Bitwise operators: How do I clear the most significat bit? - objective-c

Question

I'm working on a problem where I need to convert an integer into a special text encoding. The requirements state the I pack the int into bytes and then clear the most significant bit. I am using the bitwise operator I am unsure of how to clear the most significant bit. Here is the problem and my method that I'm working with so far:
PROBLEM:
For this task, you need to write a small program including a pair of functions that can
convert an integer into a special text encoding
The Encoding Function
This function needs to accept a signed integer in the 14-bit range [-8192..+8191] and return a 4 character string.
The encoding process is as follows:
Add 8192 to the raw value, so its range is translated to [0..16383]
2.Pack that value into two bytes such that the most significant bit of each is cleared
Unencoded intermediate value (as a 16-bit integer):
00HHHHHH HLLLLLLL
Encoded value:
0HHHHHHH 0LLLLLLL
1 of 3
Format the two bytes as a single 4-character hexadecimal string and return it.
Sample values:
Unencoded (decimal) | Intermediate (decimal) | Intermediate (hex) | Encoded (hex)
0 | 8192 | 2000 | 4000
-8192 | 0 | 0000 | 0000
8191 | 16383 | 3fff | 7F7F
2048 | 10240 | 2800 | 5000
-4096 | 4096 | 1000 | 2000
My function
-(NSString *)encodeValue{
// get the input value
int decValue = [_inputValue.text intValue];
char* bytes = (char*)&decValue;
NSNumber *number = #(decValue+8192); //Add 8192 so that the number can't be negative, because we're about to lose the sign.
u_int16_t shortNumber = [number unsignedShortValue]; //Convert the integer to an unsigned short (2 bytes) using NSNumber.
shortNumber = shortNumber << 1; // !!!! This is what I'm doing to clear the MSB !!!!!!!
NSLog(#"%hu", shortNumber);
NSString *returnString = [NSString stringWithFormat:#"%x", shortNumber]; //Convert the 2 byte number to a hex string using format specifiers
return returnString;
}
I'm using the shift bitwise operator to clear the MSB and I get the correct answer for a couple of the values, but not every time.

Answer 1

If I am understanding you correctly then I believe you are after something like this:
u_int16_t number;
number = 0xFFFF;
number &= ~(1 << ((sizeof(number) * 8) - 1));
NSLog(#"%x", number); // Output will be 7fff
How it works:
sizeof(number) * 8 gives you the number of bits in the input number (eg. 16 for a u_int16_t)
1 << (number of bits in number - 1) gives you a mask with only the MSB set (eg. 0x8000)
~(mask) gives you the bitwise NOT of the mask (eg. 0x7fff)
ANDing the mask with your number then clears only the MSB leaving all others as they were

Answer 2

You are misunderstanding your task.
You are not supposed to clear the most significant bit anywhere. You have 14 bits. You are supposed to separate these 14 bits into two groups of seven bits. And since a byte has 8 bits, storing 7 bits into a byte will leave the most significant bit cleared.
PS. Why on earth are you using an NSNumber? If this is homework, I would fail you for the use of NSNumber alone, no matter what the rest of the code does.
PS. What is this char* bytes supposed to be good for?
PS. You are not clearing any most significant bit anywhere. You have an unsigned short containing 14 significant bits, so the two most significant bits are cleared. You shift the number to the left, so the most significant bit, which was always cleared, remains cleared, but the second most significant bit isn't. And all this has nothing to do with your task.