Objective-c – Convert hex data string to NSData in Objective C (cocoa)

cocoahexiphonensdataobjective c

fairly new iPhone developer here. Building an app to send RS232 commands to a device expecting them over a TCP/IP socket connection. I've got the comms part down, and can send ASCII commands fine. It's the hex code commands I'm having trouble with.

So lets say I have the following hex data to send (in this format):

\x1C\x02d\x00\x00\x00\xFF\x7F

How do I convert this into an NSData object, which my send method expects?

Obviously this does not work for this hex data (but does for standard ascii commands):

NSString *commandascii;
NSData *commandToSend;
commandascii = @"\x1C\x02d\x00\x00\x00\xFF\x7F";
commandToSend = [commandascii dataUsingEncoding:NSStringEncoding];

For a start, some of the \x hex codes are escape characters, and I get an "input conversion stopped…" warning when compiling in XCode. And NSStringEncoding obviously isn't right for this hex string either.

So the first problem is how to store this hex string I guess, then how to convert to NSData.

Any ideas?

Best Answer

Code for hex in NSStrings like "00 05 22 1C EA 01 00 FF". 'command' is the hex NSString.

command = [command stringByReplacingOccurrencesOfString:@" " withString:@""];
NSMutableData *commandToSend= [[NSMutableData alloc] init];
unsigned char whole_byte;
char byte_chars[3] = {'\0','\0','\0'};
for (int i = 0; i < ([command length] / 2); i++) {
    byte_chars[0] = [command characterAtIndex:i*2];
    byte_chars[1] = [command characterAtIndex:i*2+1];
    whole_byte = strtol(byte_chars, NULL, 16);
    [commandToSend appendBytes:&whole_byte length:1]; 
}
NSLog(@"%@", commandToSend);

Related Solutions

Python – Convert hex string to int in Python

Without the 0x prefix, you need to specify the base explicitly, otherwise there's no way to tell:

x = int("deadbeef", 16)

With the 0x prefix, Python can distinguish hex and decimal automatically.

>>> print(int("0xdeadbeef", 0))
3735928559
>>> print(int("10", 0))
10

(You must specify 0 as the base in order to invoke this prefix-guessing behavior; if you omit the second parameter int() will assume base-10.)

C# – How to convert a byte array to a hexadecimal string, and vice versa

You can use Convert.ToHexString starting with .NET 5.
There's also a method for the reverse operation: Convert.FromHexString.

For older versions of .NET you can either use:

public static string ByteArrayToString(byte[] ba)
{
  StringBuilder hex = new StringBuilder(ba.Length * 2);
  foreach (byte b in ba)
    hex.AppendFormat("{0:x2}", b);
  return hex.ToString();
}

or:

public static string ByteArrayToString(byte[] ba)
{
  return BitConverter.ToString(ba).Replace("-","");
}

There are even more variants of doing it, for example here.

The reverse conversion would go like this:

public static byte[] StringToByteArray(String hex)
{
  int NumberChars = hex.Length;
  byte[] bytes = new byte[NumberChars / 2];
  for (int i = 0; i < NumberChars; i += 2)
    bytes[i / 2] = Convert.ToByte(hex.Substring(i, 2), 16);
  return bytes;
}

Using Substring is the best option in combination with Convert.ToByte. See this answer for more information. If you need better performance, you must avoid Convert.ToByte before you can drop SubString.

Best Answer

Related Solutions

Python – Convert hex string to int in Python

C# – How to convert a byte array to a hexadecimal string, and vice versa

Related Topic