Edit char[] with variable data

[sleve]

Hello everyone,

I'm struggling with the following problem and I can't figure how to solve it by myself.

I receive volume level and I need to send this information. I need to edit char command[] with my volume variable.

I send TCP data with the following code.

        char command[] = "\x53\x25\x81\x00\x1A";
        client.write((uint8_t *)command, sizeof(command));

This code is working well.

An example to illustrate my problem :

volume = 50%, converted to HEX = 0x32. I need to replace the part x00 with x32, is it possible ?

        char command[] = "\x53\x25\x81\x32\x1A";
        client.write((uint8_t *)command, sizeof(command));

Thank you !

[UKHeliBob]

Approach your requirement from a different angle. Instead of editing the command string consider building it from the data that you need

char command[30];

void setup()
{
  Serial.begin(115200);
  char volume[] = "32";
  sprintf(command, "\\x53\\x25\\x81\\%s\\x1A", volume);
  Serial.println(command);
}
void loop()
{
}

The reason for the double backslashes is that they have a special meaning when used singly so need to be used this way to indicate that in this use case you really do want a backslash

If the volume is actually an integer you don't need to convert it to a string first

char command[30];

void setup()
{
  Serial.begin(115200);
  int volume = 32;
  sprintf(command, "\\x53\\x25\\x81\\x%d\\x1A", volume);
  Serial.println(command);
}
void loop()
{
}

Try these 2 examples and investigate the sprintf() function some more. If required you can use multiple variables in the same command to build the required string

[J-M-L]

0x32 in Hex is 50 in decimal. so seems you just want to write the volume value in the right byte
if you have

char command[] = "\X53\X25\X81\X00\X1A";

then just do command[3] = volume; // assuming volume is a byte whose value is 50

if you want to see what's in the buffer, do this

void setup() {
  Serial.begin(115200);
  char command[] = "\x53\x25\x81\x00\x1A";
  for (byte c: command) Serial.println(c, HEX);
}

void loop() {}

[J-M-L]

@UKHeliBob

Instead of editing the command string

By the look of it, it's not a really a string, it's actually a byte buffer. I would write it like that instead of using the \Xnn escape sequence notation of a cString.

byte command[]  = {0x53, 0x25, 0x81, 0x00, 0x1A, 0x00};// do we need the trailing 0x00 ?

[johnwasser]

@sleve

        char command[] = "\x53\x25\x81\x00\x1A";
        client.write((uint8_t *)command, sizeof(command));

you are sending six bytes:
byte command[6] = {0x53, 0x25, 0x81, 0x00, 0x1A, 0x00};
Did you want to send five bytes? That would be:
byte command[5] = {0x53, 0x25, 0x81, 0x00, 0x1A};

In either case, use something like this to change the value of the byte containing 0x00:

  byte precentVolume = 50;
  command[3] = precentVolume;
  client.write(command, sizeof command);

[sleve]

Thank you ! I've used your solution, problem solved !

[J-M-L]

Good, have fun!