NumericValueOfAByte

Okay, I want something simple, but yet, I can’t find the appropriate method to do it.

I have a NSData with data in it, several thousands of hexadecimal values. I want to scan through the NSData and get something from every byte.

For the first half of the byte (sometimes called nibble), I want it’s string value. For example : 6 becomes “6” and A becomes “a”… For the other half, I want it’s decimal value. For example : 6 gives me 6 and F gives me 15…

Any hint ?

– Trax

For starters play with this:

unsigned char byte = 0xff;
unsigned char fourMostSignificantBits = byte >> 4;
unsigned char fourLeastSignificantBits = byte & 0x0f;

printf("4msb 0x%02x 4lsb 0x%02x\n", fourMostSignificantBits, fourLeastSignificantBits);
printf("4msb %i 4lsb %i\n", fourMostSignificantBits, fourLeastSignificantBits);
printf("4msb <%c> 4lsb <%c>\n", fourMostSignificantBits, fourLeastSignificantBits);

After you feel comfortable with these operations, you can move to pointers:

unsigned char *bytes = (unsigned char *)[dataObject bytes];
int i;
int length = ([dataObject length] < 100) ? [dataObject length] : 100;
for (i = 0; i < length; i++) {

    unsigned char fmsb = bytes[i] >> 4;    // fmsb - four most significant bits
    unsigned char flsb = bytes[i] & 0x0f;  // flsb - four least significant bits
    unsigned char fmsbASCI = fmsb + ((fmsb > 9) ? 87 : 48);
    unsigned char flsbASCI = flsb + ((flsb > 9) ? 87 : 48);
    
    NSLog(@"fmsb: hex: %x int: %2i asci: %c     flsb: hex: %x int: %2i asci: %c", fmsb, fmsb, fmsbASCI, flsb,  flsb, flsbASCI);
    
}

This is basic bit manipulation. Any good C book should cover shifting and masking bits. When you say a string value do you mean the asci value? When you say 6 do you mean 0110? When you say A do you mean 1010?

I agree, this is a good time to use straight up c.

CocoaDev