %@",str1,str2);
        
        NSString * str3 = [[NSString alloc] initWithFormat:@"%@ %@",str1,str2];
        
        NSLog(@"format: %@",str3);
        

3.获取长度以及单个字符

        // 1 获取字符串的长度
        NSString * str = @"qianfeng";
        // 获取长度
        NSUInteger length = [str length];
        NSLog(@"%lu",length);
        
        // 2 获取单个字符
        unichar c = [str characterAtIndex:7];
        // %c
        NSLog(@"%C",c);

4.获取子串

　　4.1 index截取前/后 子串

        // 截取从index开始到结尾的字符串(包括index)
        NSString * subString1 = [str substringFromIndex:2];
        NSLog(@"%@",subString1);
        // 截取从开始到index的字符串(不包括index)
        NSString * subString2 = [str substringToIndex:3];
        NSLog(@"%@",subString2);

　　4.2 从X开始截取Y长度的子串

        // www.baidu.com
        // 截取 baidu
        NSString * strB = @"www.baidu.com";
        NSRange rangeB = NSMakeRange(4, 5);
        NSString * subB = [strB substringWithRange:rangeB];
        NSLog(@"%@",subB);

　　4.2 获取子串范围

            //3.已知字符串NSString * str2 = @“I love Beijing!”;
            //找出love的范围
            
            NSString * str = @"I love Beijing!";
            NSRange range = [str rangeOfString:@"love"];
            if (range.location != NSNotFound) {
                NSLog(@"%@",NSStringFromRange(range));
            } else {
                NSLog(@"不存在");
            }

          //5.将字符串@"When I was young, I loved a girl in neighbor class."中，从young提取到girl。但是不许数。
            NSString * str = @"When I was young, I loved a girl in neighbor class.";
            
            NSRange rangeYoung = [str rangeOfString:@"young"];
            NSRange rangeGirl = [str rangeOfString:@"girl"];
            // 1234567 6-2
            NSRange range = {rangeYoung.location,rangeGirl.location-rangeYoung.location+rangeGirl.length};
            NSString * newStr = [str substringWithRange:range];
            NSLog(@"%@",newStr);    

5.字符串的判断

　　5.1 空字符的判断

　　　　nil和@""都是空字符

        NSString * str1 = @" ";
        NSString * str = nil;
        NSString * str2 = @"";
        if (str1 == nil || [str1 length] == 0) {
            NSLog(@"空字符串");
        } else {
            NSLog(@"非空");
        }

　　5.2 判断是否以指定内容开头/结尾　　

        // 2 判断字符串是否以指定的内容开头
        NSString * str3 = @"www.qianfeng.com";
        BOOL hasPre = [str3 hasPrefix:@"www"];
        // YES NO
        NSLog(@"%d",hasPre);
        if (hasPre) {
            NSLog(@"是一个网址");
        } else {
            NSLog(@"不是一个网址");
        }
        
        // qianfeng.txt
        NSString * str4 = @"qianfeng.txt";
        BOOL hasSuf = [str4 hasSuffix:@"txt"];
        if (hasSuf) {
            NSLog(@"是txt");
        } else {
            NSLog(@"不是txt");
        }

　　5.3判断字符串是否相等

        NSString * str5 = @"qianfeng";
        NSString * str6 = [[NSString alloc] initWithFormat:@"qianfeng"];
        NSString * str7 = @"qianfeng";
        NSString * str8 = [[NSString alloc] initWithFormat:@"qianfeng"];
        NSLog(@"%p %p %p",str5,str6,str7);
        // 判断地址是否相等
        if (str5 == str6) {
            NSLog(@"相等");
        } else {
            NSLog(@"不相等");
        }
        // 判断内容是否相等
        if ([str5 isEqualToString:str6]) {
            NSLog(@"相等");
        } else {
            NSLog(@"不相等");
        }
        

　　5.4 字符串大小比较

　　　　compare方法

        NSString * str9 = @"1235";
        NSString * str10 = @"124";
        // compare
        NSComparisonResult result = [str9 compare:str10];
        switch (result) {
            case NSOrderedAscending:
                NSLog(@"左边<右边");
                break;
            case NSOrderedSame:
                NSLog(@"左边=右边");
                break;
            case NSOrderedDescending:
                NSLog(@"左边>右边");
                break;
            default:
                break;