I am using bitflag to enable/disable some functionality in my code. I have following enum.
typedef enum function {
function1 = 1 << 0,
function2 = 1 << 1,
function3 = 1 << 2,
function4 = 1 << 3
};
I know that I need to use following code to check which flag has been enabled.
if((flags & function1) == function1)
{
// do some action
}
In my enum, number of flags is large and to check every flag, I need to have that many "if" condition checks. Is there any way which can minimize the number of if statements required? I am new to objective C and looking to implement it in objective C. Thanks in advance.
Yes, combine the flags via a bitwise or:
if (flags & (function1 | function2 | function3 | function4))
{
// any of the flags has been set
}
Moreover, to check for a particular flag you don't need the part == function1, flags & function1 is sufficient as it will evaluate either to zero or function1.
Related
I was wondering how to write never claims that stand for all instances of a proctype. For example if I have the following proposition:
#define c (camera_node[SomePid]:start_publishing == 0)
Now if I instantiate 5 instances of the camera_node how can I create an atomic proposition checking if start_publishing is equal to zero for all of these 5 instances?
Well, it isn't the most beautiful, but I've done this sort of thing in the past. (Note: this code probably isn't proper Promela but you get the point)
#define NUMBER_OF_CAMERA_NODES 5
pid_t cameraPids [NUMBER_OF_CAMERA_NODES];
byte_t cameraPidIndex = 0
active [NUMBER_OF_CAMERA_NODES] proctype cameraTask () {
atomic { cameraPids[cameraPidIndex++] = _pid }
// ...
}
#define cameraCheck( index ) (0 == camera_node[cameraPids[(index)]]:start_publishing)
#define checkAllCameras (cameraCheck(0) && cameraCheck(1) && ...)
I am trying to implement the following typedef
typedef NS_OPTIONS (NSInteger, MyCellCorners) {
MyCellCornerTopLeft,
MyCellCornerTopRight,
MyCellCornerBottomLeft,
MyCellCornerBottomRight,
};
and correctly assign a value with
MyCellCorners cellCorners = (MyCellCornerTopLeft | MyCellCornerTopRight);
when drawing my cell, how can I check which of the options match so I can correctly draw it.
Use bit masking:
typedef NS_OPTIONS (NSInteger, MyCellCorners) {
MyCellCornerTopLeft = 1 << 0,
MyCellCornerTopRight = 1 << 1,
MyCellCornerBottomLeft = 1 << 2,
MyCellCornerBottomRight = 1 << 3,
};
MyCellCorners cellCorners = MyCellCornerTopLeft | MyCellCornerTopRight;
if (cellCorners & MyCellCornerTopLeft) {
// top left corner set
}
if (etc...) {
}
The correct way to check for this value is to first bitwise AND the values and then check for equality to the required value.
MyCellCorners cellCorners = MyCellCornerTopLeft | MyCellCornerTopRight;
if ((cellCorners & MyCellCornerTopLeft) == MyCellCornerTopLeft) {
// top left corner set
}
The following reference explains why this is correct and provides other insights into enumerated types.
Reference: checking-for-a-value-in-a-bit-mask
I agree with NSWill. I recently had a similar issue with wrong comparison.
The right if statement should be:
if ((cellCorners & MyCellCornerTopLeft) == MyCellCornerTopLeft){
I've got 5 states in my app, and I use BOOL flags to mark them. But it isn't straightforward, because I have to write 5 lines to change all flags when I want to change state.
Can you write some ideas or simple code to solve this problem?
code:
//need to choose second state
flag1 = false;
flag2 = true;
flag3 = false;
flag4 = false;
flag5 = false;
Also, it's to bad because I can choose 2 states one time.
P.S.
I found modern and more Apple-way. Answer below.
Use typedef enum to define all possible states using bitmasks.
Note this will give you a maximum of up to 64 different states (on most platforms). If you need more possible states, this solution will not work.
Handling this scheme will require you to fully understand and safely handle boolean algebra.
//define all possible states
typedef enum
{
stateOne = 1 << 0, // = 1
stateTwo = 1 << 1, // = 2
stateThree = 1 << 2, // = 4
stateFour = 1 << 3, // = 8
stateFive = 1 << 4 // = 16
} FiveStateMask;
//declare a state
FiveStateMask state;
//select single state
state = stateOne; // = 1
//select a mixture of two states
state = stateTwo | stateFive; // 16 | 2 = 18
//add a state
state |= stateOne; // 18 | 1 = 19
//remove stateTwo from our state (if set)
if ((state & stateTwo) == stateTwo)
{
state ^= stateTwo; // 19 ^ 2 = 17
}
//check for a single state (while others might also be selected)
if ((state & stateOne) == stateOne)
{
//stateOne is selected, do something
}
//check for a combination of states (while others might also be selected)
if ((state & (stateOne | stateTwo)) == stateOne | stateTwo)
{
//stateOne and stateTwo are selected, do something
}
//the previous check is a lot nicer to read when using a mask (again)
FiveStateMask checkMask = stateOne | stateTwo;
if ((state & checkMask) == checkMask)
{
//stateOne and stateTwo are selected, do something
}
You can always use a byte (unsigned char) size variable using its' bits
as flags (each bit acts as one BOOL flag).
Good instructions to set/clear/toggle/check a bit is here.
Offcourse you'd want to set kind of human readable names for this
flags, i.e.:
#define flag1 1
#define flag2 2
#define flag3 4
#define flag4 8
#define flag5 16
Nowadays we have got another option for flags. It is NS_ENUM.
typedef NS_ENUM(NSInteger, UITableViewCellStyle) {
UITableViewCellStyleDefault,
UITableViewCellStyleValue1,
UITableViewCellStyleValue2,
UITableViewCellStyleSubtitle
};
First arg for type and second for name.
I'm working on a Objective-C program where I'm getting bitfields over the network, and need to set boolean variables based on those bits.
Currently I'm representing the bitfields as int's, and then using bit shifting, similar to this (all the self properties are BOOL):
typedef enum {
deleteFlagIndex = 0,
uploadFlagIndex = 1,
moveFlagIndex = 2,
renameFlagIndex = 3
} PrivilegeFlagIndex;
int userFlag = 5; //for example
// this would be YES
self.delete = ((userFlag & (1 << deleteFlagIndex)) == (1 << deleteFlagIndex));
// this would be NO
self.upload = ((userFlag & (1 << uploadFlagIndex)) == (1 << uploadFlagIndex));
And this works (to the best of my knowledge) but I'm wondering - is there a more efficient concise way to code all the bit twiddling using a fancy trick/hack? I ask because I'll be doing this for a lot of flags (more than 30).
I did realize I could use this method this as well:
self.move = ((userFlag & (1 << moveFlagIndex)) > 0)
...which does reduce the amount of typing, but I don't know if there's a good reason to not do it that way.
Edit: Revised to say concise rather than efficient - I wasn't worried about execution performance, but rather tips and best practices for doing this in a smart way.
Try:
typedef enum {
deleteFlag = 1 << 0,
uploadFlag = 1 << 1,
moveFlag = 1 << 2,
renameFlag = 1 << 3
} PrivilegeFlags;
Now you can combine them using | directly.
Usually, it suffices to check against 0:
if (userFlag & deleteFlag) {
// delete...
}
You may want to try to use bitfields and let the compiler do the optimization itself.
I have an enum declaration using bit flags and I cant exactly figure out on how to use this.
enum
{
kWhite = 0,
kBlue = 1 << 0,
kRed = 1 << 1,
kYellow = 1 << 2,
kBrown = 1 << 3,
};
typedef char ColorType;
I suppose to store multiple colors in one colorType I should OR the bits together?
ColorType pinkColor = kWhite | kRed;
But suppose I would want to check if pinkColor contains kRed, how would I do this?
Anyone care to give me an example using the provided ColorType example ?
Yes, use bitwise OR (|) to set multiple flags:
ColorType pinkColor = kWhite | kRed;
Then use bitwise AND (&) to test if a flag is set:
if ( pinkColor & kRed )
{
// do something
}
The result of & has any bit set only if the same bit is set in both operands. Since the only bit in kRed is bit 1, the result will be 0 if the other operand doesn't have this bit set too.
If you need to get whether a particular flag is set as a BOOL rather than just testing it in an if condition immediately, compare the result of the bitwise AND to the tested bit:
BOOL hasRed = ((pinkColor & kRed) == kRed);