Null Values

COM methods and properties frequently return null values for which there is no direct equivalent in the APL language. Instead, the system constant ⎕NULL is used to represent a null value.

The following spreadsheet contains a number of empty cells.

Using the Excel.Application COM object, the contents of the spreadsheet can be obtained as follows:

      'EX'⎕WC'OLEClient' 'Excel.Application'
      WB←EX.Workbooks.Open 'simple.xls'

      WB.Sheets[1].UsedRange.Value2
 [Null]  [Null]  [Null]  [Null]  [Null] 
 [Null]    Year  [Null]  [Null]  [Null] 
 [Null]    1999    2000    2001    2002 
 [Null]  [Null]  [Null]  [Null]  [Null] 
 Sales      100      76     120     150 
 [Null]  [Null]  [Null]  [Null]  [Null] 
 Costs       80      60     100     110 
 [Null]  [Null]  [Null]  [Null]  [Null] 
 Margin      20      16      20      40 

To determine which of the cells are filled, you can compare the array with ⎕NULL.

      ⎕NULL≢¨WB.Sheets[1].UsedRange.Value2
0 0 0 0 0
0 1 0 0 0
0 1 1 1 1
0 0 0 0 0
1 1 1 1 1
0 0 0 0 0
1 1 1 1 1
0 0 0 0 0
1 1 1 1 1

⎕NULL should also set the values of COM properties to null.