void setup() {
  Serial.begin(115200);
}

void loop() {
  Serial.println("Enter array size: ");
  // not a clean way but for demonstration sake
  while (Serial.available() == 0) yield(); // wait for user input
  long size = Serial.parseInt();
  while (Serial.available() != 0) Serial.read(); // empty the rest of the buffer

  Serial.print("creating an array with "); Serial.print(size); Serial.println(" entries. (no error checking!)");

  byte dynamicArray[size];

  for (int i = 0; i < size; i++) dynamicArray[i] = random(i);

  Serial.write('[');
  for (int i = 0; i < size; i++) {
    Serial.print(dynamicArray[i]); Serial.write(' ');
  }
  Serial.println("]");

}