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  • A F# tail-recursive question

    - by ksharp
    Recently, I'm learning F#. I try to solve problem in different ways. Like this: (* [0;1;2;3;4;5;6;7;8] -> [(0,1,2);(3,4,5);(6,7,8)] *) //head-recursive let rec toTriplet_v1 list= match list with | a::b::c::t -> (a,b,c)::(toTriplet_v1 t) | _ -> [] //tail-recursive let toTriplet_v2 list= let rec loop lst acc= match lst with | a::b::c::t -> loop t ((a,b,c)::acc) | _ -> acc loop list [] //tail-recursive(???) let toTriplet_v3 list= let rec loop lst accfun= match lst with | a::b::c::t -> loop t (fun ls -> accfun ((a,b,c)::ls)) | _ -> accfun [] loop list (fun x -> x) let funs = [toTriplet_v1; toTriplet_v2; toTriplet_v3]; funs |> List.map (fun x -> x [0..8]) |> List.iteri (fun i x -> printfn "V%d : %A" (i+1) x) I thought the results of V2 and V3 should be the same. But, I get the result below: V1 : [(0, 1, 2); (3, 4, 5); (6, 7, 8)] V2 : [(6, 7, 8); (3, 4, 5); (0, 1, 2)] V3 : [(0, 1, 2); (3, 4, 5); (6, 7, 8)] Why the results of V2 and V3 are different?

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  • recursive grep started at / hangs

    - by Martin
    I have used following grep search pattern on multiple platforms: grep -r -I -D skip 'string_to_match' / For example on FreeBSD 8.0, FreeBSD 6.4 and Debian 6.0(squeeze). Command does a recursive search starting from root directory, assumes that binary files do not have the 'string_to_match' and skips devices, sockets and named pipes. FreeBSD 8.0 and FreeBSD 6.4 use GNU grep version 2.5.1 and Debian 6.0 uses GNU grep version 2.6.3. On FreeBSD 6.4, last information printed to stderr was "grep: /dev/cuad0: Device busy". After this grep just idles as according to "top -m io -o total" the I/O usage of grep is nonexistent. Same behavior is true under FreeBSD 8.0, but last information sent to stderr is "grep: /tmp/.wine-0: Permission denied" on my installation. In case of Debian, last output to stderr is "grep: /proc/sysrq-trigger: Input/output error". If I check the I/O usage of grep process under Debian, it is following: root@Debian:~# iotop -bp 22439 Total DISK READ: 0.00 B/s | Total DISK WRITE: 0.00 B/s TID PRIO USER DISK READ DISK WRITE SWAPIN IO COMMAND 22439 be/4 root 0.00 B/s 0.00 B/s 0.00 % 0.00 % grep -r -I -D skip 10.10.10.99 / Total DISK READ: 0.00 B/s | Total DISK WRITE: 0.00 B/s TID PRIO USER DISK READ DISK WRITE SWAPIN IO COMMAND 22439 be/4 root 0.00 B/s 0.00 B/s 0.00 % 0.00 % grep -r -I -D skip 10.10.10.99 / Total DISK READ: 0.00 B/s | Total DISK WRITE: 0.00 B/s TID PRIO USER DISK READ DISK WRITE SWAPIN IO COMMAND 22439 be/4 root 0.00 B/s 0.00 B/s 0.00 % 0.00 % grep -r -I -D skip 10.10.10.99 / ^Croot@Debian:~# What might cause this? Is there a way to view which file grep is currently processing in case lsof is not present? I'm able to use lsof under Debian and looks like the problematic file name there is "0xc6b2c230 file struct, ty=0, op=0xc0d34120". I'm not sure what this is.. I'm not able to use lsof or fstat under FreeBSD. PS: I know I could use find utility, but this is not the question.

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  • named responding recursive on norecurse queries

    - by Keks
    I have a server on which named is running. It is intercepted with another named server which it is not aware of. Querying the first named server results in timeouts. The server tries to resolve the query recursively. During that the firewall redirects the DNS Request from the first named server to the second one (the query from the first one is addressed to a e.g. a root server and has its "Recursion desired" bit set to 0). Despite that the second named responds to this request with a entirely or at least 1 level more resolved response than the first named server expects. So it ends up with a timeout even though it got a correct name server or even the full IP for the queried domain. In the first case the first name server tries to follow the authority domain ignoring the coresponding glue record and ends up in a loop it aborts: queried: google.com -> got from named#2: ns1.google.com -> ignore glue record and query: ns1.google.com -> got authority from named#2: google.com In the second case it ignores the answer section with the correct IP and instead tries to follow the name servers from the authority section, which ends up in the same dead end as case 1. So how can it be that the second named responds with recursive results even though the bit was explicitly set to 0 in the request from the first named?

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  • Cannot make bind9 forward DNS query to subdomain unless recursive enabled

    - by PP.
    I am trying to develop my own dynamic DNS. I'm running my own custom DNS for the subdomain on port 5353. ASCII diagram: INET --->:53 Bind 9 --->:5353 node.js | V zone_files I have example.com. The node.js DNS is for dyn.example.com. In my /etc/bind/named.conf.local I have: zone "example.com" { type master; file "/etc/bind/db.com.example"; allow-transfer { zonetxfrsafe; }; }; zone "dyn.example.com" IN { # DYNAMIC type forward; forwarders { 127.0.0.1 port 5353; }; forward only; }; I've even gone so far as to add a NS in my example.com zone file: $TTL 86400 @ IN SOA ns.example.com. hostmaster.example.com. ( 2013070104 ; Serial 7200 ; Refresh 1200 ; Retry 2419200 ; Expire 86400 ) ; Negative Cache TTL ; NS ns ; inet of our nameserver ns A 1.2.3.4 ; NS record for subdomain dyn NS ns When I attempt to get a record from the subdomain server it doesn't get forwarded: dig @127.0.0.1 test.dyn.example.com However if I turn recursive on in /etc/bind/named.conf.options: options { recursion yes; } .. then I CAN see the request going to the subdomain server. But I don't want recursion yes; in my Bind configuration as it is poor security practice (and allows all-and-sundry requests that are not related to my managed zones). How does one forward (proxy) zone queries for just one zone? Or do I give up on Bind altogether and find a DNS server that can actually forward specific queries?

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  • Why so much stack space used for each recursion?

    - by Harvey
    I have a simple recursive function RCompare() that calls a more complex function Compare() which returns before the recursive call. Each recursion level uses 248 bytes of stack space which seems like way more than it should. Here is the recursive function: void CMList::RCompare(MP n1) // RECURSIVE and Looping compare function { auto MP ne=n1->mf; while(StkAvl() && Compare(n1=ne->mb)) RCompare(n1); // Recursive call ! } StkAvl() is a simple stack space check function that compares the address of an auto variable to the value of an address near the end of the stack stored in a static variable. It seems to me that the only things added to the stack in each recursion are two pointer variables (MP is a pointer to a structure) and the stuff that one function call stores, a few saved registers, base pointer, return address, etc., all 32-bit (4 byte) values. There's no way that is 248 bytes is it? I don't no how to actually look at the stack in a meaningful way in Visual Studio 2008. Thanks

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  • Beautifulsoup recursive attribute

    - by Marcos Placona
    Hi, trying to parse an XML with Beautifulsoup, but hit a brick wall when trying to use the "recursive" attribute with findall() I have a pretty odd xml format shown below: <?xml version="1.0"?> <catalog> <book id="bk101"> <author>Gambardella, Matthew</author> <title>XML Developer's Guide</title> <genre>Computer</genre> <price>44.95</price> <publish_date>2000-10-01</publish_date> <description>An in-depth look at creating applications with XML.</description> <catalog>true</catalog> </book> <book id="bk102"> <author>Ralls, Kim</author> <title>Midnight Rain</title> <genre>Fantasy</genre> <price>5.95</price> <publish_date>2000-12-16</publish_date> <description>A former architect battles corporate zombies, an evil sorceress, and her own childhood to become queen of the world.</description> <catalog>false</catalog> </book> </catalog> As you can see, the catalog tag repeats inside the book tag, which causes an error when I try to to something like: from BeautifulSoup import BeautifulStoneSoup as BSS catalog = "catalog.xml" def open_rss(): f = open(catalog, 'r') return f.read() def rss_parser(): rss_contents = open_rss() soup = BSS(rss_contents) items = soup.findAll('catalog', recursive=False) for item in items: print item.title.string rss_parser() As you will see, on my soup.findAll I've added recursive=false, which in theory would make it no recurse through the item found, but skip to the next one. This doesn't seem to work, as I always get the following error: File "catalog.py", line 17, in rss_parser print item.title.string AttributeError: 'NoneType' object has no attribute 'string' I'm sure I'm doing something stupid here, and would appreciate if someone could give me some help on how to solve this problem. Changing the HTML structure is not an option, this this code needs to perform well as it will potentially parse a large XML file. Thanks in advance, Marcos

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  • Ordering recursive result set in SQL Server

    - by Ben
    I am having extreme difficulty constructing a query which returns an XML style hierarchy. We have a database table which contains a hierarchy of URLs for our website. The table contains the columns: ID, URL, DisplayName, ParentID, ItemOrder The parent ID forms a recursive relationship between the current item and it's parent. The item should site below it's parent in the hierarchy and it should also be ordered using the item order against items at the same level in the hierarchy. I have managed to get a recursive query working so it drills down the hierarchy sequentially but I cannot order this by the item order as well. My current query is below: WITH Parents AS ( SELECT MenuItemId, URL, ParentItemId, ItemOrder FROM CambsMenu UNION ALL SELECT si.MenuItemId, si.URL, si.ParentItemId, si.ItemOrder FROM CambsMenu si INNER JOIN Parents p ON si.ParentItemId = p.MenuItemId ) SELECT DISTINCT * FROM Parents

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  • recursive program

    - by wilson88
    I am trying to make a recursive program that calculates interest per year.It prompts the user for the startup amount (1000), the interest rate (10%)and number of years(1).(in brackets are samples) Manually I realised that the interest comes from the formula YT(1 + R)----- interest for the first year which is 1100. 2nd year YT(1 + R/2 + R2/2) //R squared 2nd year YT(1 + R/3 + R2/3 + 3R3/) // R cubed How do I write a recursive program that will calculate the interest? Below is the function which I tried double calculateInterest(double startUp, double rate, double duration) { double cpdInterest = (duration*startUp)*(1 + rate); if (duration == 0) { return cpdInterest; } else if (duration < 0) { cout << "Please enter a valid year"; } else { calculateInterest(cpdInterest,rate,duration); } return cpdInterest; }

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  • Time Complexities of recursive algorithms

    - by Peter
    Whenever I see a recursive solution, or I write recursive code for a problem, it is really difficult for me to figure out the time complexity, in most of the cases I just say its exponential? How is it exponential actually? How people say it is 2^n, when it is n!, when it is n^n or n^k. I have some questions in mind, let say find all permutations of a string (O(n!)) find all sequences which sum up to k in an array (exponential, how exactly do I calculate). Find all subsets of size k whose sum is 0 (will k come somewhere in complexity , it should come right?). Can any1 help me how to calculate the exact complexity of such questions, I am able to wrote code for them , but its hard understanding the exact time complexity.

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  • recursive program

    - by wilson88
    I am trying to make a recursive program that calculates interest per year.It prompts the user for the startup amount (1000), the interest rate (10%)and number of years(1).(in brackets are samples) Manually I realised that the interest comes from the formula YT(1 + R)----- interest for the first year which is 1100. 2nd year YT(1 + R/2 + R2/2) //R squared 2nd year YT(1 + R/3 + R2/3 + 3R3/) // R cubed How do I write a recursive program that will calculate the interest? Below is the function which I tried //Latest after editing double calculateInterest2(double start, double rate, int duration) { if (0 == duration) { return start; } else { return (1+rate) * calculateInterest2(start, rate, duration - 1); } }

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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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  • Get recursive data with sql server

    - by user228777
    I am trying to get recursive data. Following code returns all parents on the top and then the children. I would like to get data Parent 1 – his children then parent 2 - his children then parent3 – his children. How do I do this? USE Subscriber GO WITH Parent (ParentId, Id, Name,subscriberID) AS ( -- Anchor member definition SELECT A.ParentId,A.id, A.name,A.SubscriberId FROM Subscriber.Budget.SubscriberCategory AS A WHERE ParentId IS NULL UNION ALL -- Recursive member definition SELECT B.ParentId, B.id, B.name,B.SubscriberId FROM Subscriber.Budget.SubscriberCategory AS B INNER JOIN Parent AS P ON B.ParentId = P.Id ) -- Statement that executes the CTE SELECT parentId, id, name FROM Parent where subscriberID = '1C18093B-5031-42E4-9251-CEF69114365F' GO

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  • What does this recursive function do?

    - by null
    What does this function do? And how do you evaluate it? How to trace it? How to understand recursive method? I just can't understand the recursion, and the exam date is coming soon, and I know in order to understand recursion, I must first understand recursion. But I just can't write a slightly complex recursive method, can anyone help me out using the simplest English words. public class BalancedStrings { public static void printBalanced(String prefix, int a, int b) { if (a > 0) printBalanced(prefix + "a", a - 1, b); if (b > 0) printBalanced(prefix + "b", a, b - 1); if (a == 0 && b == 0) System.out.println(prefix); } public static void printBalanced(int n) { if (n % 2 == 0) { printBalanced("", n / 2, n / 2); } } public static void main(String[] args) { printBalanced(4); } }

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  • How can i optimize this recursive method

    - by Tirdyr
    Hi there. I'm trying to make a word puzzle game, and for that i'm using a recursive method to find all possible words in the given letters. The letters is in a 4x4 board. Like this: ABCD EFGH HIJK LMNO The recursive method is called inside this loop: for (int y = 0; y < width; y++) { for (int x = 0; x < height; x++) { myScabble.Search(letters, y, x, width, height, "", covered, t); } } letters is a 2D array of chars. y & x is ints that shows where in the board width & height is also int, that tells the dimensions of the board "" is the string we are trying to make (the word) covered is an array of bools, to check if we allready used that square. t is a List (wich contains all the words to check against). The recursive method that need optimizing: public void Search(char[,] letters, int y, int x, int width, int height, string build, bool[,] covered, List<aWord> tt) { // Dont get outside the bounds if (y >= width || y < 0 || x >= height || x < 0) { return; } // Dont deal with allrady covered squares if (covered[x, y]) { return; } // Get Letter char letter = letters[x, y]; // Append string pass = build + letter; // check if its a possibel word //List<aWord> t = myWords.aWord.Where(w => w.word.StartsWith(pass)).ToList(); List<aWord> t = tt.Where(w => w.word.StartsWith(pass)).ToList(); // check if the list is emphty if (t.Count < 10 && t.Count != 0) { //stop point } if (t.Count == 0) { return; } // Check if its a complete word. if (t[0].word == pass) { //check if its allrdy present in the _found dictinary if (!_found.ContainsKey(pass)) { //if not add the word to the dictionary _found.Add(pass, true); } } // Check to see if there is more than 1 more that matches string pass // ie. are there more words to find. if (t.Count > 1) { // make a copy of the covered array bool[,] cov = new bool[height, width]; for (int i = 0; i < width; i++) { for (int a = 0; a < height; a++) { cov[a, i] = covered[a, i]; } } // Set the current square as covered. cov[x, y] = true; // Continue in all 8 directions. Search(letters, y + 1, x, width, height, pass, cov, t); Search(letters, y, x + 1, width, height, pass, cov, t); Search(letters, y + 1, x + 1, width, height, pass, cov, t); Search(letters, y - 1, x, width, height, pass, cov, t); Search(letters, y, x - 1, width, height, pass, cov, t); Search(letters, y - 1, x - 1, width, height, pass, cov, t); Search(letters, y - 1, x + 1, width, height, pass, cov, t); Search(letters, y + 1, x - 1, width, height, pass, cov, t); } } The code works as i expected it to do, however it is very slow.. it takes about 2 mins to find the words. EDIT: i clarified that the letters array is 2D

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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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  • Generating scala AST for recursive method.

    - by scout
    I am generating the scala AST using the following code: val setting = new Settings(error) val reporter = new ConsoleReporter(setting, in, out) { override def displayPrompt = () } val compiler = new Global(setting, reporter) with ASTExtractor{ override def onlyPresentation = true } //setting.PhasesSetting("parser", "parserPhase") val run = new compiler.Run val sourceFiles:List[String] = List("Test.scala") run.compile(sourceFiles.toList) I guess this is the standard code used to run the compiler in the code and generate the AST to work with. The above code worked fine for any valid scala code in Test.scala till now. When I use a recursive function in Test.scala, like def xMethod(x:Int):Int = if(x == 0) -1 else xMethod(x-1) It gives me a java.lang.NullPointerException. The top few lines of the stack trace look like this at scala.tools.nsc.typechecker.Typers$Typer.checkNoDoubleDefsAndAddSynthetics$1(Typers.scala:2170) at scala.tools.nsc.typechecker.Typers$Typer.typedStats(Typers.scala:2196) at scala.tools.nsc.typechecker.Typers$Typer.typedBlock(Typers.scala:1951) at scala.tools.nsc.typechecker.Typers$Typer.typed1(Typers.scala:3815) at scala.tools.nsc.typechecker.Typers$Typer.typed(Typers.scala:4124) at scala.tools.nsc.typechecker.Typers$Typer.typed(Typers.scala:4177) at scala.tools.nsc.transform.TailCalls$TailCallElimination.transform(TailCalls.scala:199) The code works fine for a method like def aMethod(c:Int):Int = { bMethod(c) } def bMethod(x:Int):Int = aMethod(x) Please let me know if recursive functions need any other setting.

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  • Linked list recursive reverse

    - by Phoenix
    I was looking at the code below from stanford library: void recursiveReverse(struct node** head_ref) { struct node* first; struct node* rest; /* empty list */ if (*head_ref == NULL) return; /* suppose first = {1, 2, 3}, rest = {2, 3} */ first = *head_ref; rest = first->next; /* List has only one node */ if (rest == NULL) return; /* put the first element on the end of the list */ recursiveReverse(&rest); first->next->next = first; /* tricky step -- see the diagram */ first->next = NULL; /* fix the head pointer */ *head_ref = rest; } What I don't understand is in the last recursive step for e.g if list is 1-2-3-4 Now for the last recursive step first will be 1 and rest will be 2. So if you set *head_ref = rest .. that makes the head of the list 2 ?? Can someone please explain how after reversing the head of the list becomes 4 ??

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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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  • How to recursive rake? -- or suitable alternatives

    - by TerryP
    I want my projects top level Rakefile to build things using rakefiles deeper in the tree; i.e. the top level rakefile says how to build the project (big picture) and the lower level ones build a specific module (local picture). There is of course a shared set of configuration for the minute details of doing that whenever it can be shared between tasks: so it is mostly about keeping the descriptions of what needs building, as close to the sources being built. E.g. /Source/Module/code.foo and cie should be built using the instructions in /Source/Module/Rakefile; and /Rakefile understands the dependencies between modules. I don't care if it uses multiple rake processes (ala recursive make), or just creates separate build environments. Either way it should be self-containable enough to be processed by a queue: so that non-dependent modules could be built simultaneously. The problem is, how the heck do you actually do something like that with Rake!? I haven't been able to find anything meaningful on the Internet, nor in the documentation. I tried creating a new Rake::Application object and setting it up, but whatever methods I try invoking, only exceptions or "Don't know how to build task ':default'" errors get thrown. (Yes, all rakefiles have a :default). Obviously one could just execute 'rake' in a sub directory for a :modulename task, but that would ditch the options given to the top level; e.g. think of $(MAKE) and $(MAKEFLAGS). Anyone have a clue on how to properly do something like a recursive rake?

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  • "Recursive" Wildcard DNS for Amazon Route 53

    - by Brian
    The title of the question might be misleading because I'm not an expert and I'm trying to learn the proper terminology. That being said, I'm wondering if it's possible to setup a wildcard DNS record for any number of dot separation in domain names and have them all point to the network root. This is for WordPress multisite, users will have the option of choosing a mapped domain name, I want to configure my DNS so that both mysite.co.uk.network.com and mysite.com.network.com are valid (I realize this is a but ugly, but WordPress multisite requires that each site have a unique site_url and I'd prefer to preserve the period-delimited appearance if it's possible).

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