Can a field name be changed with an incremented value based on a condition?

I want to replace "X" in

  var chk = this.getField("PX.myCheck.chkAmount")

using an incremented value based on a condition.

My objective is to add script to this example so that if(num2.value > "") then the field name "PX.myCheck.chkAmount" will become "P1.myCheck.chkAmount". 

Example:

var num2 = this.getField("num.chk.02");

var i = 0

if(num2.value > "")(i = i + 1; var chk = this.getField("P1.myCheck.chkAmount";chk.value = num2.value);