Multiple recursion & recursion?

Ali_100

why we need recursion, only for calling method repeatedly ? well there is a factorial problem , we can get the factorial of any number by any loop,so why we need recursion? please don't give formal reason. what is multiple recursion? any example

molesworth

waqarnaeem2@hotmail.com wrote:

why we need recursion

We need recursion for the things it kills to grow in... sorry, I went all Ah Pook for a minute there :omg: We need recursion because some things are just too messy to do with iteration. Most common recursive algorithms can easily be converted to iterative versions, but will need extra state tracking, like stacks, and then become more complex to write, read and debug. Recursion might have a slight performance penalty in some cases, but the clarity of code usually outweighs that. As the dictionary says Recursion : see Recursion :D

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Colin Angus Mackay

waqarnaeem2@hotmail.com wrote:

please don't give formal reason.

What?

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Luc Pattyn

Hi, we need recursion to model things that are inherently recursive, such as a hierarchical file system, where each folder (including the root) can contain files and folders. :)

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supercat9

Any recursive algorithm can be implemented using non-recursive code if one maintains one's own stack to hold intermediate results. In many cases, however, recursive code can be more efficient and readable. For example, if one has a tree structure which represents an expression (each either holds a value, or else an operation and up to two operands which will yield a value) one may very easily define something like (ignoring the constructor and various other things):

Class Plus
Inherits OpNode
Dim LeftOp, RightOp as OpNode
Function Eval as Double
Return LeftOp.Eval + RightOp.Eval
End Function
End Class

While there would be ways to handle such things non-recursively, they would be decidedly awkward. Recursion allows things to be handled very smoothly. If one would be worried about stack overflow, one could do something like:

Class Plus
Inherits OpNode
Dim LeftOp, RightOp as OpNode
Function Eval(MaxDepth as Integer) as Double
If MaxDepth < 0 then Throw New WhateverException("Expression too complicated")
Return LeftOp.Eval(MaxDepth-1) + RightOp.Eval(MaxDepth-1)
End Function
End Class

Still much nicer than maintaining a "to-do list" manually.

Baran M

try this [^]

Pascal Ganaye

There are a number of problems where a recursive algorithm is both simpler and faster than an loop. For example QuickSort[^] is one of the fastest sorting algorithm that have been found and it is using recursion. http://en.wikipedia.org/wiki/Tower_of_Hanoi[^] is another example where recursion is really paying. There are many more situation where recursion is needed. It makes sense each time you can solve a problem by applying the same methods on its bits. One of the typical example is a function that copy a folder somewhere else. If you were to program it without recursion you would have to write:

function copyfolder(folder)
for each file in folder
copy file
next
for each subfolder in folder
for each file in subfolder
copy file
next

for each subsubfolder in subfolder
  for each file in subsubfolder
    copy file
  next
  ... what if there is some subsubsubfolders?
next

next
end function

Using a recursive algorithm you would have less code

function copyfolder(folder)
for each file in folder
copy file
next
for each subfolder in folder
copyfolder(subfolder) ... this line is where the recursion happen
next
end function

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kanagarajm

I have wrote a program to get the file system with folders and files up to leaf nodes without using recursion. If you are interested you can send a mail to kanagarajmk@gmail.com