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There are two common methods for working with arrays of unknown size. One is to set the size of the array to an excessively large value, like
Dim $aData[500]
when you know there will be less than 500 elements. A better way is to use ReDim, as Brad referenced..
 Code:
Dim $aData         ; declare array var, no size
Dim $Ptr           ; pointer var

$Ptr = -1          ; set to negative first

; Time to add an element to the array, possibly in a loop
$Line = ReadLine(9)
While Not @ERROR
  $Ptr = $Ptr + 1  ; increase pointer
  ; increase the array size to hold the new data
  ReDim Preserve $aData[$Ptr]
  ; put the data from the file in the array
  $aData[$Ptf] = $Line
  ; read the next data line
  ReadLine(9)
Loop
This method is much preferred as it accurately maintains the size of the array. Of course, you might add logic to this to only add certain lines of data to the array, but the basic concept is the same

Glenn
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Actually I am a Rocket Scientist! \:D