week 2 using stack in calculator
return(special character) is in edit -> special characters
enter action is no argument and any object
func enter(){
userInput = false
}
note: right click to disconnect the appendDigit connection
initialize a operandStack and its data type is double
when you click the return button, the display value wiil push into operandStack
you may use a method to transfer display.text to double by using get & set
computed property: stored property: