optimize the conversion function (string to byte) - arduino

Question

Hy community, I'm writing a program with arduino. I have write a function that convert string in byte but it it occupies so much memory. My question is if there is a function that I can use and it does not occupy so much memory on my arduino.
I post the code below.
byte strConv(String str)
{
byte a = 0;
char n = 2;
char g = 3;
char stringa1;
char stringa2;
char m;
stringa1 = str.charAt(n);
stringa2 = str.charAt(g);
m = costanti (stringa1);
if (m >= '0' && m <= '9' || m >= 'A' && m <= 'F') {
a += m;
} else {
a += m;
}
a = a << 4;
m = 0;
m = costanti (stringa2);
if (m >= '0' && m <= '9' || m >= 'A' && m <= 'F') {
a += m;
} else {
a += m;
}
return a;
}
and the second function:
char costanti(char n) {
char num;
switch (n) {
case '0':
num = 0;
break;
case '1':
num = 1;
break;
case '2':
num = 2;
break;
case '3':
num = 3;
break;
case '4':
num = 4;
break;
case '5':
num = 5;
break;
case '6':
num = 6;
break;
case '7':
num = 7;
break;
case '8':
num = 8;
break;
case '9':
num = 9;
break;
case 'A':
num = 10;
break;
case 'B':
num = 11;
break;
case 'C':
num = 12;
break;
case 'D':
num = 13;
break;
case 'E':
num = 14;
break;
case 'F':
num = 15;
break;
}
return num;
}
Can I reduce this two functions in a new optimized function?

Answer 1

As far as I understand you want something like this:
char strConv(String str)
{
if(str.length() > 2)
return -1; // return something which makes sense to you for "invalid"
return static_cast<char>(strtol(str.c_str(), NULL, 16));
}
String myString(26, HEX); // creates the string "1A" for you
void setup() {
Serial.begin(9600);
}
void loop() {
Serial.println("String: \""+myString+'"');
char val = strConv(myString); //gives you the 26 again
Serial.print("int: ");
Serial.println(static_cast<int>(val));
delay(1000);
}
This prompts me the following:
String: "1a"
int: 26
String: "1a"
int: 26
...