bitwise operations - objective-c

Question

- (void)pushDigitalJoin: (NSString*)joinNumber
{
char joinByteArray[] = {
0x05, 0x00, 0x06, 0x00, 0x00, 0x03, 0x27
};
int joinIntNumber = ([joinNumber intValue] - 1);
char *upperByte;
char *lowerByte;
NSString *decimalString = [NSString stringWithFormat:#"%i", 0xff];
*upperByte = joinIntNumber & [decimalString intValue];
*lowerByte = joinIntNumber >> 8;
joinByteArray[7]= *upperByte;
joinByteArray[8] = *lowerByte;
int i;
for (i = 0; i < sizeof(joinByteArray); i++) {
NSLog(#"joinByteArray: position-%i | value-%i",i,joinByteArray[i]);
}
}
basically i have the byte array
i need to change the last 2 bytes based on the "joinNumber"
then add those 2 bytes to the array
however i get compile errors on the joinIntNumber >> 8 and the operation above that which uses the and operator doesnt seem to work. (output always shows 39)
so how do i correctly use these bitwise operators and get my 2bytes added to the array?
CHANGES MADE TO REFLECT COMMENTS AND SHOW OUTPUT (ANSWER(:
- (void)pushDigitalJoin: (NSString*)joinNumber
{
char joinByteArray[] = {
0x05, 0x00, 0x06, 0x00, 0x00, 0x03, 0x27, 0x00, 0x00
};
int joinIntNumber = ([joinNumber intValue] - 1);
char upperByte = nil;
char lowerByte = nil;
// NSString *decimalString = [NSString stringWithFormat:#"%i", 0xff];
upperByte = joinIntNumber & 0xff;//[decimalString intValue];
lowerByte = joinIntNumber >> 8;
joinByteArray[7]= upperByte;
joinByteArray[8] = lowerByte;
int i;
for (i = 0; i < sizeof(joinByteArray); i++) {
NSLog(#"joinByteArray: position-%i | value-%x",i,joinByteArray[i]);
}
}
OUTPUT (joinnumber = 5):
2011-08-26 11:06:07.554 Cameleon[2213:40b] joinByteArray: position-0 | value-5
2011-08-26 11:06:07.555 Cameleon[2213:40b] joinByteArray: position-1 | value-0
2011-08-26 11:06:07.557 Cameleon[2213:40b] joinByteArray: position-2 | value-6
2011-08-26 11:06:07.558 Cameleon[2213:40b] joinByteArray: position-3 | value-0
2011-08-26 11:06:07.559 Cameleon[2213:40b] joinByteArray: position-4 | value-0
2011-08-26 11:06:07.561 Cameleon[2213:40b] joinByteArray: position-5 | value-3
2011-08-26 11:06:07.562 Cameleon[2213:40b] joinByteArray: position-6 | value-27
2011-08-26 11:06:07.563 Cameleon[2213:40b] joinByteArray: position-7 | value-4
2011-08-26 11:06:07.564 Cameleon[2213:40b] joinByteArray: position-8 | value-0

Answer 1

so how do i correctly use these bitwise operators and get my 2bytes added to the array?
You don't. The array is declared on the stack and has fixed size (7 bytes). If you try to add values onto the end, you'll wind up stomping on other values on the stack and probably corrupting the stack.
Unrelated, but also problematic is this:
NSString *decimalString = [NSString stringWithFormat:#"%i", 0xff];
*upperByte = joinIntNumber & [decimalString intValue];
That really doesn't make any sense... why are you creating a string from an int only to take it's intValue? It'd be better to write:
*upperByte = joinIntNumber & 0xff;
And another thing... you're declaring upperByte and lowerByte as character pointes (char*), but you don't set them to point at anything in particular. So when you try to set the characters that they point to as in the above line, you're going to end up putting the values into random places.
If you want a C array that you can modify, you should declare one that's large enough to hold any values that you're going to add, in this case:
char joinByteArray[] = {
0x05, 0x00, 0x06, 0x00, 0x00, 0x03, 0x27, 0x00, 0x00
};
You could also create it on the heap with malloc() and friends, but again you'd need to make it large enough at the outset to hold your extra values, or else grow the array as necessary with realloc() before adding your new values. In any case, don't write past the end of your array.

Answer 2

First problem: joinByteArray is only 7-elements long, so you can't assign to indices 7 and 8. Not sure what you're trying to do here. Is this array supposed to grow over time as you receive more numbers? If so, you're better off using a NSMutableArray or NSMutableData and storing that in an instance variable:
char initialBytes[] = {
0x05, 0x00, 0x06, 0x00, 0x00, 0x03, 0x27
};
NSMutableData *joinBytes = [[NSMutableData alloc] initWithBytes:initialBytes length:7];
Second problem: you declare upperByte and lowerByte as pointers, but they should just be stack-allocated variables. Or better yet, use an array for this to make appending the data easier later on:
char newBytes[2];
newBytes[0] = joinIntNumber & 0xff;
newBytes[1] = joinIntNumber >> 8;
Once you've got that, you can append to the data:
[joinBytes appendBytes:newBytes length:2];
I'm not sure what it is you're trying to do, but you may want to consider endianess (see, for example, NSSwapHostIntToBig).

Answer 3

This may be closer to what you are looking for.
- (void)pushDigitalJoin: (NSString*)joinNumber
{
//You are appending 2 more values so you need to specify
//that jointByteArray is 9
unsigned char joinByteArray[9] = {
0x05, 0x00, 0x06, 0x00, 0x00, 0x03, 0x27
};
int joinIntNumber = ([joinNumber intValue] - 1);
//Upper and lower byte do not need to be char*
//unless you want to needlessly malloc memory for them
char upperByte;
char lowerByte;
//0xff is an int (unsigned) so this is useless
//NSString *decimalString = [NSString stringWithFormat:#"%i", 0xff];
//To get upper byte you need to know the size of int
//use int32_t to specify >> 24 so you don't need to use sizeof
upperByte = joinIntNumber >> ((sizeof(joinIntNumber) - 1) * 8);
lowerByte = joinIntNumber & 0xFF;
joinByteArray[7] = upperByte;
joinByteArray[8] = lowerByte;
int i;
for (i = 0; i < sizeof(joinByteArray); i++) {
//Should log hex since you are manipulating bytes
NSLog(#"joinByteArray: position-%X | value-%X",i,joinByteArray[i]);
}
}