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  • Please help with very simple android widget button click. Getting very frustrated. :(

    - by Little Wendy
    I have poured over every example that I can find and I have gone through the official documentation from google. All I am trying to do is create a toast notification from a button click on a widget. I can get the widget (and button) to appear but I can't get my intent to fire. I have seen several examples that show doing this different ways but I just can't get it to work. I haven't felt this helpless with coding in a long time and I'm starting to feel dumb. This is what I have so far: public class simpleclick extends AppWidgetProvider { /** Called when the activity is first created. */ @Override public void onUpdate(Context context, AppWidgetManager appWidgetManager, int[] appWidgetIds) { final int N = appWidgetIds.length; Toast.makeText(context, "doing on update", Toast.LENGTH_SHORT).show(); for (int i=0; i<N; i++) { int appWidgetId = appWidgetIds[i]; Intent intent = new Intent(context, simpleclick.class); intent.setAction("ham"); PendingIntent pendingIntent = PendingIntent.getActivity(context, 0, intent, 0); RemoteViews views = new RemoteViews(context.getPackageName(), R.layout.main); views.setOnClickPendingIntent(R.id.Timm, pendingIntent); appWidgetManager.updateAppWidget(appWidgetId, views); } } //@Override public void onReceive(Context context, Intent intent) { // TODO Auto-generated method stub Toast.makeText(context, "action is: " + intent.getAction(), Toast.LENGTH_SHORT).show(); super.onReceive(context, intent); } } My manifest: <?xml version="1.0" encoding="utf-8"?> <manifest xmlns:android="http://schemas.android.com/apk/res/android" package="com.tblabs.simpleclick" android:versionCode="1" android:versionName="1.0"> <application android:icon="@drawable/icon" android:label="@string/app_name"> <receiver android:name="simpleclick"> <intent-filter> <action android:name="android.appwidget.action.APPWIDGET_UPDATE" /> </intent-filter> <meta-data android:name="android.appwidget.provider" android:resource="@xml/simpleclick" /> </receiver> </application> <uses-sdk android:minSdkVersion="5" /> </manifest> I would appreciate some help! Thanks, Wendy

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  • How to design database for tests in online test application

    - by Kien Thanh
    I'm building an online test application, the purpose of app is, it can allow teacher create courses, topics of course, and questions (every question has mark), and they can create tests for students and students can do tests online. To create tests of any courses for students, first teacher need to create a test pattern for that course, test pattern actually is a general test includes the number of questions teacher want it has, then from that test pattern, teacher will generate number of tests corresponding with number of students will take tests of that course, and every test for student will has different number of questions, although the max mark of test in every test are the same. Example if teacher generate tests for two students, the max mark of test will be 20, like this: Student A take test with 20 questions, student B take test only has 10 questions, it means maybe every question in test of student A only has mark is 1, but questions in student B has mark is 2. So 20 = 10 x 2, sorry for my bad English but I don't know how to explain it better. I have designed tables for: - User (include students and teachers account) - Course - Topic - Question - Answer But I don't know how to define associations between user and test pattern, test, question. Currently I only can think these: Test pattern table: name, description, dateStart, dateFinish, numberOfMinutes, maxMarkOfTest Test table: test_pattern_id And when user (is Student) take tests, I think i will have one more table: Result: user_id, test_id, mark but I can't set up associations among test pattern and test and question. How to define associations?

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  • Object desing problem for simple school application

    - by Aragornx
    I want to create simple school application that provides grades,notes,presence,etc. for students,teachers and parents. I'm trying to design objects for this problem and I'm little bit confused - because I'm not very experienced in class designing. Some of my present objects are : class PersonalData() { private String name; private String surename; private Calendar dateOfBirth; [...] } class Person { private PersonalData personalData; } class User extends Person { private String login; private char[] password; } class Student extends Person { private ArrayList<Counselor> counselors = new ArrayList<>(); } class Counselor extends Person { private ArrayList<Student> children = new ArrayList<>(); } class Teacher extends Person { private ArrayList<ChoolClass> schoolClasses = new ArrayList<>(); private ArrayList<Subject> subjects = new ArrayList<>(); } This is of course a general idea. But I'm sure it's not the best way. For example I want that one person could be a Teacher and also a Parent(Counselor) and present approach makes me to have two Person objects. I want that user after successful logging in get all roles that it has (Student or Teacher or (Teacher & Parent) ). I think I should make and use some interfaces but I'm not sure how to do this right. Maybe like this: interface Role { } interface TeacherRole implements Role { void addGrade( Student student, Grade grade, [...] ); } class Teacher implements TeacherRole { private Person person; [...] } class User extends Person{ ArrayList<Role> roles = new ArrayList<>(); } Please if anyone could help me to make this right or maybe just point me to some literature/article that covers practical objects design.

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  • Where can a self-teacher learn general good programming habits and conventions?

    - by lucid
    A few mistakes and general childishness in early adulthood have left me in a situation where I work a menial job, with no possibility (in the near future) of attending school. I aspire to one day work in the programming field (gaming specifically), after proving myself on the indie end of things. I've gotten very confident in C++, java, and python, and I find I'm able to solve any problem I want either from previous experience, or from scouring the web for help. The solutions work, and with each attempt they become more readable, maintainable, and extensible. But this is because I'm learning from mistakes and bad programming and design habits I feel I might have avoided with actual schooling. General tips like: "if it's hard to read or getting long, or you're writing it twice, it should be in one or more functions." or "design all your classes before you start coding, so you don't have to rewrite classes later when you discover an unforeseen dependency" Is there a good book or website for learning general good programming practices and design habits? Also, naming and format conventions. I realize sometimes development houses have their own conventions, but things like "Classes in python usually have the first letter of each word capitalized". I'd like to be able to show some source code to a potential employer, and be prepared when for what's expected on a team. Is there some central database of naming and formatting conventions somewhere? Also, feel free to give any thoughts on whether or not the self-teach, garner some indie sales, use them as your resume' route is realistic

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  • Accessing the relationship of a relationship with Entity Framework

    - by J. Pablo Fernández
    I the School class I have this code: from student in this.Students where student.Teacher.Id == id select student The Student class there are two relationships: Teacher and School. In the School class I'm trying to find out all the students whose Teacher has a given id. The problem is that I get System.NullReferenceException: Object reference not set to an instance of an object. in the statement student.Teacher.Id I thought of doing this.Students.Include("Teacher"), but this.Students doesn't have such a method. Any ideas how can I perform that query?

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  • Django design question: extending User to make users that can't log in

    - by jobrahms
    The site I'm working on involves teachers creating student objects. The teacher can choose to make it possible for a student to log into the site (to check calendars, etc) OR the teacher can choose to use the student object only for record keeping and not allow the student to log in. In the student creation form, if the teacher supplies a username and a password, it should create an object of the first kind - one that can log in, i.e. a regular User object. If the teacher does not supply a username/password, it should create the second type. The other requirement is that the teacher should be able to go in later and change a non-logging-in student to the other kind. What's the best way to design for this scenario? Subclass User and make username and password not required? What else would this affect?

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  • How to set up two models having a has_many association with each other

    - by daz13
    I'm looking for a suggestion on how to set up two models, Teacher and Subject. A Teacher can have many Subjects, and a Subject can have many Teachers. Another thing to consider in the relationship between the two models is that a Teacher can create a Subject and add other Teachers to the Subject. I think I'm solid on the basics of the set up for each model: for teacher.rb: has_many :subjects for subject.rb: has_many :teachers and the teachers table should have a subject_id column and the subject table should have a teacher_id column. What I'm not sure about is how to set up the views (and corresponding controller methods) to allow the addition of a Teacher to a Subject. Any suggestions (or links to examples) are greatly appreciated. I haven't been able to find anything on this exact case.

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  • How does this code break the Law of Demeter?

    - by Dave Jarvis
    The following code breaks the Law of Demeter: public class Student extends Person { private Grades grades; public Student() { } /** Must never return null; throw an appropriately named exception, instead. */ private synchronized Grades getGrades() throws GradesException { if( this.grades == null ) { this.grades = createGrades(); } return this.grades; } /** Create a new instance of grades for this student. */ protected Grades createGrades() throws GradesException { // Reads the grades from the database, if needed. // return new Grades(); } /** Answers if this student was graded by a teacher with the given name. */ public boolean isTeacher( int year, String name ) throws GradesException, TeacherException { // The method only knows about Teacher instances. // return getTeacher( year ).nameEquals( name ); } private Grades getGradesForYear( int year ) throws GradesException { // The method only knows about Grades instances. // return getGrades().getForYear( year ); } private Teacher getTeacher( int year ) throws GradesException, TeacherException { // This method knows about Grades and Teacher instances. A mistake? // return getGradesForYear( year ).getTeacher(); } } public class Teacher extends Person { public Teacher() { } /** * This method will take into consideration first name, * last name, middle initial, case sensitivity, and * eventually it could answer true to wild cards and * regular expressions. */ public boolean nameEquals( String name ) { return getName().equalsIgnoreCase( name ); } /** Never returns null. */ private synchronized String getName() { if( this.name == null ) { this.name == ""; } return this.name; } } Questions How is the LoD broken? Where is the code breaking the LoD? How should the code be written to uphold the LoD?

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  • Internet Protocol Suite: Transition Control Protocol (TCP) vs. User Datagram Protocol (UDP)

    How do we communicate over the Internet?  How is data transferred from one machine to another? These types of act ivies can only be done by using one of two Internet protocols currently. The collection of Internet Protocol consists of the Transition Control Protocol (TCP) and the User Datagram Protocol (UDP).  Both protocols are used to send data between two network end points, however they both have very distinct ways of transporting data from one endpoint to another. If transmission speed and reliability is the primary concern when trying to transfer data between two network endpoints then TCP is the proper choice. When a device attempts to send data to another endpoint using TCP it creates a direct connection between both devices until the transmission has completed. The direct connection between both devices ensures the reliability of the transmission due to the fact that no intermediate devices are needed to transfer the data. Due to the fact that both devices have to continuously poll the connection until transmission has completed increases the resources needed to perform the transmission. An example of this type of direct communication can be seen when a teacher tells a students to do their homework. The teacher is talking directly to the students in order to communicate that the homework needs to be done.  Students can then ask questions about the assignment to ensure that they have received the proper instructions for the assignment. UDP is a less resource intensive approach to sending data between to network endpoints. When a device uses UDP to send data across a network, the data is broken up and repackaged with the destination address. The sending device then releases the data packages to the network, but cannot ensure when or if the receiving device will actually get the data.  The sending device depends on other devices on the network to forward the data packages to the destination devices in order to complete the transmission. As you can tell this type of transmission is less resource intensive because not connection polling is needed,  but should not be used for transmitting data with speed or reliability requirements. This is due to the fact that the sending device can not ensure that the transmission is received.  An example of this type of communication can be seen when a teacher tells a student that they would like to speak with their parents. The teacher is relying on the student to complete the transmission to the parents, and the teacher has no guarantee that the student will actually inform the parents about the request. Both TCP and UPD are invaluable when attempting to send data across a network, but depending on the situation one protocol may be better than the other. Before deciding on which protocol to use an evaluation for transmission speed, reliability, latency, and overhead must be completed in order to define the best protocol for the situation.  

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  • What is the use of Association, Aggregation and Composition (Encapsulation) in Classes

    - by SahilMahajanMj
    I have gone through lots of theories about what is encapsulation and the three techniques of implementing it, which are Association, Aggregation and Composition. What i found is, Encapsulation Encapsulation is the technique of making the fields in a class private and providing access to the fields via public methods. If a field is declared private, it cannot be accessed by anyone outside the class, thereby hiding the fields within the class. For this reason, encapsulation is also referred to as data hiding. Encapsulation can be described as a protective barrier that prevents the code and data being randomly accessed by other code defined outside the class. Access to the data and code is tightly controlled by an interface. The main benefit of encapsulation is the ability to modify our implemented code without breaking the code of others who use our code. With this feature Encapsulation gives maintainability, flexibility and extensibility to our code. Association Association is a relationship where all object have their own lifecycle and there is no owner. Let’s take an example of Teacher and Student. Multiple students can associate with single teacher and single student can associate with multiple teachers but there is no ownership between the objects and both have their own lifecycle. Both can create and delete independently. Aggregation Aggregation is a specialize form of Association where all object have their own lifecycle but there is ownership and child object can not belongs to another parent object. Let’s take an example of Department and teacher. A single teacher can not belongs to multiple departments, but if we delete the department teacher object will not destroy. We can think about “has-a” relationship. Composition Composition is again specialize form of Aggregation and we can call this as a “death” relationship. It is a strong type of Aggregation. Child object dose not have their lifecycle and if parent object deletes all child object will also be deleted. Let’s take again an example of relationship between House and rooms. House can contain multiple rooms there is no independent life of room and any room can not belongs to two different house if we delete the house room will automatically delete. The question is: Now these all are real world examples. I am looking for some description about how to use these techniques in actual class code. I mean what is the point for using three different techniques for encapsulation, How these techniques could be implemented and How to choose which technique is applicable at time.

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  • login not working when changing from mysql to mysqli

    - by user1438647
    I have a code below where it logs a teacher in by matching it's username and password in the database, if correct, then log in, if incorrect, then display a message. <?php session_start(); $username="xxx"; $password="xxx"; $database="mobile_app"; $link = mysqli_connect('localhost',$username,$password); mysqli_select_db($link, $database) or die( "Unable to select database"); foreach (array('teacherusername','teacherpassword') as $varname) { $$varname = (isset($_POST[$varname])) ? $_POST[$varname] : ''; } ?> <form action="<?php echo htmlentities($_SERVER['PHP_SELF']); ?>" method="post" id="teachLoginForm"> <p>Username</p><p><input type="text" name="teacherusername" /></p> <!-- Enter Teacher Username--> <p>Password</p><p><input type="password" name="teacherpassword" /></p> <!-- Enter Teacher Password--> <p><input id="loginSubmit" type="submit" value="Login" name="submit" /></p> </form> <?php if (isset($_POST['submit'])) { $query = " SELECT * FROM Teacher t WHERE (t.TeacherUsername = '".mysqli_real_escape_string($teacherusername)."') AND (t.TeacherPassword = '".mysqli_real_escape_string($teacherpassword)."') "; $result = mysqli_query($link, $query); $num = mysqli_num_rows($result); $loged = false; while($row = mysqli_fetch_array($result)) { if ($_POST['teacherusername'] == ($row['TeacherUsername']) && $_POST['teacherpassword'] == ($row['TeacherPassword'])) { $loged = true; } $_SESSION['teacherforename'] = $row['TeacherForename']; $_SESSION['teachersurname'] = $row['TeacherSurname']; $_SESSION['teacherusername'] = $row['TeacherUsername']; } if ($loged == true){ header( 'Location: menu.php' ) ; }else{ echo "The Username or Password that you Entered is not Valid. Try Entering it Again."; } mysqli_close($link); } ?> Now the problem is that even if the teacher has entered in the correct username and password, it still doesn't let the teacher log in. When the code above was the old mysql() code, it worked fine as teacher was able to login when username and password match, but when trying to change the code into mysqli then it causes login to not work even though username and password match. What am I doing wrong?

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  • How should I structure a site with content dependent on visitor type (not user)?

    - by Pedr
    I have a website that displays different content depending on two selections made by a visitor: Whether they are a teacher or student, and their learning level (from 4 options). Everything is public and they don't need to authenticate to access the content. Depending on their selection, different content is displayed across the whole site, other than a contact and about page. The tone of the language changes depending on whether the visitor is a student or teacher and the materials available on each page also change depending on the learning level, however in all cases, the structure of the site is identical. Currently I'm using a cookie to store the visitor's selections and render different content appropriately, so I have a single set of URLs which display different content depending on the cookie, with one of the permutations as default. I appreciate this is far from ideal, but what is the better option? Would I be better using a distinguishing segment for each selection, for example: http://example.com/teacher/lv3/resources/activities http://example.com/teacher/lv4/resources/activities http://example.com/student/lv4/resources/activities etc. What is the most sensible way to handle this situation?

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  • How do I set up a server for SSH?

    - by Rob S.
    At my workplace (a university) we have two Ubuntu servers. One is a teacher server and the other is a student server. I am currently setting up a new server to replace the existing teacher server. The new server is currently online and I am trying to get it setup so I can SSH into it from my home. Right now, I can SSH into the existing teacher or student server and then SSH into the new server. However, I can not SSH directly into the new server. What must I do so that I can SSH directly into the new server? Thank you very much in advance for your time.

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  • Mix Audio tracks with offset in SOX

    - by Laramie
    From ASP.Net, I am using FFMPEG to convert flv files on a Flash Media Server to wavs that I need to mix into a single MP3 file. I originally attempted this entirely with FFMPEG but eventually gave up on the mixing step because I don't believe it it possible to combine audio only tracks into a single result file. I would love to be wrong. I am now using FFMPEG to access the FLV files and extract the audio track to wav so that SOX can mix them. The problem is that I must offset one of the audio tracks by a few seconds so that they are synchronized. Each file is one half of a conversation between a student and a teacher. For example teacher.wav might need to begin 3.3 seconds after student.wav. I can only figure out how to mix the files with SOX where both tracks begin at the same time. My best attempt at this point is: ffmpeg -y -i rtmp://server/appName/instance/student.flv -ac 1 student.wav ffmpeg -y -i rtmp://server/appName/instance/teacher.flv -ac 1 teacher.wav sox -m student.wav teacher.wav combined.mp3 splice 3.3 These tools (FFMEG/SoX) were chosen based on my best research, but are not required. Any working solution would allow an ASP.Net service to input the two FMS flvs and create a combined MP3 using open-source or free tools.

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  • Nested attributes form for model which belongs_to few models

    - by ExiRe
    I have few models - User, Teacher and TeacherLeader. class User < ActiveRecord::Base attr_accessible ..., :teacher_attributes has_one :teacher has_one :teacher_leader accepts_nested_attributes_for :teacher_leader end class Teacher < ActiveRecord::Base belongs_to :user has_one :teacher_leader end class TeacherLeader < ActiveRecord::Base belongs_to :user belongs_to :teacher end I would like to fill TeacherLeader via nested attributes. So, i do such things in controller: class TeacherLeadersController < ApplicationController ... def new @user = User.new @teacher_leader = @user.build_teacher_leader @teachers_collection = Teacher.all.collect do |t| [ "#{t.teacher_last_name} #{t.teacher_first_name} #{t.teacher_middle_name}", t.id ] end @choosen_teacher = @teachers_collection.first.last unless @teachers_collection.empty? end end And also have such view (new.html.erb): <%= form_for @user, :url => teacher_leaders_url, :html => {:class => "form-horizontal"} do |f| %> <%= field_set_tag do %> <% f.fields_for :teacher_leader do |tl| %> <div class="control-group"> <%= tl.label :teacher_id, "Teacher names", :class => "control-label" %> <div class="controls"> <%= select_tag( :teacher_id, options_for_select( @teachers_collection, @choosen_teacher )) %> </div> </div> <% end %> <div class="control-group"> <%= f.label :user_login, "Login", :class => "control-label" %> <div class="controls"> <%= f.text_field :user_login, :placeholder => @everpresent_field_placeholder %> </div> </div> <div class="control-group"> <%= f.label :password, "Pass", :class => "control-label" %> <div class="controls"> <%= f.text_field :password, :placeholder => @everpresent_field_placeholder %> </div> </div> <% end %> <%= f.submit "Create", :class => "btn btn-large btn-success" %> <% end %> Problem is that select form here does NOT appear. Why? Do i do something wrong?

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  • Grails UnitTest

    - by Tomáš
    Hi (it is propably stupid question) how can acquire Domain class from database in test? class PollServiceTests extends GrailsUnitTestCase { Integer id = 1 void testSomething() { Teacher teacher1 = Teacher.get(id) assert teacher1 != null } } I always get null or No signature of method: cz.jak.Teacher.get() is applicable for argument types: (java.lang.Integer) values: [1] thanks a lot Tom

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  • MEF instance management

    - by Lawrence A. Contreras
    I am working on application that has multiple modules and sub modules. Here's what I need to do: ModuleA has a sub module SubModuleA, SubModuleB, SubModuleC. I want to export ModuleA, SubModuleA, SubModuleB, SubModuleC. ModuleA can have multiple instances. Whenever I import ModuleA inside the sub modules, I want to get the correct instance and also when I import SubModuleA,SubModuleB and SubModuleC inside other classes. How can I manage that scenario using MEF? Do I really need to use MEF. Updated: Here's a quick example: public class School { List<Classroom> classRooms { get; set; } } [Export] public class Classroom { List<Teacher> teachers { get; set; } } public class Teacher { List<Student> students { get; set; } } public class Student { [Import] Classroom classroom { get; set; } } As you can see, I want to export the classroom class because I need to import it in the Student class, let's just say that I really need the classroom class inside the student class. I want to skip the part where we pass the classroom to the teacher and from the teacher we'll pass it to the student. But when I import classroom, I want to have the correct instance where that class room contains the teacher of the student.

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  • Looking for MSSQL Table Design Sanity Check for Profile Tables with Dynamic Columns.

    - by Code Sherpa
    I just want a general sanity check regarding database design. We are building a web system that has both Teachers and Students. Both have accounts in the system. Both have profiles in the system. My question is about the table design of those Profile tables. The Teacher profile is pretty static regarding the metadata associated with it. Each teacher has a set number of fields that exposes information about that individual (schools, degrees, etc). The students, however, are a different case. We are using a windows service to pull varying data about the students from an endless stream of excel spreadsheets. The data gets moved into our database and then the fields appear in association with the student's profile. Accordingly, each and every student may have very different fields in their profile. I originally started with the concept of three tables: Accounts ---------- AccountID TeacherProfiles ---------- TeacherProfileID AccountID SecondarySchool University YearsTeaching Etc... StudentProfiles ---------- StudentProfileID AccountID Header Value The StudentProfiles table would hold the name of the column headers from the excel spreadsheets and the associated values. I have since evolved the design a little to treat Profiles more generically per the attached ERD image. The Teacher and Student "Headers" are stored in a table called "ProfileAttributeTypes" and responses (either from the excel document or via input fields on the web form) are put in a ProfileAttributes table. This way both Student and Teacher profiles can be associated with a dynamic flow of profile fields. The "Permissions" table tells us whether we are dealing with a Student or a Teacher. Since this system is likely to grow quickly, I want to make sure the foundation is solid. Can you please provide feedback about this design and let me know if it seems sound or if you could see problems it might create and, if so, what might be a better approach? Thanks in advance.

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  • NHibernate SchemaUpdate adding existing foreign keys again?

    - by afsharm
    I'm using SchemaUpdate to synchronize my hbms with existing database. Database has recently created based on hbms and is completely up-to-date. But SchemaUpdate generates all foreign key constraints again. For example suppose you have Student and Teacher. Student has association to Teacher with name ArtTeacher. ArtTeacher is a foreign key from Student to Teacher. Suppose database is up-to-date and currently holde Student, Teacher and their foreign key relation. So HBM and Database are equivalent. Know SchemaUpdate must not do anything but when I see its generated scripts, it re-produce that foreign key again. Why this happens? Is there any way to avoid it?

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  • Can i use a model object directly in a find

    - by user340100
    Hi, Can i pass a_teacher directly into the find? or do i have to compare each of its attributes as i have done here? thanks a_teacher = Techer.new(:name => "Bob", :age => 30) self.classes.all(:conditions => ["teacher.name = ? AND teacher.age = ?", a_teacher.name,a_teacher.age], :joins => :teacher)

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  • block access to certain website types

    - by frustrated teacher
    Need to block access to certain website types without listing each URL to block. Students at secondary school are going to porn sites. Need to be able to block all such access without having to list each possible site URL. Having the Content -- Ratings tab set to None for all categories on the ratings files listed on my computers does not prevent access. Unchecking users may access sites with no rating, even with the security settings set to High, still allows the porn sites to come up. If that is checked, then ONLY listed sites can open and students would not be able to do any research via google, for example. I would rather not have to continue checking each computer and blocking sites as they find them.

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  • More Fun With Math

    - by PointsToShare
    More Fun with Math   The runaway student – three different ways of solving one problem Here is a problem I read in a Russian site: A student is running away. He is moving at 1 mph. Pursuing him are a lion, a tiger and his math teacher. The lion is 40 miles behind and moving at 6 mph. The tiger is 28 miles behind and moving at 4 mph. His math teacher is 30 miles behind and moving at 5 mph. Who will catch him first? Analysis Obviously we have a set of three problems. They are all basically the same, but the details are different. The problems are of the same class. Here is a little excursion into computer science. One of the things we strive to do is to create solutions for classes of problems rather than individual problems. In your daily routine, you call it re-usability. Not all classes of problems have such solutions. If a class has a general (re-usable) solution, it is called computable. Otherwise it is unsolvable. Within unsolvable classes, we may still solve individual (some but not all) problems, albeit with different approaches to each. Luckily the vast majority of our daily problems are computable, and the 3 problems of our runaway student belong to a computable class. So, let’s solve for the catch-up time by the math teacher, after all she is the most frightening. She might even make the poor runaway solve this very problem – perish the thought! Method 1 – numerical analysis. At 30 miles and 5 mph, it’ll take her 6 hours to come to where the student was to begin with. But by then the student has advanced by 6 miles. 6 miles require 6/5 hours, but by then the student advanced by another 6/5 of a mile as well. And so on and so forth. So what are we to do? One way is to write code and iterate it until we have solved it. But this is an infinite process so we’ll end up with an infinite loop. So what to do? We’ll use the principles of numerical analysis. Any calculator – your computer included – has a limited number of digits. A double floating point number is good for about 14 digits. Nothing can be computed at a greater accuracy than that. This means that we will not iterate ad infinidum, but rather to the point where 2 consecutive iterations yield the same result. When we do financial computations, we don’t even have to go that far. We stop at the 10th of a penny.  It behooves us here to stop at a 10th of a second (100 milliseconds) and this will how we will avoid an infinite loop. Interestingly this alludes to the Zeno paradoxes of motion – in particular “Achilles and the Tortoise”. Zeno says exactly the same. To catch the tortoise, Achilles must always first come to where the tortoise was, but the tortoise keeps moving – hence Achilles will never catch the tortoise and our math teacher (or lion, or tiger) will never catch the student, or the policeman the thief. Here is my resolution to the paradox. The distance and time in each step are smaller and smaller, so the student will be caught. The only thing that is infinite is the iterative solution. The race is a convergent geometric process so the steps are diminishing, but each step in the solution takes the same amount of effort and time so with an infinite number of steps, we’ll spend an eternity solving it.  This BTW is an original thought that I have never seen before. But I digress. Let’s simply write the code to solve the problem. To make sure that it runs everywhere, I’ll do it in JavaScript. function LongCatchUpTime(D, PV, FV) // D is Distance; PV is Pursuers Velocity; FV is Fugitive’ Velocity {     var t = 0;     var T = 0;     var d = parseFloat(D);     var pv = parseFloat (PV);     var fv = parseFloat (FV);     t = d / pv;     while (t > 0.000001) //a 10th of a second is 1/36,000 of an hour, I used 1/100,000     {         T = T + t;         d = t * fv;         t = d / pv;     }     return T;     } By and large, the higher the Pursuer’s velocity relative to the fugitive, the faster the calculation. Solving this with the 10th of a second limit yields: 7.499999232000001 Method 2 – Geometric Series. Each step in the iteration above is smaller than the next. As you saw, we stopped iterating when the last step was small enough, small enough not to really matter.  When we have a sequence of numbers in which the ratio of each number to its predecessor is fixed we call the sequence geometric. When we are looking at the sum of sequence, we call the sequence of sums series.  Now let’s look at our student and teacher. The teacher runs 5 times faster than the student, so with each iteration the distance between them shrinks to a fifth of what it was before. This is a fixed ratio so we deal with a geometric series.  We normally designate this ratio as q and when q is less than 1 (0 < q < 1) the sum of  + … +  is  – 1) / (q – 1). When q is less than 1, it is easier to use ) / (1 - q). Now, the steps are 6 hours then 6/5 hours then 6/5*5 and so on, so q = 1/5. And the whole series is multiplied by 6. Also because q is less than 1 , 1/  diminishes to 0. So the sum is just  / (1 - q). or 1/ (1 – 1/5) = 1 / (4/5) = 5/4. This times 6 yields 7.5 hours. We can now continue with some algebra and take it back to a simpler formula. This is arduous and I am not going to do it here. Instead let’s do some simpler algebra. Method 3 – Simple Algebra. If the time to capture the fugitive is T and the fugitive travels at 1 mph, then by the time the pursuer catches him he travelled additional T miles. Time is distance divided by speed, so…. (D + T)/V = T  thus D + T = VT  and D = VT – T = (V – 1)T  and T = D/(V – 1) This “strangely” coincides with the solution we just got from the geometric sequence. This is simpler ad faster. Here is the corresponding code. function ShortCatchUpTime(D, PV, FV) {     var d = parseFloat(D);     var pv = parseFloat (PV);     var fv = parseFloat (FV);     return d / (pv - fv); } The code above, for both the iterative solution and the algebraic solution are actually for a larger class of problems.  In our original problem the student’s velocity (speed) is 1 mph. In the code it may be anything as long as it is less than the pursuer’s velocity. As long as PV > FV, the pursuer will catch up. Here is the really general formula: T = D / (PV – FV) Finally, let’s run the program for each of the pursuers.  It could not be worse. I know he’d rather be eaten alive than suffering through yet another math lesson. See the code run? Select  “Catch Up Time” in www.mgsltns.com/games.htm The host is running on Unix, so the link is case sensitive. That’s All Folks

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  • communication network applications in Ubuntu 12.04

    - by soporte.cs
    I have installed edubuntu on 2 computers that are networked. I installed an educational software to keep track of the teams (one student and one teacher) but the teacher software is not any student team, as I have indicated in the product support should be enabled for proper telnet performance but despite being able to telnet between computers not get it to work. I wonder which options should be active in ubuntu for this type of software to operate smoothly. Thank you.

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