Best practice to use bit operations to set some flags - objective-c

Question

I would like to turn on/off 3 stars that represent a level of difficulty. I don't want to make usage of several if condition, would it be possible to do so by just using bitwise operation?
Let's say i have declared an enum like this:
enum
{
EASY = 0,
MODERATE,
CHALLENGING
} Difficulty;
I would like to find a bit operation that let me find which star to turn on or off:
e.g:
level 2 (challenging)
star 0 -> 1
star 1 -> 1
star 2 -> 1
level 1 (moderate)
star 0 -> 1
star 1 -> 1
star 2 -> 0
level 0 (easy)
star 0 -> 1
star 1 -> 0
star 2 -> 0

Answer 1

In the case if you want to have 3 bits to save your stars states, like instead of having three boolean flags, than you should do:
typedef enum
{
DifficultyEasy = 1 << 0,
DifficultyModerate = 1 << 1,
DifficultyChallenging = 1 << 2
} Difficulty;
Difficulty state = 0; // default
To set Easy:
state |= DifficultyEasy;
To add Challenging:
state |= DifficultyChallenging;
To reset Easy:
state &= ~DifficultyEasy;
To know is Challenging set:
BOOL isChallenging = DifficultyChallenging & state;
In the case somebody needs an explanation how it works:
1 << x means set x bit to 1 (from right);
// actually it means move 0b00000001 left by x, but I said 'set' to simplify it
1 << 5 = 0b00100000; 1 << 2 = 0b00000100; 1 << 0 = 0b00000001;
0b00001111 | 0b11000011 = 0b11001111 (0 | 0 = 0, 1 | 0 = 1, 1 | 1 = 1)
0b00001111 & 0b11000011 = 0b00000011 (0 & 0 = 0, 1 & 0 = 0, 1 & 1 = 1)
~0b00001111 = 0b11110000 (~0 = 1, ~1 = 0)

Answer 2

You would want to do something like this:
typedef enum Difficulty : NSUInteger
{
EASY = 1 << 0,
MODERATE = 1 << 1,
CHALLENGING = 1 << 2
} Difficulty;
And then to check it:
- (void) setStarsWithDifficulty:(Difficulty)diff
{
star0 = (diff & (EASY | MODERATE | CHALLENGING));
star1 = (diff & (MODERATE | CHALLENGING));
star2 = (diff & CHALLENGING);
}

Answer 3

Are you talking about something like:
star0 = 1
star1 = value & CHALLENGING || value & MODERATE
star2 = value & CHALLENGING

Answer 4

#define STAR0 1
#define STAR1 2
#define STAR2 4
#define EASY STAR0
#define MODERATE STAR1|STAR0
#define CHALLENGING STAR0|STAR1|STAR2
Detection a value d with and and compare against 0 will produce the required mapping, some of the above samples will give you the mapped value instead, take a look:
int d = EASY;
NSLog(#"Star 0 %d",(d&STAR0)!=0);
NSLog(#"Star 1 %d",(d&STAR1)!=0);
NSLog(#"Star 2 %d",(d&STAR2)!=0);
d=MODERATE;
NSLog(#"Star 0 %d",(d&STAR0)!=0);
NSLog(#"Star 1 %d",(d&STAR1)!=0);
NSLog(#"Star 2 %d",(d&STAR2)!=0);
d=CHALLENGING;
NSLog(#"Star 0 %d",(d&STAR0)!=0);
NSLog(#"Star 1 %d",(d&STAR1)!=0);
NSLog(#"Star 2 %d",(d&STAR2)!=0);