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PROGRAMMING POWER
Recursion: the good, the bad, and the alternatives
By Ian Murray
Sometime back, we looked at recursion as a way to solve some problems not easily solved in other ways. Unfortunately LotusScript recursion does have its limitations and in this article I will be discussing how to discover what those limitations mean for you and your code, and how you can avoid them.
To recap, recursion is where a process is designed to solve some problem by breaking the problem down to a single indivisible case and calling itself to deal with the "rest of the problem", where the "rest of the problem" is a smaller, similar problem to the original problem. This kind of effect can be seen in the natural world.
For example, with reference to an apple tree, a "tree part" is a piece of wood with either several smaller tree parts coming from one end, or with an apple or leaf attached to the end. If you set the initial tree part to be the trunk, then we have successfully described a tree -- almost. We are forgetting about the root. In fact the same process can be applied to the root, as this can be recursively described.
To find every apple on the tree, we could have some pseudo-code like:
Sub ExamineTreepart(Treepart):
If Treepart has apple then
Pick Apple
Else
For all attached treeparts
ExamineTreepart(attachedTreePart)
End forall
End if
End
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Trees are everywhere in computing: filesystems, indexing (B-Tree, in Domino's case) and pretty much any kind of system hierarchy -- so it's important to be able to deal with them, from a programming point of view.
Most agents take a linear path from the start of the program through to the end, although we may have jump out to take advantage of re-usable code such as objects and subs/functions. We may also perform loops, such as "while... end while" or "forall". This is known as iterative programming. Any iterative method can be reproduced using recursion.
For example, printing a string could be described as "print the first character, then print the rest of the string by printing the first character of the remainder, then print the rest of..." as you can see here:
Sub PrintAscii(Byval sToPrint As String)
' A "string of characters" can be described as a single character followed by nothing or a string of characters
Print Asc(Left$(sToPrint,1)) 'The first character
If Len(sToPrint) > 1 Then
PrintAscii(Mid$(SToPrint,2)) '... The rest
End If
End Sub
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