codice:<form method="post" name="form1" id="form1"> <table align="center"> <tr valign="baseline"> <td nowrap="nowrap" align="right">Cognome:</td> <td><input type="text" name="Cognome" size="32" /></td> </tr> <tr valign="baseline"> <td nowrap="nowrap" align="right">Nome:</td> <td><input type="text" name="Nome" size="32" /></td> </tr> <tr valign="baseline"> <td nowrap="nowrap" align="right">Card:</td> <td><input type="text" name="card" size="32" /></td> </tr> <tr valign="baseline"> <td> <input name="crediti" id="crediti" placeholder="Valore da moltiplicare" onKeyup="document.querySelector('#result').value=Number(this.value)*Number(document.querySelector('#costomensa').dataset.price)"><br></td> <tr valign="baseline"> <td nowrap="nowrap" align="right">mensa:</td> <td><select name="costomensa" id="costomensa"> <option value="1" data-price="3.5">3,5 - lettera</option> <option value="2" data-price="5">5 - allega</option> <option value="3" data-price="2">2 - Sura</option> </select></td> </tr> < <input type="submit" value="Aggiorna" /> </p> <p> </p> </td> </tr> </table> </form>
Nota che nel tag select, anzi nei tags option, ho usato un data-attribute che contiene il prezzo espresso usando il punto come separatore dei decimali.