NSString *string = @"今天是个好日子，忘记穿秋裤了";
    
    NSString *string1 = [string substringToIndex:7];//截取掉下标7之后的字符串
    
    NSString *string2 = [string substringFromIndex:2];//截取掉下标2之前的字符串
    
//    NSString *string3 = [string substringWithRange:range];//截取范围类的字符串
    
    NSString *string4 = [string substringFromIndex:string.length- 8 ];//截取某串字符串的后8位

    NSLog(@"\n\n%@\n\n%@\n\n%@",string1,string2,string4);