Arduino: Convert string array to byte array

Question

I am working on a project where I first need to build up the payload as a string then later on convert it to byte when sending the payload. I am having issues with conversion.

Reson why payload build-up is in string is due to the way it generates each single string, "02", "12", "31", "03" and "36" are the result after diffrent methods of calculations.

String lockHexString[5];  
lockHexString[0] = "02"; 
lockHexString[1] = "12"; 
lockHexString[2] = "31"; 
lockHexString[3] = "03"; 
lockHexString[4] = "36"

Now when I have lockHexString, I need convert the lockHexString to byteArray, so It looks like

byte lockByte[5] = {0x02, 0x12, 0x31, 0x03, 0x36};

Does this answer your question? [Convert string of hex to char array c++](https://stackoverflow.com/questions/47204749/convert-string-of-hex-to-char-array-c) — kaylum, Jun 26 '21 at 23:36
@kaylum Those answers are useless on the system having 2kB of RAM. No std at all. — 0___________, Jun 26 '21 at 23:40
@0___________ I probably should have linked directly to the answer that that post is duplicated to. It has an answer without `std`. https://stackoverflow.com/questions/17261798/converting-a-hex-string-to-a-byte-array — kaylum, Jun 26 '21 at 23:42
no sorry, that method takes too much memory, as I only have 2kb. — Nabil Akhlaque, Jun 27 '21 at 21:30
Maybe you shouldn't use the String to create the payload to begin with. If you use `char*` strings, then `atoi()` would be a simple solution. or Maybe if you could store your string as decString instead of hexString, then at least you could easily convert it to a byte with `(uint8_t) String.toInt()`. — hcheung, Jun 28 '21 at 01:14

score 1 · Accepted Answer · answered Jun 26 '21 at 23:39

If they are always 2 chars I would not use heavy standard functions on Arduino. Simple:

unsigned getdigit(const char ch)
{
    switch(ch)
    {
        case '0' ... '9':
          return ch - '0';
        case 'a' ... 'f':
          return ch - 'a';
        case 'A' ... 'F':
          return ch - 'A';
        default:
          return 0;
    }
}
unsigned char convHEX(const char *str)
{
    return getdigit(str[0]) * 16 + getdigit(str[1]);
}

Arduino: Convert string array to byte array

1 Answers1