As the others say, you're getting an array returned. The real problem is the order you want the fields back: there's no guarantee that it's the same order as you've drawn them on the form! (and if one's blank, you can't tell which: you only know that you're only passed three fields.)

This really is a case for dynamically constructing the field names (note, that's symbolic field names, not symbolic variable names): 17cardnumber_1 through 17cardnumber_4, for example