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• Functional Programming - Lots of emphasis on recursion, why?

  - by peakit

  I am getting introduced to Functional Programming [FP] (using Scala). One thing that is coming out from my initial learnings is that FPs rely heavily on recursion. And also it seems like, in pure FPs the only way to do iterative stuff is by writing recursive functions. And because of the heavy usage of recursion seems the next thing that FPs had to worry about were StackoverflowExceptions typically due to long winding recursive calls. This was tackled by introducing some optimizations (tail recursion related optimizations in maintenance of stackframes and @tailrec annotation from Scala v2.8 onwards) Can someone please enlighten me why recursion is so important to functional programming paradigm? Is there something in the specifications of functional programming languages which gets "violated" if we do stuff iteratively? If yes, then I am keen to know that as well. PS: Note that I am newbie to functional programming so feel free to point me to existing resources if they explain/answer my question. Also I do understand that Scala in particular provides support for doing iterative stuff as well.

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• Tail recursion in C++

  - by Phenom

  Can someone show me a simple tail-recursive function in C++? Why is tail recursion better, if it even is? What other kinds of recursion are there besides tail recursion?

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• What's a nice explanation for recursion?

  - by Gulshan

  This question is inspired by What's a nice explanation for pointers? So, what can be a nice explanation of the recursion? Update: The idea of recursion is not very common in real world. So, it seems a bit confusing to the novice programmers. Though, I guess, they become used to the concept gradually. So, what can be a nice explanation for them to grasp the idea easily? I expected some detailed answers. N.B. : Even I am going to post one answer. But to be fair, it will be few hours later.

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• Recursion VS memory allocation

  - by Vladimir Kishlaly

  Which approach is most popular in real-world examples: recursion or iteration? For example, simple tree preorder traversal with recursion: void preorderTraversal( Node root ){ if( root == null ) return; root.printValue(); preorderTraversal( root.getLeft() ); preorderTraversal( root.getRight() ); } and with iteration (using stack): Push the root node on the stack While the stack is not empty Pop a node Print its value If right child exists, push the node's right child If left child exists, push the node's left child In the first example we have recursive method calls, but in the second - new ancillary data structure. Complexity is similar in both cases - O(n). So, the main question is memory footprint requirement?

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• Performance: recursion vs. iteration in Javascript

  - by mastazi

  I have read recently some articles (e.g. http://dailyjs.com/2012/09/14/functional-programming/) about the functional aspects of Javascript and the relationship between Scheme and Javascript (the latter was influenced by the first, which is a functional language, while the O-O aspects are inherited from Self which is a prototyping-based language). However my question is more specific: I was wondering if there are metrics about the performance of recursion vs. iteration in Javascript. I know that in some languages (where by design iteration performs better) the difference is minimal because the interpreter / compiler converts recursion into iteration, however I guess that probably this is not the case of Javascript since it is, at least partially, a functional language.

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• Resources for improving your comprehension of recursion?

  - by Andrew M

  I know what recursion is (when a patten reoccurs within itself, typically a function that calls itself on one of its lines, after a breakout conditional... right?), and I can understand recursive functions if I study them closely. My problem is, when I see new examples, I'm always initially confused. If I see a loop, or a mapping, zipping, nesting, polymorphic calling, and so on, I know what's going just by looking at it. When I see recursive code, my thought process is usually 'wtf is this?' followed by 'oh it's recursive' followed by 'I guess it must work, if they say it does.' So do you have any tips/plans/resources for building up your skills in this area? Recursion is kind of a wierd concept so I'm thinking the way to tackle it may be equally wierd and inobvious.

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• XSLT multiple string replacement with recursion

  - by John Terry

  I have been attempting to perform multiple (different) string replacement with recursion and I have hit a roadblock. I have sucessfully gotten the first replacement to work, but the subsequent replacements never fire. I know this has to do with the recursion and how the with-param string is passed back into the call-template. I see my error and why the next xsl:when never fires, but I just cant seem to figure out exactly how to pass the complete modified string from the first xsl:when to the second xsl:when. Any help is greatly appreciated. <xsl:template name="replace"> <xsl:param name="string" select="." /> <xsl:choose> <xsl:when test="contains($string, '&#13;&#10;')"> <xsl:value-of select="substring-before($string, '&#13;&#10;')" /> <br/> <xsl:call-template name="replace"> <xsl:with-param name="string" select="substring-after($string, '&#13;&#10;')"/> </xsl:call-template> </xsl:when> <xsl:when test="contains($string, 'TXT')"> <xsl:value-of select="substring-before($string, '&#13;TXT')" /> <xsl:call-template name="replace"> <xsl:with-param name="string" select="substring-after($string, '&#13;')" /> </xsl:call-template> </xsl:when> <xsl:otherwise> <xsl:value-of select="$string"/> </xsl:otherwise> </xsl:choose> </xsl:template>

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• Scheme accumulative recursion with lists

  - by John Retallack

  How can I pass a list as a parameter to a function adding elements to it recursively,and have it unmodified when it comes out of recursion?

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• tail recursion vs. forward recursion

  - by REALFREE

  Can someone give me the difference between these two kinds recursions and example? specifically in ocaml. Thanks

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• Are there advantages for using recursion over iteration - other than sometimes readability and elegance?

  - by Prog

  I am about to make two assumptions. Please correct me if they're wrong: There isn't a recursive algorithm without an iterative equivalent. Iteration is always cheaper performance-wise than recursion (at least in general purpose languages such as Java, C++, Python etc.). If it's true that recursion is always more costly than iteration, and that it can always be replaced with an iterative algorithm (in languages that allow it) - than I think that the two remaining reasons to use recursion are: elegance and readability. Some algorithms are expressed more elegantly with recursion. E.g. scanning a binary tree. However apart from that, are there any reasons to use recursion over iteration? Does recursion have advantages over iteration other than sometimes elegance and readability?

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• How is this recursion properly working when it is iterated through [on hold]

  - by Rakso Zrobin

  Here is my code right now: hasht= {"A":["B", "D", "E"], "B":["C"], "C":["D", "E"], "D":["C", "E"], "E":["B"]} paths=[] def recusive(start, finish, started=true): if start==finish and !started: return start else: for i in hasht[start]: path= start+ recusive(i,finish,false) paths.append(path) print (recusive("C","C",1)) print paths # [CDC, CDEBC, CEBC] I am trying to generate a table like the one on the bottom, but am running into the problem of the string and the array not being able to concatenate. When I just return however, it returns CDC and works, however, exiting the function as return is meant to do. I am wondering how I can improve my code here to (1) make it work, (2) why my logic was faulty. For example, I understand that it generates say [DC], but I am confused as to how to go around that. perhaps index the value returned?

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• How can I best study a problem to determine whether recursion can/should be used?

  - by user10326

  In some cases, I fail to see that a problem could be solved by the divide and conquer method. To give a specific example, when studying the find max sub-array problem, my first approach is to brute force it by using a double loop to find the max subarray. When I saw the solution using the divide and conquer approach which is recursion-based, I understood it but ok. From my side, though, when I first read the problem statement, I did not think that recursion is applicable. When studying a problem, is there any technique or trick to see that a recursion based (i.e. divide and conquer) approach can be used or not?

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• Is recursion ever faster than looping?

  - by Carson Myers

  I know that recursion is sometimes a lot cleaner than looping, and I'm not asking anything about when I should use recursion over iteration, I know there are lots of questions about that already. What I'm asking is, is recursion ever faster than a loop? To me it seems like, you would always be able to refine a loop and get it to perform more quickly than a recursive function because the loop is absent constantly setting up new stack frames. I'm specifically looking for whether recursion is faster in applications where recursion is the right way to handle the data, such as in some sorting functions, in binary trees, etc.

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• Can someone describe through code a practical example of backtracking with iteration instead of recursion?

  - by chrisapotek

  Recursion makes backtracking easy as it guarantees that you won't go through the same path again. So all ramifications of your path are visited just once. I am trying to convert a backtracking tail-recursive (with accumulators) algorithm to iteration. I heard it is supposed to be easy to convert a perfectly tail-recursive algorithm to iteration. But I am stuck in the backtracking part. Can anyone provide a example through code so myself and others can visualize how backtracking is done? I would think that a STACK is not needed here because I have a perfectly tail-recursive algorithm using accumulators, but I can be wrong here.

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• Recursion Vs Loops

  - by sachin

  I am trying to do work with examples on Trees as given here: http://cslibrary.stanford.edu/110/BinaryTrees.html These examples all solve problems via recursion, I wonder if we can provide a iterative solution for each one of them, meaning, can we always be sure that a problem which can be solved by recursion will also have a iterative solution, in general. If not, what example can we give to show a problem which can be solved only by recursion/Iteration? --

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• Process arbitrarily large lists without explicit recursion or abstract list functions?

  - by Erica Xu

  This is one of the bonus questions in my assignment. The specific questions is to see the input list as a set and output all subsets of it in a list. We can only use cons, first, rest, empty?, empty, lambda, and cond. And we can only define exactly once. But after a night's thinking I don't see it possible to go through the arbitrarily long list without map or foldr. Is there a way to perform recursion or alternative of recursion with only these functions?

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• Recursion in an ASP.NET MVC view

  - by Dennis Palmer

  I have a nested data object for a set of items within categories. Each category can contain sub categories and there is no set limit to the depth of sub categories. (A file system would have a similar structure.) It looks something like this: class category { public int id; public string name; public IQueryable<category> categories; public IQueryable<item> items; } class item { public int id; public string name; } I am passing a list of categories to my view as IQueryable<category>. I want to output the categories as a set of nested unordered list (<ul>) blocks. I could nest foreach loops, but then the depth of sub categories would be limited by the number of nested foreach blocks. In WinForms, I have done similar processing using recursion to populate a TreeView, but I haven't seen any examples of using recursion within an ASPX MVC view. Can recursion be done within an ASPX view? Are there other view engines that include recursion for view output?

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• SQL server recursive query error.The maximum recursion 100 has been exhausted before statement completion

  - by ienax_ridens

  I have a recursive query that returns an error when I run it; in other databases (with more data) I have not the problem. In my case this query returns 2 colums (ID_PARENT and ID_CHILD) doing a recursion because my tree can have more than one level, bit I wanna have only "direct" parent. NOTE: I tried to put OPTION (MAXRECURSION 0) at the end of the query, but with no luck. The following query is only a part of the entire query, I tried to put OPTION only at the end of the "big query" having a continous running query, but no errors displayed. Error have in SQL Server: "The statement terminated.The maximum recursion 100 has been exhausted before statement completion" The query is the following: WITH q AS (SELECT ID_ITEM, ID_ITEM AS ID_ITEM_ANCESTOR FROM ITEMS_TABLE i JOIN ITEMS_TYPES_TABLE itt ON itt.ID_ITEM_TYPE = i.ID_ITEM_TYPE UNION ALL SELECT i.ID_ITEM, q.ID_ITEM_ANCESTOR FROM q JOIN ITEMS_TABLE i ON i.ID_ITEM_PADRE = q.ID_ITEM JOIN ITEMS_TYPES_TABLE itt ON itt.ID_ITEM_TYPE = i.ID_ITEM_TYPE) SELECT ID_ITEM AS ID_CHILD, ID_ITEM_ANCESTOR AS ID_PARENT FROM q I need a suggestion to re-write this query to avoid the error of recursion and see the data, that are few.

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• Is there any limit to recursion in lisp?

  - by Isaiah

  I enjoy using recursion whenever I can, it seems like a much more natural way to loop over something then actual loops. I was wondering if there is any limit to recursion in lisp? Like there is in python where it freaks out after like 1000 loops? Could you use it for say, a game loop? Testing it out now, simple counting recursive function. Now at 7000000! Thanks alot

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• Python recursive function error: "maximum recursion depth exceeded"

  - by user283169

  I solved Problem 10 of Project Euler with the following code, which works through brute force: def isPrime(n): for x in range(2, int(n**0.5)+1): if n % x == 0: return False return True def primeList(n): primes = [] for i in range(2,n): if isPrime(i): primes.append(i) return primes def sumPrimes(primelist): prime_sum = sum(primelist) return prime_sum print (sumPrimes(primeList(2000000))) The three functions work as follows: isPrime checks whether a number is a prime; primeList returns a list containing a set of prime numbers for a certain range with limit 'n', and; sumPrimes sums up the values of all numbers in a list. (This last function isn't needed, but I liked the clarity of it, especially for a beginner like me.) I then wrote a new function, primeListRec, which does exactly the same thing as primeList, to help me better understand recursion: def primeListRec(i, n): primes = [] #print i if (i != n): primes.extend(primeListRec(i+1,n)) if (isPrime(i)): primes.append(i) return primes return primes The above recursive function worked, but only for very small values, like '500'. The function caused my program to crash when I put in '1000'. And when I put in a value like '2000', Python gave me this: RuntimeError: maximum recursion depth exceeded. What did I do wrong with my recursive function? Or is there some specific way to avoid a recursion limit?

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• problem string recursion antlr lexer token

  - by barrozz

  How do I build a token in lexer that can handle recursion inside as this string: ${*anythink*${*anything*}*anythink*} ? thanks

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• Code an integer multiplication function using recursion (in C)

  - by Madhan

  Can any body tell how to write a multiplication function (in C) using recursion?

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• Recursion problem in C

  - by jake

  Hi there. I've been trying to solve this problem for a few days now but it seems I haven't grasped the concept of recursion,yet. I have to build a program in C (recursion is a must here but loops are allowed as well) which does the following: The user inputs 2 different strings.For example: String 1 - ABC String 2 - DE The program is supposed to print strings which are combined of the ones the user has entered. the rule is that the inner order of the letters in each string (1&2) must remain. That's the output for string1=ABC & string2=DE ": abcde abdce abdec adbce adbec adebc dabce dabec daebc deabc If anyone could give me a hand here, it would be great. Thanks guys.

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• Help needed inorder to avoid recursion

  - by Srivatsa

  Hello All I have a method, which gives me the required number of Boxes based on number of devices it can hold.Currently i have implemented this logic using recursion private uint PerformRecursiveDivision(uint m_oTotalDevices,uint m_oDevicesPerBox, ref uint BoxesRequired) { if (m_oTotalDevices< m_oDevicesPerBox) { BoxesRequired = 1; } else if ((m_oTotalDevices- m_oDevicesPerBox>= 0) && (m_oTotalDevices- m_oDevicesPerBox) < m_oDevicesPerBox) { //Terminating condition BoxesRequired++; return BoxesRequired; } else { //Call recursive function BoxesRequired++; return PerformRecursiveDivision((m_oTotalDevices- m_oDevicesPerBox), m_oDevicesPerBox, ref BoxesRequired); } return BoxesRequired; } Is there any better method to implement the same logic without using recursion. Cos this method is making my application very slow for cases when number of devices exceeds 50000. Thanks in advance for your support Constant learner

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• merge sort recursion tree height

  - by Tony

  Hello! I am learning about recursion tree's and trying to figure out how the height of the tree is log b of n where n = 2 and one has 10 elements as input size. I am working with Merge sort. The number of times the split is done is the height of the tree as far as I understood, and the number of levels in the tree is height + 1. But if you take (for merge sort) log2 of 10 you get 1, where if you draw the tree you get at least 2 times that the recursion occurs. Where have I gone wrong? (I hope I am making sense here) NOTE: I am doing a self study, this is not homework!

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