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Reversible numerical calculations in Prolog

- by user8472

While reading SICP I came across logic programming chapter 4.4. Then I started looking into the Prolog programming language and tried to understand some simple assignments in Prolog. I found that Prolog seems to have troubles with numerical calculations. Here is the computation of a factorial in standard Prolog: f(0, 1). f(A, B) :- A > 0, C is A-1, f(C, D), B is A*D. The issues I find is that I need to introduce two auxiliary variables (C and D), a new syntax (is) and that the problem is non-reversible (i.e., f(5,X) works as expected, but f(X,120) does not). Naively, I expect that at the very least C is A-1, f(C, D) above may be replaced by f(A-1,D), but even that does not work. My question is: Why do I need to do this extra "stuff" in numerical calculations but not in other queries? I do understand (and SICP is quite clear about it) that in general information on "what to do" is insufficient to answer the question of "how to do it". So the declarative knowledge in (at least some) math problems is insufficient to actually solve these problems. But that begs the next question: How does this extra "stuff" in Prolog help me to restrict the formulation to just those problems where "what to do" is sufficient to answer "how to do it"?

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Is the size of a struct required to be an exact multiple of the alignment of that struct?

- by Steve314

Once again, I'm questioning a longstanding belief. Until today, I believed that the alignment of the following struct would normally be 4 and the size would normally be 5... struct example { int m_Assume_32_Bits; char m_Assume_8_Bit_Bytes; }; Because of this assumption, I have data structure code that uses offsetof to determine the distance in bytes between two adjacent items in an array. Today, I spotted some old code that was using sizeof where it shouldn't, couldn't understand why I hadn't had bugs from it, coded up a unit test - and the test surprised me by passing. A bit of investigation showed that the sizeof the type I used for the test (similar to the struct above) was an exact multiple of the alignment - ie 8 bytes. It had padding after the final member. Here is an example of why I never expected this... struct example2 { example m_Example; char m_Why_Cant_This_Be_At_Offset_6_Bytes; }; A bit of Googling showed examples that make it clear that this padding after the final member is allowed - for example http://en.wikipedia.org/wiki/Data_structure_alignment#Data_structure_padding (the "or at the end of the structure" bit). This is a bit embarrassing, as I recently posted this comment - Use of struct padding (my first comment to that answer). What I can't seem to determine is whether this padding to an exact multiple of the alignment is guaranteed by the C++ standard, or whether it is just something that is permitted and that some (but maybe not all) compilers do. So - is the size of a struct required to be an exact multiple of the alignment of that struct according to the C++ standard? If the C standard makes different guarantees, I'm interested in that too, but the focus is on C++.

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Spring security - same page to deliver different content based on user role

- by Ramesh

Hello, i tried to search for any previous post related to my issue but couldnt find any. I have a scenario where in page handles 3 different scenarios and one of them not working. This page returns different content depending on if the user is authenticated or anonymous. localhost:8080/myApp/muUrl?test=authenticatedContent - used for Scenario 1 & 2 localhost:8080/myApp/muUrl?test=anonymousContent - used for Scenario 3 Scenario: 1) Authenticated user accesing the page url - the user gets displayed correct information. Works fine 2) Anonymous user accesing page URL with parameters that requires authentication - If anonymous, there is second level of check on the content they are accessing. for example, based on the GET parameters, there is custom logic to determine if the user has to be authenticated. In which case the page gets redirected to login page (WORKS fine). 3) Anonymous user accessing page URL with parameters that doesnt need authentication - in this case i get the SAvedRequest and redirect to the URL which is taking me to an infinite loop. Am i missing something very obvious or is there a way in AuthenticationProcessFilterEntryPoint to say "DON'T redirect to LOGIN page but process it" ? thanks.

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Parsing custom time format with SimpleDateFormat

- by ggrigery

I'm having trouble parsing a date format that I'm getting back from an API and that I have never seen (I believe is a custom format). An example of a date: /Date(1353447000000+0000)/ When I first encountered this format it didn't take me long to see that it was the time in milliseconds with a time zone offset. I'm having trouble extracting this date using SimpleDateFormat though. Here was my first attempt: String weirdDate = "/Date(1353447000000+0000)/"; SimpleDateFormat sdf = new SimpleDateFormat("'/Date('SSSSSSSSSSSSSZ')/'"); Date d1 = sdf.parse(weirdDate); System.out.println(d1.toString()); System.out.println(d1.getTime()); System.out.println(); Date d2 = new Date(Long.parseLong("1353447000000")); System.out.println(d2.toString()); System.out.println(d2.getTime()); And output: Tue Jan 06 22:51:41 EST 1970 532301760 Tue Nov 20 16:30:00 EST 2012 1353447000000 The date (and number of milliseconds parsed) is not even close and I haven't been able to figure out why. After some troubleshooting, I discovered that the way I'm trying to use SDF is clearly flawed. Example: String weirdDate = "1353447000000"; SimpleDateFormat sdf = new SimpleDateFormat("S"); Date d1 = sdf.parse(weirdDate); System.out.println(d1.toString()); System.out.println(d1.getTime()); And output: Wed Jan 07 03:51:41 EST 1970 550301760 I can't say I've ever tried to use SDF in this way to just parse a time in milliseconds because I would normally use Long.parseLong() and just pass it straight into new Date(long) (and in fact the solution I have in place right now is just a regular expression and parsing a long). I'm looking for a cleaner solution that I can easily extract this time in milliseconds with the timezone and quickly parse out into a date without the messy manual handling. Anyone have any ideas or that can spot the errors in my logic above? Help is much appreciated.

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Simulating a contains If-Else using RewriteCond

- by hlissner

Hi! I'm trying to get my head around RewriteCond, and want to rewrite any requests either to a static html page (if it exists), or to a specific index.php (so long as the requested file doesn't exist). To illustrate the logic: if HTTP_HOST is '(www\.)?mydomain.com' if file exists: "/default/static/{REQUEST_URI}.html", then rewrite .* to /default/static/{REQUEST_URI}.html else if file exists: {REQUEST_FILENAME}, then do not rewrite else rewrite .* to /default/index.php I don't seem to have much trouble doing it when I don't need to test for the HTTP_HOST. Ultimately, this one .htaccess file will be handling requests for several domains. I know I could get around this with vhosts, but I'd like to figure out how to do it this way. Here's where I am at now: RewriteCond %{HTTP_HOST} ^(www\.)?mydomain\.com$ [NC] RewriteCond /default/static/%{REQUEST_URI}.html -f RewriteRule . /default/static/%{REQUEST_URI}.html [L,NC] RewriteCond %{REQUEST_FILENAME} !-f RewriteRule . /default/index.php [L,QSA] I'm not too familiar with some of the other flags, will any of them be of use here (like chain|C, next|N or skip|S)? Thanks in advance!

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structDelete doesn't affect the shallow copy?

- by Travis

I was playing around onError so I tried to create an error using a large xml document object. <cfset XMLByRef = variables.parsedXML.XMLRootElement.XMLChildElement> <cfset structDelete(variables.parsedXML, "XMLRootElement")> <cfset startXMLShortLoop = getTickCount()> <cfloop from = "1" to = "#arrayLen(variables.XMLByRef)#" index = "variables.i"> <cfoutput>#variables.XMLByRef[variables.i].id.xmltext#</cfoutput><br /> </cfloop> <cfset stopXMLShortLoop = getTickCount()> I expected to get an error because I deleted the structure I was referencing. From LiveDocs: Variable Assignment - Creates an additional reference, or alias, to the structure. Any change to the data using one variable name changes the structure that you access using the other variable name. This technique is useful when you want to add a local variable to another scope or otherwise change a variable's scope without deleting the variable from the original scope. instead I got 580df1de-3362-ca9b-b287-47795b6cdc17 25a00498-0f68-6f04-a981-56853c0844ed ... ... ... db49ed8a-0ba6-8644-124a-6d6ebda3aa52 57e57e28-e044-6119-afe2-aebffb549342 Looped 12805 times in 297 milliseconds <cfdump var = "#variables#"> Shows there's nothing in the structure, just parsedXML.xmlRoot.xmlName with the value of XMLRootElement. I also tried <cfset structDelete(variables.parsedXML.XMLRootElement, "XMLChildElement")> as well as structClear for both. More information on deleting from the xml document object. http://help.adobe.com/en_US/ColdFusion/9.0/Developing/WSc3ff6d0ea77859461172e0811cbec22c24-78e3.html Can someone please explain my faulty logic? Thanks.

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Encapsulating user input of data for a class (C++)

- by Dr. Monkey

For an assignment I've made a simple C++ program that uses a superclass (Student) and two subclasses (CourseStudent and ResearchStudent) to store a list of students and print out their details, with different details shown for the two different types of students (using overriding of the display() method from Student). My question is about how the program collects input from the user of things like the student name, ID number, unit and fee information (for a course student) and research information (for research students): My implementation has the prompting for user input and the collecting of that input handled within the classes themselves. The reasoning behind this was that each class knows what kind of input it needs, so it makes sense to me to have it know how to ask for it (given an ostream through which to ask and an istream to collect the input from). My lecturer says that the prompting and input should all be handled in the main program, which seems to me somewhat messier, and would make it trickier to extend the program to handle different types of students. I am considering, as a compromise, to make a helper class that handles the prompting and collection of user input for each type of Student, which could then be called on by the main program. The advantage of this would be that the student classes don't have as much in them (so they're cleaner), but also they can be bundled with the helper classes if the input functionality is required. This also means more classes of Student could be added without having to make major changes to the main program, as long as helper classes are provided for these new classes. Also the helper class could be swapped for an alternative language version without having to make any changes to the class itself. What are the major advantages and disadvantages of the three different options for user input (fully encapsulated, helper class or in the main program)?

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Intermittent Issue Writing to Google Appengine Datastore

- by user242153

Hi, I have a functioning app and recently have had intermittent problems writing to the datastore. I did not make any relevant code changes, however in the last few days my attempts to write to the datastore sometimes work and sometimes don't. I am trying to save an object that is in a many to one relationship with an existing persisted parent. So, the logic works like this: 1) Parent pulled from the datastore 2) Child created / instantiated using constructor 3) Parent.addSingleChild(child); // the "addSingleChild" method just adds the object argument to the collection of children 4) child.setParent(Parent); // sets the Parent object to the parent field I am using transactions as explained in the documentation ending with "finally {if (tx.isActive()) {tx.rollback(); } }" When the servlet is called, the parent is called from the datastore and the child object is created and added to the many to one mapping to the pre-existing parent. The child should automatically be persisted, since the parent is already persistent, and the child is added to the collection of children that map to the parent. And it worked this way in the past. However, to be sure, i did add a pm.makePersistent(child). Doesn't seem to help, still have the intermittent problem. Any suggestions would be appreciated, and if you need to see the actual code I can post. Thanks

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jQuery AJAX chained calls + Celery in Django

- by user1029968

Currently clicking one of the links in my application, triggers AJAX call (GET) that - if succeeds - triggers the second one and this second one - if succeeds - calls the third one. This way user can be informed which part of process started when clicking the link is currently ongoing. So in the template file in Django project, click callback body for link mentioned looks like below: $("#the-link").click(function(item)) { // CALL 1 $.ajax({ url: {% url ajax_call_1 %}, data: { // something } }) .done(function(call1Result) { // CALL 2 $.ajax({ url: {% url ajax_call_1 %}, data: { // call1Result passed here to CALL 2 } }) .done(function(call2Result) { // CALL 3 $.ajax({ url: {%url ajax_call_3 %}, data: { // call2Result passed here to CALL 3 } }) .done(function(call3Result) { // expected result if everything went fine console.log("wow, it worked!"); console.log(call3Result); }) .fail(function(errorObject) { console.log("call3 failed"); console.log(errorObject); } }) .fail(function(errorObject)) { console.log("call2 failed"); console.log(errorObject); } }) .fail(function(errorObject) { console.log("call1 failed"); console.log(errorObject); }); }); This works fine for me. The thing is, I'd like to prevent interrupting the following calls if the user closes the browser and the calls are not finished (as it will take some time to finish all three), as there is some additional logic in Django view functions called in each GET request. For example, if user clicks the link and closes the browser during CALL 1, is it possible to somehow go on with the following CALL 2 and CALL 3? I know that normally I'd be able to use Celery Task to process the function but is it still possible here with the chained calls mentioned? Any help is much appreciated!

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Testing InlineFormset clean methods

- by Rory

I have a Django project, with 2 models, a Structure and Bracket, the Bracket has a ForeignKey to a Structure (i.e. one-to-many, one Structure has many Brackets). I created a TabularInline for the admin site, so that there would be a table of Brackets on the Structure. I added a custom formset with some a custom clean method to do some extra validation, you can't have a Bracket that conflicts with another Bracket on the same Structure etc. The admin looks like this: class BracketInline(admin.TabularInline): model = Bracket formset = BracketInlineFormset class StructureAdmin(admin.ModelAdmin): inlines = [ BracketInline ] admin.site.register(Structure, StructureAdmin) That all works, and the validation works. However now I want to write some unittest to test my complex formset validation logic. My first attempt to validate known-good values is: data = {'form-TOTAL_FORMS': '1', 'form-INITIAL_FORMS': '0', 'form-MAX_NUM_FORMS': '', 'form-0-field1':'good-value', … } formset = BracketInlineFormset(data) self.assertTrue(formset.is_valid()) However that doesn't work and raises the exception: ====================================================================== ERROR: testValid (appname.tests.StructureTestCase) ---------------------------------------------------------------------- Traceback (most recent call last): File "/paht/to/project/tests.py", line 494, in testValid formset = BracketInlineFormset(data) File "/path/to/django/forms/models.py", line 672, in __init__ self.instance = self.fk.rel.to() AttributeError: 'BracketInlineFormset' object has no attribute 'fk' ---------------------------------------------------------------------- The Django documentation (for formset validation) implies one can do this. How come this isn't working? How do I test the custom clean()/validation for my inline formset?

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How to write this snippet in Python?

- by morpheous

I am learning Python (I have a C/C++ background). I need to write something practical in Python though, whilst learning. I have the following pseudocode (my first attempt at writing a Python script, since reading about Python yesterday). Hopefully, the snippet details the logic of what I want to do. BTW I am using python 2.6 on Ubuntu Karmic. Assume the script is invoked as: script_name.py directory_path import csv, sys, os, glob # Can I declare that the function accepts a dictionary as first arg? def getItemValue(item, key, defval) return !item.haskey(key) ? defval : item[key] dirname = sys.argv[1] # declare some default values here weight, is_male, default_city_id = 100, true, 1 # fetch some data from a database table into a nested dictionary, indexed by a string curr_dict = load_dict_from_db('foo') #iterate through all the files matching *.csv in the specified folder for infile in glob.glob( os.path.join(dirname, '*.csv') ): #get the file name (without the '.csv' extension) code = infile[0:-4] # open file, and iterate through the rows of the current file (a CSV file) f = open(infile, 'rt') try: reader = csv.reader(f) for row in reader: #lookup the id for the code in the dictionary id = curr_dict[code]['id'] name = row['name'] address1 = row['address1'] address2 = row['address2'] city_id = getItemValue(row, 'city_id', default_city_id) # insert row to database table finally: f.close() I have the following questions: Is the code written in a Pythonic enough way (is there a better way of implementing it)? Given a table with a schema like shown below, how may I write a Python function that fetches data from the table and returns is in a dictionary indexed by string (name). How can I insert the row data into the table (actually I would like to use a transaction if possible, and commit just before the file is closed) Table schema: create table demo (id int, name varchar(32), weight float, city_id int); BTW, my backend database is postgreSQL

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Java: Help constructing a fillTextFields() method

- by Holly

I have a Java project where I am to connect to a database and create buttons (next, new, save, delete, previous) to navigate through the database content where there are appropriate text fields and labels for the specific information. I'll use the code below as an example (each button is set up very similar)... I have it as follows: JButton jbtnNext = new JButton("Next ->"); jbtnNext.addActionListener(this); if (e.getSource() == jbtnNext) jbtnNext_Click(); private void jbtnNext_Click() { JOptionPane.showMessageDialog(null, "Next" ,"Button Pressed", JOptionPane.INFORMATION_MESSAGE); try { if (rset.next()) { fillTextFields(true); }else{ //Display result in a dialog box JOptionPane.showMessageDialog(null, "Not found"); } } catch (SQLException ex) { ex.printStackTrace(); } } The professor gave the following outline of logic to construct the fillTextFields() method: Construct the method to provide reusable code that would fill the JTextFields on the GUI with appropriate values from current record from the database (when "Previous or "Next" buttons are pressed) or blank values (when the new Button is pressed). To determine when the current record was to provide values (next and previous) or the value would be blank (new button) pass a boolean argument into the method. If data from the current record was to be used as fill values, pass true for both previous and next button code after moving the record pointer. If the new button was pressed and want to fill with blank values, pass false to the method. Inside the method, use a conditional expression to evaluate the boolean variable. If true, The appropriate get----() resultset method is used to fill the JTextFields. If false, fill them with "". The .setText() method of the JTextField is used to fill each JTextField. Make sure the fillTextFields method throws the appropriate exception. I understand and have the previous and next button methods passing true, while the new button method is passing false, but I don't quite understand how to set up the fillTextFields() method correctly or how to "throw the appropriate exception"... Any help would really be appreciated, thank you!

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bool as object vs string as object testing equality

- by Ray Pendergraph

I am relatively new to C# and I noticed something interesting today that I guess I have never noticed or perhaps I am missing something. Here is an NUnit test to give an example: object boolean1 = false; object booloan2 = false; Assert.That(boolean1 == booloan2); This unit test fails, but this one passes: object string1 = "string"; object string2 = "string"; Assert.That(string1 == string2); I'm not that surprised in and of itself that the first one fails seeing as boolean1 and boolean2 are different references. But it is troubling to me that the first one fails and the second one passes. I read (on MSDN somewhere) that some magic was done to the String class to facilitate this. I think my question really is why wasn't this behavior replicated in bool? As a note... if the boolean1 and 2 are declared as "bool" then there is no problem. Does anyone know the reason for these differences or why it was implemented that way? Can anyone think of a situation where you would want to reference a bool object for anything except its value?

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find contiguous stretches of equal data in a vector

- by mariotomo

I have a numeric vector, it contains patches of elements that are repeating, something like: R> data <- c(1,1,1,2,2,2,3,3,2,2,2,2,2,3,3,1,1,1,1,1) R> data [1] 1 1 1 2 2 2 3 3 2 2 2 2 2 3 3 1 1 1 1 1 R> I need to extract contiguous patches of elements equals to a specific value... but I'm only interested in the patch around a specific position. so, my input is: (1) the numeric vector, (2) the desired value, (3) the position. I want to return a logic vector indicating which positions satisfy the request. if at that position the data does not equal the value, I return all FALSE. possible outcomes that are not all F would be: [1] 1 1 1 2 2 2 3 3 2 2 2 2 2 3 3 1 1 1 1 1 [1] T T T F F F F F F F F F F F F F F F F F [2] F F F T T T F F F F F F F F F F F F F F [3] F F F F F F T T F F F F F F F F F F F F [4] F F F F F F F F T T T T T F F F F F F F [5] F F F F F F F F F F F F F T T F F F F F [6] F F F F F F F F F F F F F F F T T T T T

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LINQ to SQL: Reusable expression for property?

- by coenvdwel

Pardon me for being unable to phrase the title more exact. Basically, I have three LINQ objects linked to tables. One is Product, the other is Company and the last is a mapping table Mapping to store what Company sells which products and by which ID this Company refers to this Product. I am now retrieving a list of products as follows: var options = new DataLoadOptions(); options.LoadWith<Product>(p => p.Mappings); context.LoadOptions = options; var products = ( from p in context.Products select new { ProductID = p.ProductID, //BackendProductID = p.BackendProductID, BackendProductID = (p.Mappings.Count == 0) ? "None" : (p.Mappings.Count > 1) ? "Multiple" : p.Mappings.First().BackendProductID, Description = p.Description } ).ToList(); This does a single query retrieving the information I want. But I want to be able to move the logic behind the BackendProductID into the LINQ object so I can use the commented line instead of the annoyingly nested ternary operator statements for neatness and re-usability. So I added the following property to the Product object: public string BackendProductID { get { if (Mappings.Count == 0) return "None"; if (Mappings.Count > 1) return "Multiple"; return Mappings.First().BackendProductID; } } The list is still the same, but it now does a query for every single Product to get it's BackendProductID. The code is neater and re-usable, but the performance now is terrible. What I need is some kind of Expression or Delegate but I couldn't get my head around writing one. It always ended up querying for every single product, still. Any help would be appreciated!

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How can I send rich emails using the user's mail client ?

- by Brann

I need my .net program to send rich emails (usually containing table data, around 20 columns x 10 rows) using the user's mail infrastructure, allowing him to review/edit the mail before sending it, and storing the mail in his 'sent items' folder. mailto: seems the obvious choice, but unfortunately, it doesn't support neither attachments nor html bodies. It seems some clients support some extra features (e.g. Outlook 97 used to support a &Attach tag, but this is not the case for more recent versions). I could use mailto and try to format the text body to look nice (using tabs, etc), but this isn't really elegant and wouldn't support huge data. using automation seems a very huge task, as I would need to automate dozens of clients (4 or 5 versions of outlook, lotusnotes, thunderbid, etc.) ... This would be a huge task and it's not really my core business ... I could send emails through code and write my own mail form to let the user edit the mail, but this would have a lot of drawbacks : the user would need to manually configure the mail server settings he wouldn't have access to his contact directory the mail wouldn't be sent in his sent items folder This seems a quite common issue, but I haven't found any satisfying solution yet ; does someone knows of a library supporting this (ie containing automation logic for most mainstream email clients?). Or an alternative to mailto ?

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Object Design: How to Organize/Structure a "Collection Class"

- by CrimsonX

I'm currently struggling to understand how I should organize/structure a class which I have already created. The class does the following: As its input in the constructor, it takes a collection of logs In the constructor it validates and filters the logs through a series of algorithms implementing my business logic After all filtering and validation is complete, it returns a collection (a List) of the valid and filtered logs which can be presented to the user graphically in a UI. Here is some simplified code describing what I'm doing: class FilteredCollection { public FilteredCollection( SpecialArray<MyLog> myLog) { // validate inputs // filter and validate logs in collection // in end, FilteredLogs is ready for access } Public List<MyLog> FilteredLogs{ get; private set;} } However, in order to access this collection, I have to do the following: var filteredCollection = new FilteredCollection( secialArrayInput ); //Example of accessing data filteredCollection.FilteredLogs[5].MyLogData; Other key pieces of input: I foresee only one of these filtered collections existing in the application (therefore should I make it a static class? Or perhaps a singleton?) Testability and flexibility in creation of the object is important (Perhaps therefore I should keep this an instanced class for testability?) I'd prefer to simplify the dereferencing of the logs if at all possible, as the actual variable names are quite long and it takes some 60-80 characters to just get to the actual data. My attempt in keeping this class simple is that the only purpose of the class is to create this collection of validated data. I know that there may be no "perfect" solution here, but I'm really trying to improve my skills with this design and I would greatly appreciate advice to do that. Thanks in advance.

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need to change TextBox.Text inside TextChanged, something forces form close

- by Istrebitel

I am making a TextBox behave like if it could store a null value. In order to do that, i have a variable NullMode that indicates wether the value is stored is Null, and in TextChanged i set that to false, and on specific user action i set it to true and Text to a value that indicates that there is null inside the textbox. Then, based on NullMode, textbox is drawn differently. Now, i have a semaphore-like approach in order to prevent event handle from firing when i dont need it. Here is how it looks: private void input_TextChanged(object sender, EventArgs e) { if (_preventTextBoxEvents) return; _preventTextBoxEvents = true; //if (NullMode) // Text = ""; NullMode = false; ValidateInput(); _preventTextBoxEvents = false; } Now, if i need to set a textbox text to something that should show while in nullmode, i just set _preventTextBoxEvents before i do to true and it works all right. BUT! I need to also remove the text when user tries to input something into the textbox! So i need to set Text to "". Problem is, if i uncomment that, form is closed after the event handler exits. I cannot prevent it (e.Cancel = true in FormClosing doesnt help!) and do not understand what can be causing it. There is no error message too (and i'm not doing try-catch). My logic, when i do Text="". OnTextChanged should fire, it should call my TextChanged and it will see _preventTextBoxEvents is true and exit, so there would be no stack overflow / infinite recursion. What is going on?

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Numerical calculations in Prolog

- by user8472

While reading SICP I came across logic programming chapter 4.4. Then I started looking into the Prolog programming language and tried to understand some simple assignments in Prolog. I found that Prolog seems to have troubles with numerical calculations. Here is the computation of a factorial in standard Prolog: f(0, 1). f(A, B) :- A > 0, C is A-1, f(C, D), B is A*D. The issues I find is that I need to introduce two auxiliary variables (C and D), a new syntax (is) and that the problem is non-reversible (i.e., f(5,X) works as expected, but f(X,120) does not). Naively, I expect that at the very least C is A-1, f(C, D) above may be replaced by f(A-1,D), but even that does not work. My question is: Why do I need to do this extra "stuff" in numerical calculations but not in other queries? I do understand (and SICP is quite clear about it) that in general information on "what to do" is insufficient to answer the question of "how to do it". So the declarative knowledge in (at least some) math problems is insufficient to actually solve these problems. But that begs the next question: How does this extra "stuff" in Prolog help me to restrict the formulation to just those problems where "what to do" is sufficient to answer "how to do it"?

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Deal with undefined values in code or in the template?

- by David

I'm writing a web application (in Python, not that it matters). One of the features is that people can leave comments on things. I have a class for comments, basically like so: class Comment: user = ... # other stuff where user is an instance of another class, class User: name = ... # other stuff And of course in my template, I have <div>${comment.user.name}</div> Problem: Let's say I allow people to post comments anonymously. In that case comment.user is None (undefined), and of course accessing comment.user.name is going to raise an error. What's the best way to deal with that? I see three possibilities: Use a conditional in the template to test for that case and display something different. This is the most versatile solution, since I can change the way anonymous comments are displayed to, say, "Posted anonymously" (instead of "Posted by ..."), but I've often been told that templates should be mindless display machines and not include logic like that. Also, other people might wind up writing alternate templates for the same application, and I feel like I should be making things as easy as possible for the template writer. Implement an accessor method for the user property of a Comment that returns a dummy user object when the real user is undefined. This dummy object would have user.name = 'Anonymous' or something like that and so the template could access it and print its name with no error. Put an actual record in my database corresponding to a user with user.name = Anonymous (or something like that), and just assign that user to any comment posted when nobody's logged in. I know I've seen some real-world systems that operate this way. (phpBB?) Is there a prevailing wisdom among people who write these sorts of systems about which of these (or some other solution) is the best? Any pitfalls I should watch out for if I go one way vs. another? Whoever gives the best explanation gets the checkmark.

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Where do objects merge/join data in a 3-tier model?

- by BerggreenDK

Its probarbly a simple 3-tier problem. I just want to make sure we use the best practice for this and I am not that familiary with the structures yet. We have the 3 tiers: GUI: ASP.NET for Presentation-layer (first platform) BAL: Business-layer will be handling the logic on a webserver in C#, so we both can use it for webforms/MVC + webservices DAL: LINQ to SQL in the Data-layer, returning BusinessObjects not LINQ. DB: The SQL will be Microsoft SQL-server/Express (havent decided yet). Lets think of setup where we have a database of [Persons]. They can all have multiple [Address]es and we have a complete list of all [PostalCode] and corresponding citynames etc. The deal is that we have joined a lot of details from other tables. {Relations}/[tables] [Person]:1 --- N:{PersonAddress}:M --- 1:[Address] [Address]:N --- 1:[PostalCode] Now we want to build the DAL for Person. How should the PersonBO look and when does the joins occure? Is it a business-layer problem to fetch all citynames and possible addressses pr. Person? or should the DAL complete all this before returning the PersonBO to the BAL ? Class PersonBO { public int ID {get;set;} public string Name {get;set;} public List<AddressBO> {get;set;} // Question #1 } // Q1: do we retrieve the objects before returning the PersonBO and should it be an Array instead? or is this totally wrong for n-tier/3-tier?? Class AddressBO { public int ID {get;set;} public string StreetName {get;set;} public int PostalCode {get;set;} // Question #2 } // Q2: do we make the lookup or just leave the PostalCode for later lookup? Can anyone explain in what order to pull which objects? Constructive criticism is very welcome. :o)

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WPF validation on Enter Key Up

- by Martin

I'm trying to validate a UI change when Enter key is pressed. The UI element is a textbox, which is data binded to a string. My problem is that the data binding hasn't updated TestText when Enter key is Up. It is only updated when I press the button which brings up a message box. /// <summary> /// Interaction logic for Window1.xaml /// </summary> public partial class Window1 : Window, INotifyPropertyChanged { String _testText = new StringBuilder("One").ToString(); public string TestText { get { return _testText; } set { if (value != _testText) { _testText = value; OnPropertyChanged("TestText"); } } } public Window1() { InitializeComponent(); myGrid.DataContext = this; } private void OnPropertyChanged(string property) { if (PropertyChanged != null) { PropertyChanged(this, new PropertyChangedEventArgs(property)); } } public event PropertyChangedEventHandler PropertyChanged; private void onKeyUp(object sender, KeyEventArgs e) { if (e.Key != System.Windows.Input.Key.Enter) return; System.Diagnostics.Trace.WriteLine(TestText); } private void button1_Click(object sender, RoutedEventArgs e) { MessageBox.Show(TestText); } } Window XAML: Window x:Class="VerificationTest.Window1" xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation" xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml" Title="Window1" Height="300" Width="300" KeyUp="onKeyUp" TextBox XAML: TextBox Name="myTextBox" Text="{Binding TestText}" Button XAML: Button Name="button1" Click="button1_Click"

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Chrome targeted CSS

- by Chris

I have some CSS code that hides the cursor on a web page (it is a client facing static screen with no interaction). The code I use to do this is below: *, html { cursor: url('/web/resources/graphics/blank.cur'), pointer; } Blank.cur is a totally blank cursor file. This code works perfectly well in all browsers when I host the web files on my local server but when I upload to a Windows CE webserver (our production unit) the cursor represents itself as a black box. Odd. After some testing it seems that chrome only has a problem with totally blank cursor files when served from WinCE web server, so I created a blank cursor with one pixel as white, specifically for chrome. How do I then target this CSS rule to chrome specifically? i.e. *, html { cursor: url('/web/resources/graphics/blank.cur'), pointer; } 

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ASP.NET MVC editor template for property

- by Idsa

Usually I render my forms by @Html.RenderModel, but this time I have a complex rendering logic and I render it manually. I decided to create a editor template for one property. Here is the code (copy pasted from default object editor template implementation): <%@ Control Language="C#" Inherits="System.Web.Mvc.ViewUserControl" %> <% var modelMetadata = ViewData.ModelMetadata; %> <% if (modelMetadata.HideSurroundingHtml) { %> <%= Html.Editor(modelMetadata.PropertyName) %> <% } else { %> <% if (!String.IsNullOrEmpty(Html.Label(modelMetadata.PropertyName).ToHtmlString())) { %> <div class="editor-label"><%= Html.Label(modelMetadata.PropertyName) %></div> <% } %> <div class="editor-field"> <%= Html.Editor(modelMetadata.PropertyName) %> <%= Html.ValidationMessage(modelMetadata.PropertyName) %> </div> <% } %> And here is how I use it: @Html.EditorFor(x => x.SomeProperty, "Property") //"Property" is template above But it didn't work: labels are rendered regardless of DisplayName and editors are not rendered at all (in Watches Html.Editor(modelMetadata.PropertyName shows empty string). What am I doing wrong?

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How do I generate reports in R?

- by Maiasaura

I've been struggling for a week now trying to figure out how to generate reports in R using either Sweave or Brew. I should say right at the beginning that I have never used Tex before but I understand the logic of it. I have read this document several times. However, I cannot even get a simple example to parse. Brew successfully converts a simple markup file (just a title and some text) to a .tex file (no error). But it never ever converts tex to a pdf. > library(tools) > library(brew) > brew("population.brew", "population.tex") > texi2dvi("population.tex", pdf = TRUE) The last step always fails with: Error in texi2dvi("population.tex", pdf = TRUE) : Running 'texi2dvi' on 'population.tex' failed. What am I doing wrong? The report I am trying to build is fairly simple. I have 157 different analysis to summarize. Each one has 4 plots, 1 table and a summary. I just want output plot 1,2,3,4 output table \pagebreak ... that's it. Can anyone help me get further? I use osx, don't have Tex installed. thanks

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