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  • Does MATLAB perform tail call optimization?

    - by Shea Levy
    I've recently learned Haskell, and am trying to carry the pure functional style over to my other code when possible. An important aspect of this is treating all variables as immutable, i.e. constants. In order to do so, many computations that would be implemented using loops in an imperative style have to be performed using recursion, which typically incurs a memory penalty due to the allocation a new stack frame for each function call. In the special case of a tail call (where the return value of a called function is immediately returned to the callee's caller), however, this penalty can be bypassed by a process called tail call optimization (in one method, this can be done by essentially replacing a call with a jmp after setting up the stack properly). Does MATLAB perform TCO by default, or is there a way to tell it to?

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  • Is there a better way to serially submit multiple AJAX requests?

    - by friedo
    I have a page with multiple forms that I submit via Ajax POSTs serially. At first, I tried using synchronous XHR requests, but this causes the browser to lock up for the duration of the request, and breaks my DOM-manipulation effects, which is unacceptable. So the pattern I ended up using is basically this: var fcount = 0; // incremented for each form to be submitted function submit_form( num ) { var fdata = { ... }; // data from form # num $.ajax( { async: true, url: '/index.cgi', data: fdata, type: 'POST', success: function() { if ( num < fcount ) { submit_form( ++num ); } } } ); } $( '#submit_form_btn' ).click( function() { submit_form( 1 ) } ); The recursion strikes me as a bit of an ugly solution to what is essentially an iterative problem. Is there a cleaner or more elegant way that this could be handled?

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  • What is the possible benefit (if any) of allowing recursive contructors?

    - by Penang
    In Java, constructors cannot be recursive. Compile time error: "recursive constructor invocation". Let's assume that we did not have this restriction. Things to keep in mind: The return type of a constructor is void. Since it is a void method you can't harness the complete power of recursion. A constructor can invoke itself (or any other constructor) using this(). But a "call to this must be first statement in constructor" We could use non local data between consecutive calls to still have some possible gain from recursive constructors. Would there be any benefit from allowing recursive constructors?

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  • Creating a program to find longest path

    - by stef89
    Hi everyone, I'm fairly new to programming and I'm having a few problems. Basically I have to make a function that will find the lengths of all the paths through a network diagram. I've been working on this for hours but I can't seem to get anywhere. Using recursion I can navigate through every path but I'm just unsure as to how I should record the lengths of the paths. Any help would be greatly appreciated. Thanks! $dependencies = array("","","","1","3","4,2"); $durations = array("5","3","4","11","9","10"); function tracePath($path,$dependencies,$durations,$returnArray=array()) { if($dependencies[$path] != "") { $currentTaskDependencies = preg_split("/[\s]*[,][\s]*/", $dependencies[$path]); for($i=0;$i<count($currentTaskDependencies);$i++) { tracePath($currentTaskDependencies[$i]-1,$dependencies,$durations,$returnArray[$i]); } } return $returnArray; } tracePath(6,$dependencies,$durations);

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  • How did this get 8?

    - by David
    Here's the code: class qual { public static int fibonacci(int n) { if (n == 0 || n == 1) { return 1; } else { return fibonacci(n-1) + fibonacci(n-2); } } public static void main(String[] arg) { System.out.println(fibonacci(5)); } } The output was 8. The output should be 8 but when I look at this I think it should be 7 ((5-1) +(5-2)). Why was the output 8? I think the reasoning behind getting 8 will make recursion maybe stop being confusing for me.

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  • Better data-structure design

    - by Tempname
    Currently in my application I have a single table that is giving me a bit of trouble. The issue at hand is I have a value object that is mapped to this table. When the data is returned to me as an array of value objects, I have to then loop through this array and begin my recursion by matching the ParentID to parent ObjectID's. The column ParentID is either null (acts a parent) or it holds the value of an ObjectID. I know there has to be a better way to create this data structure so that I do not have to do recursive loops to match ParentID's with their ObjectID's. Any help with this is greatly appreciated. Here is the table in describe form: +----------------+------------------+------+-----+---------------------+-----------------------------+ | Field | Type | Null | Key | Default | Extra | +----------------+------------------+------+-----+---------------------+-----------------------------+ | ObjectID | int(11) unsigned | NO | PRI | NULL | auto_increment | | ObjectHeight | decimal(6,2) | NO | | NULL | | | ObjectWidth | decimal(6,2) | NO | | NULL | | | ObjectX | decimal(6,2) | NO | | NULL | | | ObjectY | decimal(6,2) | NO | | NULL | | | ObjectLabel | varchar(255) | NO | | NULL | | | TemplateID | int(11) unsigned | NO | MUL | NULL | | | ObjectTypeID | int(11) unsigned | NO | MUL | NULL | | | ParentID | int(11) unsigned | YES | MUL | NULL | | | CreationDate | datetime | YES | | 0000-00-00 00:00:00 | | | LastModifyDate | timestamp | YES | | NULL | on update CURRENT_TIMESTAMP | +----------------+------------------+------+-----+---------------------+-----------------------------+e

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  • What are some practical uses of generating all permutations of a list, such as ['a', 'b', 'c'] ?

    - by Jian Lin
    I was asked by somebody in an interview for web front end job, to write a function that generates all permutation of a string, such as "abc" (or consider it ['a', 'b', 'c']). so the expected result from the function, when given ['a', 'b', 'c'], is abc acb bac bca cab cba Actually in my past 20 years of career, I have never needed to do something like that, especially when doing front end work for web programming. What are some practical use of this problem nowadays, in web programming, front end or back end, I wonder? As a side note, I kind of feel that expecting a result in 3 minutes might be "either he gets it or he doesn't", especially I was thinking of doing it by a procedural, non-recursive way at first. After the interview, I spent another 10 minutes and thought of how to do it using recursion, but expecting it to be solved within 3 minutes... may not be a good test of how qualified he is, especially for front end work.

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  • Recursive Functions for Beginners?

    - by cam
    I'm looking for a few recursive function examples, preferably ones that show increase in complexity. I understand basic recursive functions, but I'm having trouble implementing them in my code. I've never used them in my code before, and I know it doesn't always call for it, but I'd like to try. Is there a good resource with examples, and maybe challenges of some sort? Or even some simple math problems (Project Euler-style?) that I could use recursion on? For examples, I prefer C#, but anything works.

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  • Understanding linear linked list

    - by ArtWorkAD
    Hi, I have some problems understanding the linear linked list data structure. This is how I define a list element: class Node{ Object data; Node link; public Node(Object pData, Node pLink){ this.data = pData; this.link = pLink; } } To keep it simple we say that a list are linked nodes so we do not need to define a class list (recursion principle). My problem is that I am really confused in understanding how nodes are connected, more precisely the sequence of the nodes when we connect them. Node n1 = new Node(new Integer(2), null); Node n2 = new Node(new Integer(1), n1); What is link? Is it the previous or the next element? Any other suggestions to help me understanding this data structure?

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  • C: Recursive function for inverting an int

    - by Jorge
    I had this problem on an exam yesterday. I couldn't resolve it so you can imagine the result... Make a recursive function: int invertint( int num) that will receive an integer and return it but inverted, example: 321 would return as 123 I wrote this: int invertint( int num ) { int rest = num % 10; int div = num / 10; if( div == 0 ) { return( rest ); } return( rest * 10 + invert( div ) ) } Worked for 2 digits numbers but not for 3 digits or more. Since 321 would return 1 * 10 + 23 in the last stage. Thanks a lot! PS: Is there a way to understand these kind of recursion problems in a faster manner or it's up to imagination of one self?

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  • Traverse multi dimensional array recusively without using foreach

    - by ejaz
    I have an array like this and the code using foreach loop. $arr = array( array ( array( 'CAR_TIR', 'Tires', 100 ), array( 'CAR_OIL', 'Oil', 10 ), array( 'CAR_SPK', 'Spark Plugs', 4 ) ), array ( array( 'VAN_TIR', 'Tires', 120 ), array( 'VAN_OIL', 'Oil', 12 ), array( 'VAN_SPK', 'Spark Plugs', 5 ) ), array ( array( 'TRK_TIR', 'Tires', 150 ), array( 'TRK_OIL', 'Oil', 15 ), array( 'TRK_SPK', 'Spark Plugs', 6 ) ) ); function recarray($array) { foreach($array as $key=>$value) { if(is_array($value)) { RecArray($value); } else { echo "key = $key value = $value"; } } } recarray($arr); I have to traverse the array using recursion and without using foreach. I would appreciate it if any one can help me

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  • Tail-recursive merge sort in OCaml

    - by CFP
    Hello world! I’m trying to implement a tail-recursive list-sorting function in OCaml, and I’ve come up with the following code: let tailrec_merge_sort l = let split l = let rec _split source left right = match source with | [] -> (left, right) | head :: tail -> _split tail right (head :: left) in _split l [] [] in let merge l1 l2 = let rec _merge l1 l2 result = match l1, l2 with | [], [] -> result | [], h :: t | h :: t, [] -> _merge [] t (h :: result) | h1 :: t1, h2 :: t2 -> if h1 < h2 then _merge t1 l2 (h1 :: result) else _merge l1 t2 (h2 :: result) in List.rev (_merge l1 l2 []) in let rec sort = function | [] -> [] | [a] -> [a] | list -> let left, right = split list in merge (sort left) (sort right) in sort l ;; Yet it seems that it is not actually tail-recursive, since I encounter a "Stack overflow during evaluation (looping recursion?)" error. Could you please help me spot the non tail-recursive call in this code? I've searched quite a lot, without finding it. Cout it be the let binding in the sort function? Thanks a lot, CFP.

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  • Scheme Depth-first search of a graph function

    - by John Retallack
    This is a homework question,I'm trying to do a Depth-first search function in Scheme,Here's the code I've written so far: (define explore (?(node visited) (let* ([neighbors (force (cdr node))] [next (nextNode visited neighbors)] [is-visited (member? node visited)]) (cond ;if I have no unvisited neighbours print current node and go up one level [(equal? next #f) (begin (display (car node)) (display " "))] ;if current node is not visited and I have unvisited neighbors ;print current node,mark as visited and visit it's neighbors [(and (equal? is-visited #f) (not (equal? next #f))) (begin (display (car node)) (display " ") (explore next (cons node visited)))]) ;go and visit the next neighbor (if (not (equal? (nextNode (cons next visited) neighbors) #f )) (explore (nextNode (cons next visited) neighbors) (cons node visited)))))) 'node' is the current node 'visited' is a list in witch I keep track of the nodes I visited 'nextNode' is a function that returns the first unvisited neighbor if any or #f otherwise 'member?' test's if a node is in the visited list The Graph representation is using adjacent made using references to nodes with letrec so that's why I use force in 'neighbors': Eg: (letrec ([node1 (list "NY" (delay (list node2 node3)))],where node2 and node3 are defined as node1 The problem witch I'm dealing with is that my visited lists looses track of some of the nodes I visited when I come out of recursion,How can I fix this ?

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  • FIlling a Java Bean tree structure from a csv flat file

    - by Clem
    Hi, I'm currently trying to construct a list of bean classes in Java from a flat description file formatted in csv. Concretely : Here is the structure of the csv file : MES_ID;GRP_PARENT_ID;GRP_ID;ATTR_ID M1 ; ;G1 ;A1 M1 ; ;G1 ;A2 M1 ;G1 ;G2 ;A3 M1 ;G1 ;G2 ;A4 M1 ;G2 ;G3 ;A5 M1 ; ;G4 ;A6 M1 ; ;G4 ;A7 M1 ; ;G4 ;A8 M2 ; ;G1 ;A1 M2 ; ;G1 ;A2 M2 ; ;G2 ;A3 M2 ; ;G2 ;A4 It corresponds to the hierarchical data structure : M1 ---G1 ------A1 ------A2 ------G2 ---------A3 ---------A4 ---------G3 ------------A5 ------G4 ---------A7 ---------A8 M2 ---G1 ------A1 ------A2 ---G2 ------A3 ------A4 Remarks : A message M can have an infinite number of groups G and attributes A A group G can have an infinite number of attributes and an infinite number of under-groups each of them having under-groups too That beeing said, I'm trying to read this flat csv decription to store it in this structure of beans : Map<String, MBean> messages = new HashMap<String, Mbean>(); == public class MBean { private String mes_id; private Map<String, GBean> groups; } public class GBean { private String grp_id; private Map<String, ABean> attributes; private Map<String, GBean> underGroups; } public class ABean { private String attr_id; } Reading the csv file sequentially is ok and I've been investigating how to use recursion to store the description data, but couldn't find a way. Thanks in advance for any of your algorithmic ideas. I hope it will put you in the mood of thinking about this ... I've to admit that I'm out of ideas :s

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  • Regex For Finding Ctypes with Int32

    - by Stefan H
    (Hey all, I am looking for a little regex help... I am trying to find all CType(expression,Int32) s and replace them with CInt(expression) This, however, is proving quite difficult, considering there could be a nested Ctype(expression, Int32) within the regex match. Does anyone have any ideas for how to best go about doing this? Here is what I have now: Dim str As String = "CType((original.Width * CType((targetSize / CType(original.Height, Single)), Single)), Int32)" Dim exp As New Regex("CType\((.+), Int32\)") str = exp.Replace(str, "CInt($1)") But this will match the entire string and replace it. I was thinking of doing a recursive function to find the outer most match, and then work inwards, but that still presents a problem with things like CType(replaceChars(I), Int32)), Chr(CType(replacementChars(I), Int32) Any tips would be appreciated. Input returnString.Replace(Chr(CType(replaceChars(I), Int32)), Chr(CType(replacementChars(I), Int32))) Output: returnString.Replace(Chr(CInt(replaceChars(I))),Chr(CInt(replacementChars(I)))) Edit: Been working on it a little more and have a recursive function that I'm still working out the kinks in. Recursion + regex. it kinda hurts. Private Function FindReplaceCInts(ByVal strAs As String) As String System.Console.WriteLine(String.Format("Testing : {0}", strAs)) Dim exp As New Regex("CType\((.+), Int32\)") If exp.Match(strAs).Success Then For Each match As Match In exp.Matches(strAs) If exp.Match(match.Value.Substring(2)).Success Then Dim replaceT As String = match.Value.Substring(2) Dim Witht As String = FindReplaceCInts(match.Value.Substring(2)) System.Console.WriteLine(strAs.IndexOf(replaceT)) strAs.Replace(replaceT, Witht) End If Next strAs = exp.Replace(strAs, "CInt($1)") End If Return strAs End Function Cheers,

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  • Which languages support *recursive* function literals / anonymous functions?

    - by Hugh Allen
    It seems quite a few mainstream languages support function literals these days. They are also called anonymous functions, but I don't care if they have a name. The important thing is that a function literal is an expression which yields a function which hasn't already been defined elsewhere, so for example in C, &printf doesn't count. EDIT to add: if you have a genuine function literal expression <exp>, you should be able to pass it to a function f(<exp>) or immediately apply it to an argument, ie. <exp>(5). I'm curious which languages let you write function literals which are recursive. Wikipedia's "anonymous recursion" article doesn't give any programming examples. Let's use the recursive factorial function as the example. Here are the ones I know: JavaScript / ECMAScript can do it with callee: function(n){if (n<2) {return 1;} else {return n * arguments.callee(n-1);}} it's easy in languages with letrec, eg Haskell (which calls it let): let fac x = if x<2 then 1 else fac (x-1) * x in fac and there are equivalents in Lisp and Scheme. Note that the binding of fac is local to the expression, so the whole expression is in fact an anonymous function. Are there any others?

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  • Returning recursive ternary freaks out

    - by David Titarenco
    Hi, assume this following function: int binaryTree::findHeight(node *n) { if (n == NULL) { return 0; } else { return 1 + max(findHeight(n->left), findHeight(n->right)); } } Pretty standard recursive treeHeight function for a given binary search tree binaryTree. Now, I was helping a friend (he's taking an algorithms course), and I ran into some weird issue with this function that I couldn't 100% explain to him. With max being defined as max(a,b) ((a)>(b)?(a):(b)) (which happens to be the max definition in windef.h), the recursive function freaks out (it runs something like n^n times where n is the tree height). This obviously makes checking the height of a tree with 3000 elements take very, very long. However, if max is defined via templating, like std does it, everything is okay. So using std::max fixed his problem. I just want to know why. Also, why does the countLeaves function work fine, using the same programmatic recursion? int binaryTree::countLeaves(node *n) { if (n == NULL) { return 0; } else if (n->left == NULL && n->right == NULL) { return 1; } else { return countLeaves(n->left) + countLeaves(n->right); } } Is it because in returning the ternary function, the values a => countLeaves(n->left) and b => countLeaves(n->right) were recursively double called simply because they were the resultants? Thank you!

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  • PHP: Recursively get children of parent

    - by Nic Hubbard
    I have a function which gets the ids of all children of a parent from my DB. So, if I looked up id 7, it might return an array with 5, 6 and 10. What I then want to do, is recursively find the children of those returned ids, and so on, to the final depth of the children. I have tried to write a function to do this, but I am getting confused about recursion. function getChildren($parent_id) { $tree = Array(); $tree_string; if (!empty($parent_id)) { // getOneLevel() returns a one-dimentional array of child ids $tree = $this->getOneLevel($parent_id); foreach ($tree as $key => $val) { $ids = $this->getChildren($val); array_push($tree, $ids); //$tree[] = $this->getChildren($val); $tree_string .= implode(',', $tree); } return $tree_string; } else { return $tree; } }//end getChildren() After the function is run, I would like it to return a one-dimentional array of all the child ids that were found.

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  • Java: how to get all subdirs recursively?

    - by HH
    Before debugging the late-hour-out-of-bound-recursive-function: is there a command to get subdirs? giveMeSubDirs(downToPath)? // WARNING: RECURSION out of bound public HashSet<FileObject> getAllDirs(String path) { HashSet<FileObject> checkedDirs = new HashSet<FileObject>(); HashSet<FileObject> allDirs = new HashSet<FileObject>(); String startingPath = path; File fileThing = new File(path); FileObject fileObject = new FileObject(fileThing); for (FileObject dir : getDirsInDir(path)) { // SUBDIR while ( !checkedDirs.contains(dir) && !(getDirsInDir(dir.getFile().getParent()).size() == 0)) { // DO NOT CHECK TOP DIRS if any bottom dir UNCHECKED! while ( uncheckedDirsOnLevel(path, checkedDirs).size() > 0) { while (getDirsInDir(path).size() == 0 || (numberOfCheckedDirsOnLevel(path, checkedDirs)==getDirsInDir(path).size())) { allDirs.add(new FileObject(new File(path))); checkedDirs.add(new FileObject(new File(path))); if(traverseDownOneLevel(path) == startingPath ) return allDirs; //get nearer to the root path = traverseDownOneLevel(path); } path = giveAnUncheckedDir(path, checkedDirs); if ( path == "NoUnchecked.") { checkedDirs.add(new FileObject( (new File(path)).getParentFile() )); break; } } } } return allDirs; }

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  • How can I make this method more Scalalicious

    - by Neil Chambers
    I have a function that calculates the left and right node values for some collection of treeNodes given a simple node.id, node.parentId association. It's very simple and works well enough...but, well, I am wondering if there is a more idiomatic approach. Specifically is there a way to track the left/right values without using some externally tracked value but still keep the tasty recursion. /* * A tree node */ case class TreeNode(val id:String, val parentId: String){ var left: Int = 0 var right: Int = 0 } /* * a method to compute the left/right node values */ def walktree(node: TreeNode) = { /* * increment state for the inner function */ var c = 0 /* * A method to set the increment state */ def increment = { c+=1; c } // poo /* * the tasty inner method * treeNodes is a List[TreeNode] */ def walk(node: TreeNode): Unit = { node.left = increment /* * recurse on all direct descendants */ treeNodes filter( _.parentId == node.id) foreach (walk(_)) node.right = increment } walk(node) } walktree(someRootNode) Edit - The list of nodes is taken from a database. Pulling the nodes into a proper tree would take too much time. I am pulling a flat list into memory and all I have is an association via node id's as pertains to parents and children. Adding left/right node values allows me to get a snapshop of all children (and childrens children) with a single SQL query. The calculation needs to run very quickly in order to maintain data integrity should parent-child associations change (which they do very frequently). In addition to using the awesome Scala collections I've also boosted speed by using parallel processing for some pre/post filtering on the tree nodes. I wanted to find a more idiomatic way of tracking the left/right node values. After looking at the answers listed I have settled on this synthesised version: def walktree(node: TreeNode) = { def walk(node: TreeNode, counter: Int): Int = { node.left = counter node.right = treeNodes .filter( _.parentId == node.id) .foldLeft(counter+1) { (counter, curnode) => walk(curnode, counter) + 1 } node.right } walk(node,1) }

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  • Convert a nested array into a flat array with PHP

    - by Ben Fransen
    Hello all, I'm trying to create a generic database mapping class with PHP. Collecting the data through my functions is going well, but as expected I'm retrieving a nested set. A print_r of my received array looks like: Array ( [table] => Session [columns] => Array ( [0] => `Session`.`ID` AS `Session_ID` [1] => `Session`.`User` AS `Session_User` [2] => `Session`.`SessionID` AS `Session_SessionID` [3] => `Session`.`ExpiresAt` AS `Session_ExpiresAt` [4] => `Session`.`CreatedAt` AS `Session_CreatedAt` [5] => `Session`.`LastActivity` AS `Session_LastActivity` [6] => `Session`.`ClientIP` AS `Session_ClientIP` ) [0] => Array ( [table] => User [columns] => Array ( [0] => `User`.`ID` AS `User_ID` [1] => `User`.`UserName` AS `User_UserName` [2] => `User`.`Password` AS `User_Password` [3] => `User`.`FullName` AS `User_FullName` [4] => `User`.`Address` AS `User_Address` ) [0] => Array ( [table] => Address [columns] => Array ( [0] => `Address`.`ID` AS `Address_ID` [1] => `Address`.`UserID` AS `Address_UserID` [2] => `Address`.`Street` AS `Address_Street` [3] => `Address`.`City` AS `Address_City` ) ) ) ) To simplify things I want to recreate this nested array to a flat array so I can easily loop through it and use the 'columns' key to create my SELECT query. I'm kinda struggling with this for a while now and figures, maybe some users at SO can help me out here. I've tried multiple things with recursion, all without luck so far... Any help is much appriciated! Thanks in advance, Ben Fransen

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  • Traversing through an arbitrary dictionary tree structure in C#

    - by Rudism
    I am trying to write a recursive C# function that will operate on generic dictionaries of the form IDictionary<string, T> where T is either another IDictionary<string, T> or a string. My first failed attempt looked something like this: public string HandleDict(IDictionary<string, string> dict){ // handle the leaf-node here } public string HandleDict<T>(IDictionary<string, IDictionary<string, T>> dict){ // loop through children foreach(string key in dict.Keys){ HandleDict(dict[key]); } } I also tried variants of HandleDict<T>(IDictionary<string, T>) where T : IDictionary<string, T> but that also doesn't quite work. I know that what I want to do could be achieved through a custom class as the parameter instead of dictionaries, and that my attempts are conceptually flawed (there's no end to the recursion in the generic function). I would still like to know if there is actually a way to do what I want here using just generic IDictionaries.

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  • Stop an executing recursive javascript function

    - by DA
    Using jQuery, I've build an image/slide rotator. The basic setup is (in pseudocode): function setupUpSlide(SlideToStartWith){ var thisSlide = SlideToStartWith; ...set things up... fadeInSlide(thisSlide) } function fadeInSlide(thisSlide){ ...fade in this slide... fadeOutSlide(thisSlide) } function fadeOutSlide(thisSlide){ ...fade out this slide... setupUpSlide(nextSlide) } I call the first function and pass in a particular slide index, and then it does its thing calling chain of functions which then, in turn, calls the first function again passing in the next index. This then repeats infinitely (resetting the index when it gets to the last item). This works just fine. What I want to do now is allow someone to over-ride the slide show by being able to click on a particular slide number. Therefore, if slide #8 is showing and I click #3, I want the recursion to stop and then call the initial function passing in slide #3, which then, in turn, will start the process again. But I'm not sure how to go about that. How does one properly 'break' a recursive script. Should I create some sort of global 'watch' variable that if at any time is 'true' will return: false and allow the new function to execute?

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  • Java function changing input

    - by Doodle
    I would like to go from one number to another. For example if I started at 6 and my goal was 10 I want a function that on every pass would bring me from 6 towards 10 or if I had the number 14 and my goal was 9 it would count down from 14 towards 9.So far I have (this is written in Processing a Java Api but there is essentially no difference from regualr Java, draw is just a continuous loop) int x=100; void draw(){ x=towards(x,10); println(x); } int towards(int current ,int target){ if(current!=target){ if (current <target){ current=current+1; } else { current=current-1; } } return current; } this gives me the results I would like but I would like to have everything in side of the towards() function. When I replace X with a variable it of course resets it self to the static variable. To sum it up how can I pass a variable to a function and have that variable thats been passed change on every subsequent pass. I have looked into recursion as a solution but that of just brings me to a final solution. I can pass the count to an array but wouldn't like to do that either.

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  • Should not a tail-recursive function also be faster?

    - by Balint Erdi
    I have the following Clojure code to calculate a number with a certain "factorable" property. (what exactly the code does is secondary). (defn factor-9 ([] (let [digits (take 9 (iterate #(inc %) 1)) nums (map (fn [x] ,(Integer. (apply str x))) (permutations digits))] (some (fn [x] (and (factor-9 x) x)) nums))) ([n] (or (= 1 (count (str n))) (and (divisible-by-length n) (factor-9 (quot n 10)))))) Now, I'm into TCO and realize that Clojure can only provide tail-recursion if explicitly told so using the recur keyword. So I've rewritten the code to do that (replacing factor-9 with recur being the only difference): (defn factor-9 ([] (let [digits (take 9 (iterate #(inc %) 1)) nums (map (fn [x] ,(Integer. (apply str x))) (permutations digits))] (some (fn [x] (and (factor-9 x) x)) nums))) ([n] (or (= 1 (count (str n))) (and (divisible-by-length n) (recur (quot n 10)))))) To my knowledge, TCO has a double benefit. The first one is that it does not use the stack as heavily as a non tail-recursive call and thus does not blow it on larger recursions. The second, I think is that consequently it's faster since it can be converted to a loop. Now, I've made a very rough benchmark and have not seen any difference between the two implementations although. Am I wrong in my second assumption or does this have something to do with running on the JVM (which does not have automatic TCO) and recur using a trick to achieve it? Thank you.

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