Arduino: Convert a String hex "#FFFFFF" into 3 int

Question

I'm completely new to C/C++ and I am trying to figure out how to convert a String argument that would be in the form of a html style rgb hex such as "#ffffff" and turn that into 3 integers vars

I'm really not sure where to being.

Solution 1

All you need to do is convert the string to integers and then split them into three separate r, g, b values.

string hexstring = "#FF3Fa0";

// Get rid of '#' and convert it to integer
int number = (int) strtol( &hexstring[1], NULL, 16);

// Split them up into r, g, b values
int r = number >> 16;
int g = number >> 8 & 0xFF;
int b = number & 0xFF;

You may want to have a look at this question as well.

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Edit (thanks to James comments):

For some machine (e.g. Arduino (Uno)), ints are 16 bits instead of 32. If red values are dropping for you, use a long instead.

string hexstring = "#FF3Fa0";

// Get rid of '#' and convert it to integer
long number = strtol( &hexstring[1], NULL, 16);

// Split them up into r, g, b values
long r = number >> 16;
long g = number >> 8 & 0xFF;
long b = number & 0xFF;

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Edit (an even safer version, use strtoll instead of strtol):

long long number = strtoll( &hexstring[1], NULL, 16);

// Split them up into r, g, b values
long long r = number >> 16;
long long g = number >> 8 & 0xFF;
long long b = number & 0xFF;

Solution 2

First, you need to parse your value. You may do that this way:

void parse_hex(char* a, char* b, char* c, const char* string) {
    //certainly not the most elegant way. Note that we start at 1 because of '#'
    a[0] = string[1];
    a[1] = string[2];
    b[0] = string[3];
    b[1] = string[4];
    c[0] = string[5];
    c[1] = string[6];
}

Then, you will convert each string into it's correspondent integer. You can learn how to do that from this answer.