void setup() 
{
  Serial.begin( 115200);
  Serial.println( "add \"0\"   printf");
  Serial.println( "DEC HEX   DEC  HEX");
  Serial.println( "--  ----  --   ----");
}

void loop() 
{
  int key = random( 0, 25);

  // ------------------------------------------------
  // Print as decimal
  // ------------------------------------------------
  // Value is 10 or above ? No leading zero
  // Value is 9 or below ? Add leading zero
  if( key < 10)
  {
    Serial.print( "0");     // Add leading zero
  }
  Serial.print( key, DEC);

  // ------------------------------------------------
  // Print as hexadecimal
  // ------------------------------------------------
  // Value is 0x10 or above ? No leading zero
  // Value is 0x9 or below ? Add leading zero
  Serial.print( "  0x");
  if( key < 0x10)
  {
    Serial.print( "0");
  }
  Serial.print( key, HEX);


  // ------------------------------------------------
  // Print using printf() formatting
  // ------------------------------------------------
  // For the format, see: https://www.cplusplus.com/reference/cstdio/printf/
  //
  // The %02d has a minimal size of 2.
  // To be safe, allow the integer to be anything.
  // The value "-30000" uses 6 bytes, plus zero-terminator makes 7.
  // So a buffer of 7 should be enough, then I make it 10.
  char buffer[10];
  sprintf( buffer,"  %02d", key);
  Serial.print( buffer);
  sprintf( buffer,"  0x%02X", key);  // lowercase 0x, uppercase value
  Serial.print( buffer);

  Serial.println();
  delay( 500);
}