List box, Creating columns with dynamic variables

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4D v15

Originally introduced in 4D v14 R3

A new feature in 4D allows you to add or insert new columns dynamically at runtime. With this new feature, 4D automatically handles the required variable definitions (column, footer, and header).

Consequently, three 4D commands have been updated:

LISTBOX INSERT COLUMN
LISTBOX INSERT COLUMN FORMULA
LISTBOX DUPLICATE COLUMN

New syntax (Nil pointer)

The syntax for these commands has been updated to handle dynamic variable creation.

LISTBOX INSERT COLUMN ( {* ;} object ; colPosition ; colName ; colVariable ; headerName ; headerVar {; footerName ; footerVar} )
LISTBOX INSERT COLUMN FORMULA ( {* ;} object ; colPosition ; colName ; formula ; dataType ; headerName ; headerVar {; footerName ; footerVar} )
LISTBOX DUPLICATE COLUMN ( {* ;} object ; colPosition; colName ; colVariable ; headerName ; headerVar {; footerName ; footerVar} )

Parameter	Type		Description
...
colVariable	Array, Field, Variable, Nil pointer	->	Column array name or field or variable
headerVar	Integer variable *or Nil pointer*	->	Column header variable
...
footerVar	Variable *or Nil pointer*	->	Column footer variable

These commands now accept a Nil pointer (->[]) as a value for the colVariable (when applicable), headerVar and footerVar parameters. In this case, 4D will create the required variables dynamically when the commands are executed (see the "Dynamic variables" section in the 4D Language manual for more information).

Note that header and footer variables are always created with a specific type (respectively, long integer and text). However, the column variable cannot be typed at creation because the listbox accepts different types of arrays for this variable (text array, integer array, etc.) so you must set the type of the array manually. For example:

 C_POINTER($NilPtr)

 LISTBOX INSERT COLUMN(*;"MyListBox";1;"MyNewColumn";$NilPtr;"MyNewHeader";$NilPtr)

 ColPtr:=OBJECT Get pointer(Object named;"MyNewColumn")

 ARRAY TEXT(ColPtr->;0) // Replace 0 by a positive value if you want to preallocate elements in the array

It is important to do this typing before using commands like LISTBOX INSERT ROWS to insert new elements in the array. Alternatively, it is possible to use APPEND TO ARRAY both to type the array and start inserting elements at the same time.