Dear Forum,
I am looking for a way to convert a string with hex characters inside into readable ASCII.
For example the hex string would be 6167653d41726b68616d and the expected output is age=Arkham
I tried some suggestions i found onlie but struggle with the dynamic size of the string as well as the conversion into one string with the message in the end.
Does anyone know a awy?
Thanks so much and have a great weekend 
Is the original data in a String or a C style string ?
Show us the code you have so far.
(in code tags)
make a minimal sketch which shows which variable types you are using and what you plan with the new value after having it as ASCII.
edit:
by the way, quite interesting to read what the forum comes up with:
In og. String format
"6167653d41726b68616d"
I am on the go atm
But can show some code but the code itself is hug
isn't a hex string already ascii?
there are 10 bytes. what data type is this? array of bytes?
look this over
output
x = 0x1a2b4
z = 6167653d41726b68616d
long x = 0x01a2b4;
byte z [] = { 0x61, 0x67, 0x65, 0x3d, 0x41, 0x72, 0x6b, 0x68, 0x61, 0x6d };
const int Zlen = sizeof(z);
char s [90];
// -----------------------------------------------------------------------------
void
setup (void)
{
Serial.begin (9600);
sprintf (s, "x = 0x%lx", x);
Serial.println (s);
int j = 0;
for (int i = 0; i < Zlen; i++, j += 2)
sprintf (& s[j], "%02x", z [i]);
s [j] = '\0';
Serial.print ("z = ");
Serial.println (s);
}
void loop (void) { }
like this you mean?
void setup() {
char hex_str[]="6167653d41726b68616d";
char char_str[(strlen(hex_str)+2)/2]={0};
int i=0,j=0;
Serial.begin(115200);
for(i=0; i < strlen(hex_str); i+=2){
char temp[3] ={0};
memcpy(temp,&hex_str[i],2);
char_str[j++]=strtol(temp, NULL, 16);
}
Serial.print("Hex string: ");
Serial.println(hex_str);
Serial.print("\nCoverted Hex string: ");
Serial.println(char_str);
}
void loop() {
// put your main code here, to run repeatedly:
}
Output:
Hex string: 6167653d41726b68616d
Coverted Hex string: age=Arkham
hope that helps....
thank you for the suggestion.
The issue is I cant get my string (got it in a variable of type String) to a char [] as it can't know the size:
Compilation error: initializer fails to determine size of 'hex_str'
i tried putting the length of the String in to it but get the error
Compilation error: array must be initialized with a brace-enclosed initializer
so I put {} around it and get error
Compilation error: cannot convert 'String' to 'char' in initialization
this is the last one char hex_str[strURI.length()]= {strURI};
did you try using c_str?
yes I did:
char convertStringURI = strURI.c_str();
char hex_str[]= convertStringURI;
Compilation error: invalid conversion from 'const char*' to 'char' [-fpermissive]
also tried
char* convertStringURI = strURI.c_str();
char hex_str[]= convertStringURI;
Compilation error: invalid conversion from 'const char*' to 'char*' [-fpermissive]
That's the point.
If YOU had posted a short demo sketch as suggested, you might have gotten an example with a String object not strings...
here's an example using c_str()
void setup() {
char *str_p;
String Str = "this is a string";
str_p = &Str.c_str()[0];
Serial.begin(115200);
Serial.println(str_p);
Serial.println(strlen(str_p));
}
void loop() {
// put your main code here, to run repeatedly:
}
I let you combine this with code in post #7! 
hope that helps...
I can do a shortened version:
String strURI ="";
strURI = "6167653d41726b68616d"; //example value as value comes from a nfc card and is written to the strURI pair by pair
really don't know what more I should add as code.
The whole code is a mess and huge as I need to clean-up the nfc card functions (reader)
do you want me to post all the code?
no.
I just wanted you to do the ground work
String hexToAscii( String hex )
{
uint16_t len = hex.length();
String ascii = "";
for ( uint16_t i = 0; i < len; i += 2 )
ascii += (char)strtol( hex.substring( i, i+2 ).c_str(), NULL, 16 );
return ascii;
}
void setup() {
Serial.begin(115200);
String strURI ="";
strURI = "6167653d41726b68616d";
String out= hexToAscii(strURI);
Serial.println(out);
}
void loop() {
// put your main code here, to run repeatedly:
}will output
age=Arkham
by the way, the function comes from another thread, you could have found if you followed the forum search.
Hi sherzaad,
I posted the code one to one in a new sketch and same error:
Compilation error: invalid conversion from 'const char*' to 'char*' [-fpermissive]
I use Wokwi and did not get any errors for the share example I did.... 
did you try MY example code from post 12?
Maybe its related to the esp32 I use? but shouldnt make a diffrence?
I tried wokwi with your code:
ifi-scan.ino: In function 'void setup()':
wifi-scan.ino:5:11: error: invalid conversion from 'const char*' to 'char*' [-fpermissive]
5 | str_p = &Str.c_str()[0];
| ^~~~~~~~~~~~~~~
| |
| const char*
Error during build: exit status 1
void setup()
{
Serial.begin(9600);
byte z [] = { 0x61, 0x67, 0x65, 0x3d, 0x41, 0x72, 0x6b, 0x68, 0x61, 0x6d };
for (int i = 0; i < sizeof z; i++)
{
Serial.print((char)z[i]);
}
}
void loop() {}
Output:
age=Arkham
ah... so ur using an ESP32....
I tested with a UNO...
fixed it for esp32! 
void setup() {
String hex_S = "6167653d41726b68616d";
const char *hex_str;
hex_str = &hex_S.c_str()[0];
char char_str[(strlen(hex_str)+2)/2]={0};
int i=0,j=0;
Serial.begin(115200);
for(i=0; i < strlen(hex_str); i+=2){
char temp[3] ={0};
memcpy(temp,&hex_str[i],2);
char_str[j++]=strtol(temp, NULL, 16);
}
Serial.print("Hex string: ");
Serial.println(hex_str);
Serial.print("\nCoverted Hex string: ");
Serial.println(char_str);
}
void loop() {
// put your main code here, to run repeatedly:
}