Best way to convert string representation of hex to byte? (Working with Arduino)

Question

I'm sending in a hex representation of a byte through the serial port and I needed to convert it to a byte to use in my application. I've approached it several different ways and it looks like there's got to be an easier way to approach it than the way I have been (and I haven't yet found anything online that resolves this issue). I'm guessing there's probably a simple function that does what I want it to do without 3 or 4 different conversions.

Does anyone have any suggestions on how to approach this?

Essentially, I'm trying to take a string (inString) with value "CE" and store the equivalent in byte form (0xCE or 206) into outByte. Below is what I'm currently working with:

Original Code:

String inString = “CE”
Byte outByte;
outByte = inString.toInt();
printData("outByte: " + String(outByte),serialchan);

Output:

outByte: 0

Additional approaches:

String inString = “CE”
Byte outByte;

char tempchar[2];
String tempstring;
inString.toCharArray(tempchar, 4); // Works with 4, not with 2 for whatever reason
printData("inString: " + String(inString),serialchan);
printData("inString hex: " + String(tempchar),serialchan);

tempstring = "0x" + String(inString);
printData("inString in string hex: " + String(tempstring),serialchan);
outByte = (byte)tempstring.toInt();
printData("outByte: " + String(outByte),serialchan);

Output:

outByte: 0                                                        
inString: CE                                                   
inString hex: CE                                                
inString in string hex: 0xCE                                     
outByte: 0

I'm expecting outByte to read 206 (I'll convert it to a hex string later, I'm guessing that's pretty straight forward). Like I said, I think I'm close . . . but there just seems like there should be a simple one or two line solution to this. If you have any suggestions, please let me know. Thanks!

You can convert a C string in various number bases to an integer using strtol. For example long val = strtol(inString, NULL, 16);. Or you can use sscanf. — Weather Vane
– Weather Vane, Commented Mar 29, 2023 at 17:12
You could copy the Arduino String to a C-style buffer first with its getBytes() method? — Weather Vane
– Weather Vane, Commented Mar 29, 2023 at 17:35
Don't use such tiny strings. The one you supplied is too small, and you told the function it is twice is actual size. — Weather Vane
– Weather Vane, Commented Mar 29, 2023 at 17:46
Sorry, C strings have a nul terminator, 2 bytes is too short. Use a buffer size 4 and pass the actual size of the buffer available. If it is length 4 then you pass 4 to the getBytes. — Weather Vane
– Weather Vane, Commented Mar 29, 2023 at 17:49

dimich · Accepted Answer · 2023-03-29 21:48:12Z

2

If you don't mind about validation (i.e. input string is always valid hexadecimal ASCII string) you can use this case-insensitive almost branchless snippet:

static unsigned char nibble(char c)
{
    return c % 16 + 9*(c >> 6);
}

unsigned int hex2num(const char *s)
{
    unsigned int r = 0;
    while (*s)
        r = (r << 4) | nibble(*s++);
    return r;
}

answered Mar 29, 2023 at 21:48

dimich

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Error_404 · Accepted Answer · 2023-03-29 21:48:03Z

1

int convertDigit(char a) {
    if (a >= '0' && a <= '9') return a - '0';
    if (a >= 'A' && a <= 'F') return a + 10 - 'A';
    assert(0 && "not valid");
}

int HexToByte(char *string) {
    unsigned int length = strlen(string);
    int value = 0;
    for (int i = 0; i < length; ++i) {
        value += convert(string[i]) * pow(16, length - i - 1);
    }
    return value;
}

I wrote this relay fast but should do what you need. Pay attention that require math for the pow function but that can be easily implemented.

[EDIT] thanks to @dimich

int convertDigit(char a) {
    if (a >= '0' && a <= '9') return a - '0';
    if (a >= 'A' && a <= 'F') return a + 10 - 'A';
    assert(0 && "not valid");
}

int HexToByte(char *str) {
    int value = 0;
    while (*str) value = (value << 4) | convert(*str++);
    return value;
}

edited Mar 29, 2023 at 21:48

answered Mar 29, 2023 at 18:38

Error_404

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2 Comments

Bob Dole Over a year ago

I essentially ended up writing something along these lines based on an example I saw. I would've thought there would be an easier way to approach it but I checked everywhere, no luck. Thanks for your help!

dimich Over a year ago

while (*str) value = (value << 4) | convert(*str++). No "pow function" needed.

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