Using the element zero of an array

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4D v18

An array always has an element zero. While element zero is not shown when an array supports a form object, there is no restriction(*) in using it with the language.

One example of the use of element zero is the case of the combo box discussed in the Arrays and Form Objects section.

Here is another example: you want to execute an action only when you click on an element other than the previously selected element. To do this, you must keep track of each selected element. One way to do this is to use a process variable in which you maintain the element number of the selected element. Another way is to use the element zero of the array:

  ` atNames scrollable area object method

 Case of

    :(Form event code=On Load)

  ` Initialize the array (as shown further above)

       ARRAY TEXT(atNames;5)

  ` ...

  ` Initialize the element zero with the number

  ` of the current selected element in its string form

  ` Here you start with no selected element

       atNames{0}:="0"

 

    :(Form event code=On Unload)

  ` We no longer need the array

       CLEAR VARIABLE(atNames)

 

    :(Form event code=On Clicked)

       If(atNames#0)

          If(atNames#Num(atNames{0}))

             vtInfo:="You clicked on: "+atNames{atNames}+" and it was not selected before."

             atNames{0}:=String(atNames)

          End if

       End if

    :(Form event code=On Double Clicked)

       If(atNames#0)

          ALERT("You double clicked on: "+atNames{atNames}

       End if

 End case

(*) However, there is one exception: in an array type List Box, the zero element is used internally to store the previous value of an element being edited, so it is not possible to use it in this particular context.