NSString

1.建议使用 [NSString stringWithxxxxx:@""];这样不用自己管理内存
2.将字符串中的字符全部变为大写:[@"Hello World" uppercaseString]; @"HELLO WORLD"
3.将字符串的的字符全部变为小写:[@"Hello World" lowercaseString];
@"hello world"
4.将字符串的首字母转换为大些:[@"Hello world" capitaliaedString];
@"Hello World"
5.判断是否相等[@"abc" isEqualToString:@"aBc"];
6.比较两个字符串:
NSComparisonResult result = [@"abc" compare:@"aBc"];
三种输出：NSOrderedDescending NSOrderedSame NSOrderedDescending
7.判断字符串是否以啥开头[@"abcedfg" hasPrefix"@"ab"];
8.判断字符串是否以啥结尾[@"abcdefg" hasSuffix:@"fg"];
9.判断字符串中是否有啥字符串[@"abcedfg" rangeOfString:@"cde"];
10.[@"abcedfg" substringFromIndex:3];//从第三个索引开始（包含第三个索）截取到最后一位
11.[@"abcdefg" substringToIndex:3];//从0开始截取到第三个索引(不包括第三个索引)
12.NSString *str = @"12.abcd.12d"; 
    NSArray *array = [str componentSeoaratedByString:@"."];
    array = [@"12", @"abcd",@"12d"];
13.类型转换
     NSLog(@"%i", [@"12" intValue]);
14.计算字符串长度
     NSLog([@"Hello World, 世界你好" length]);
15.取出指定的字符串
     NSLog([@"abc" characterAtIndex:0]);
16.OC ---> C
    const char *s =  [@"abc" UTF8String];
17.追加字符串
    [str appendString:@"abc"];
18.[str appendFormat@"我的年龄是%i"， 18]；
19.替换字符串
    NSRange range = [str rangeOfString"@"dear"];
    [str replaceCharactersInRange:range withString:@"Honey"];
20.插入字符串
    [str insertString:@"My name is Sunjian." atIndex:12];
21.删除指定字符串：
    [str deleteCharactersInRange:[str rangeOfString@"My name is Sunjian."]];