How to insert \x into NSString object - objective-c

Question

I am trying to print data to a bluetooth printer. In order to do this I need to send hex to the printer. I have been successfully doing this by sending commands such as
#"\x1b\x7b\x01"
#"\x1b\x7b\x00"
These are all static commands. I now need to take some data, a NSString, convert it to hex, and then send it to the printer. I tried several different ways, most of which from this site, but cannot get it to work.
I know this doesn't work but I would like to do the following:
for(NSUInteger i = 0; i < [str length]; i++ )
{
[hexString appendString:[NSString stringWithFormat:#"\x%#",[str characterAtIndex:i]]];
}
hexString is the string I am trying to create and return. str is the string I am trying turn into hex. For example, 11111111 or abcdefg.
I have tried a variety of things all with no luck.
Any help would be greatly appreciated!!!

Answer 1

dreamlax is right, you should use NSData. You could do something like this:
NSDictionary *hexTable = #{#"0" : #0,
#"1" : #1,
#"2" : #2,
#"3" : #3,
#"4" : #4,
#"5" : #5,
#"6" : #6,
#"7" : #7,
#"8" : #8,
#"9" : #9,
#"a" : #10,
#"b" : #11,
#"c" : #12,
#"d" : #13,
#"e" : #14,
#"f" : #15};
NSMutableData *data = [[NSMutableData alloc] init];
for(NSUInteger i = 0; i < str.length; i+=2 ) {
NSString *substring1 = [str substringWithRange:NSMakeRange(i, 1)];
NSString *substring2 = [str substringWithRange:NSMakeRange(i+1, 1)];
char valueHigh = [hexTable[substring1] charValue];
char valueLow = [hexTable[substring2] charValue];
char result = valueHigh*0x10 + valueLow;
[data appendBytes:&result length:1];
}

Answer 2

for using "\" you need to escape one "\" more in your format it should be looking like this "\\x%#"
basically \ is a escape character using in many of the languages
e.g if you use
NSLog(#"\\"); // output is one backslash
NSLog(#"\\\\"); // output is two backslashes

Answer 3

Strings are for storing text. If you want to store arbitrary bytes use NSData instead. In particular, when interfacing with external hardware, I think in general you want to avoid using NSString especially because strings are always assumed to contain text, but it appears as though your printer is expecting binary data (which you are simply only representing in hexadecimal). Things like NUL bytes (\x00 or \0) do not play nicely with NSString but they do play nicely with NSData.
const char boldOnBytes[] = { 0x1b, 0x7b, 0x01 };
NSData *boldOn = [NSData dataWithBytes:boldOnBytes length:sizeof boldOnBytes];
const char boldOffBytes[] = { 0x1b, 0x7b, 0x00 };
NSData *boldOff = [NSData dataWithBytes:boldOffBytes length:sizeof boldOffBytes];
(I have no idea what your actual codes do)
To get the bytes again, you can use for example:
// assume you have a function called sendFunction which accepts a handle to the
// printer, the bytes to send, and a length of how many bytes to send.
sendFunction(printerHandle, [boldOn bytes], [boldOn length]);