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• How can I detect whether an image is a PNG or APNG format?

  - by perlit

  APNG is backwards compatible with PNG. I opened up an apng and png file in a hex editor and the first few bytes look identical. So if a user uploads either of these formats, how do I detect what the format really is? I've seen this done on some sites that block apng. I'm guessing the ImageMagick library makes this easy, but what if I were to do the detect without the use of an image processing library (for learning purposes)? Can I look for specific bytes that tell me if the file is apng? Solutions in any language is welcome.

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• django left join with null

  - by SledgehammerPL

  The model: class Product(models.Model): name = models.CharField(max_length = 128) def __unicode__(self): return self.name class Receipt(models.Model): name = models.CharField(max_length=128) components = models.ManyToManyField(Product, through='ReceiptComponent') class Admin: pass def __unicode__(self): return self.name class ReceiptComponent(models.Model): product = models.ForeignKey(Product) receipt = models.ForeignKey(Receipt) quantity = models.FloatField(max_length=9) unit = models.ForeignKey(Unit) def __unicode__(self): return unicode(self.quantity!=0 and self.quantity or '') + ' ' + unicode(self.unit) + ' ' + self.product.genitive The idea: there are a components on stock. I'd like to find out which recipes I can made with components which I have. It's not easy - but possible - I made a SQL view, which gets the solution. But I'm learning python and Django so I'd like to make it Django-style ;D The concept of solution: get the set of recipes which has at last one component: list_of_available_components = ReceiptComponent.objects.filter(product__in=list_of_available_products).distinct() list_of_related_receipts = Receipt.objects.filter(receiptcomponent__in = list_of_available_components).distinct() get recipes (from list_of_related_receipts) which has not at last one component list_of_incomplete_recipes = (SELECT * FROM drinkbook_receiptcomponent LEFT JOIN drinkstore_stock_products USING(product_id) WHERE drinkstore_stock_products.stock_id IS NULL AND receipt_id IN (SELECT receipt_id FROM drinkbook_receiptcomponent JOIN drinkstore_stock_products USING(product_id))) get recipes (from list_of_related_receipts) which are not in "list_of_incomplete_recipes"

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• What fields have job security?

  - by computergeek6

  I can program pretty well, and I'm trying to think of a programming area that I can practice so I have a better chance of getting a job when I finish my education. I'm currently learning game development, but there are a ton of other people trying to get into game dev, so I want to find something a little more secure and sustainable to develop skills in. I've thought of things like financial systems and engineering stuff, but nothing I can think of is accessible to someone in high school. I'm trying to find something that involves physics or networking and isn't as popular a field as game dev. Does anyone have any ideas?

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• How JQuery data validation plugin works on the submit action

  - by kwokwai

  Hi all, I am learning how to use JQuery validation plugin, and came across some questions about the JQuery magic that works on the sumbit action. I got a simple HTML form which has got an input text field like this: <form class="cmxform" id="commentForm" method="get" action=""> <input id="cname" name="name" size="25" class="required" minlength="2" /> <input class="submit" type="submit" value="Submit"/> </form> The magic happened when I tried not to input any data into the field and pressed the submit button. I saw an error message in red pop up, and no matter how hard and how many times I pressed the submit button, nothing was submitted. It seemed that there was some scripts that disabled the submit action, but I just couldn't find which lines of codes in the JQuery Validatyion Plugin that did it.

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• How to use NSTableView's selectedRowIndexes?

  - by Jakub Lédl

  Hi guys, I'm just learning some basic programming in Objective C and Cocoa. I'm trying to get some data from NSTableView. Based on what I read in one tutorial, I wrote this: NSArray * items = [[itemsTableView selectedRowEnumerator] allObjects]; But then I learned that selectedRowEnumerator was deprecated already in 10.3 Panther and that I should use selectedRowIndexes. The problem is, I didn't find how to actually use the returned NSIndexSet to achieve the same result as with code written above. So, If anyone could give me a tip, I would be very grateful. Thanks.

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• How to handle similar items in rails MVC?

  - by mocker

  I'm working on building a pretty simple site mainly as an exercise in learning more about rails. You can see my rough progress at statific.com. It's working pretty much as I wanted it for keeping track of server information, but now I'd like to expand it to other things, next on the list being firewalls. I can pretty easily duplicate all the models, views, etc.. that I have for my servers. The problem I see with that is that it isn't very DRY since most of the code would look the same, the only difference would be the attributes I have setup for firewalls would be different than for servers. I know in plain ruby this is pretty simple, you can have a 'Product' w/ common attributes ('manufacturer', 'model') and then have children with more specific attributes. Does the same type of concept exist for rails, or am I just over thinking this?

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• BlueJ Chpt 4 - Auction Project - How to make removeLot method work after entering a new lot

  - by Ellaine Ryan

  Hi All I am learning Java through BlueJ I would like to add a removeLot method to the Auction Class in BlueJ Chpt 4. This involves also returning the lot with the given number or null if there is no such lot. If you enter , say 5 lots and then use the remove method, this codes removes whichever lot you specify. However if you enter a new lot after using the removeLot method - an internal error message comes up regarding lotnumbering . Any ideas? Thanks Elaine public Lot removeLot(int number) { if((number >= 1) ) { } Lot lot = getLot (number); if(lot !=null) { lots.remove(lot); } return lot; }

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• Shall i learn Assembly Language or C, to Understand how "real programming" works?

  - by Daniel Upton

  Hello, World.. I'm a web developer mostly working in Ruby and C#.. I wanna learn a low level language so i dont look like an ass infront of my (computer science expert) boss. Ive heard a lot of purist buzz about how assembly language is the only way to learn how computers actually work, but on the other hand C would probably be more useful as a language rather than just for theory. So my question is.. Would Learning C teach me enough computer science theory / low level programming to not look like a common dandy (complete tool)? Thanks! Daniel

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• Resources to learn about engineering aspects of data analytics (OLAP, warehousing, ETL, etc.)

  - by JT

  I'm a math/stats guy, interested in learning more about the engineering aspects of "data analytics" (this may be an overly broad term, this is a case of "I don't know what I don't know", so I'm not sure how to be more specific). I'm fine with manipulating and analyzing the data once it's already stored somewhere and I can access it, and I'm fine with writing scripts and SQL queries (and have a general knowledge of things like normalization). What I don't know is the whole engineering process of capturing and storing the data. For example, terms I've heard thrown about that I only vaguely understand the meaning of include: - OLAP, OLTP - Data warehousing - ETL - ??? What's a good book (or any other resource) to learn about these kinds of things? What are things I should know about database design (normalization seems kinda "obvious" to me, something I would have done even before I knew the term -- is there anything else?)? In other words, for jobs falling under the umbrella term of "analytics engineer", what kinds of things should I know?

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• Get all folder / directories list in VB.net

  - by Henry

  Hi, this is my first Stackoverflow question, im learning VB and having a few problems with getting a list of all folders/directories on the system, im using the code included here and it seems to work until it hits the recycle bin folder, and some other system folders Sub main() Dim DirList As New ArrayList GetDirectories("c:\", DirList) For Each item In DirList 'add item to listbox or text etc here Next End sub End Sub Sub GetDirectories(ByVal StartPath As String, ByRef DirectoryList As ArrayList) Dim Dirs() As String = Directory.GetDirectories(StartPath) DirectoryList.AddRange(Dirs) For Each Dir As String In Dirs GetDirectories(Dir, DirectoryList) Next End Sub Can anyone help me with this, id like to know what is causing this first, and a good fix, or alternative way to do this. Thanks in advance.

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• I have Person.Surname field implemented as a string if I change the implementation of the field to a

  - by AndyM

  This is me going right back to basics with TDD for learning purposes. I originally implemented Person.Surname as field of type object (the simplest possible way of passing the test. I then added a test setting Person.Surname stating that the return value should be a string and set Person.Surname=20. I 'fixed' the test by changing the implementation to use string rather than object. The test now long compiles due to static type checking, so I commented it out. So I'm left with no way of leaving my intention in the test. Is there a way of having a failing test in this circumstance?

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• Interested in embedded systems. Where to begin?

  - by Ala ABUDEEB

  Hello, i'm a computer systems engineering student. i'm interested in designing embedded systems but i don't know where to begin learning this, and what topics are essentially needed to proceed in this domain. So can you please tell me what topics do i have to study, and what books are available there in market or online that can help me??? please help me p.s. normally as an engineering student i have basic knowledge of circuit theory and microcontroller realm. thank you in advance.

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• iPhone Accelerometer > csv > email

  - by Bradley Powers

  Hi all, I'm trying to collect data for a machine learning project I'm working on. What I'd like to do is collect accelerometer data from an iPhone, save it to a csv and email it to myself. My app currently is able to acquire data from the accelerometer, but I'm at a bit of a loss as to how to proceed. First of all, I'd like to acquire data for a preset amount of time (after playing a sound to the user) which I don't really know how to do, and I can't find good documentation for. Also, I'd like to save that to a csv, which there is some documentation on (specifically using the NSString writeToFile method). Any recommendations/ ideas? Thanks!

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• Are there any good videos out there on Java Design Patterns?

  - by Becky Reamy

  My team would like to spend some time at lunch learning design patterns. Previously, we watched some videos on Javascript which we found very useful as a way to start discussions. We would like to do the same thing with design patterns so that we don't have to spend a lot of time (outside of work) researching individual patterns in order to give a presentation. I did a little searching and came up fairly empty handed. Any help would be appreciated. It doesn't even have to be a video, even something that we can listen to (maybe a book on tape even).

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• Python code that needs some overview

  - by fabio

  Hi guys, im currently learning python (in the very begining), so I still have some doubts about good code manners and how should I proceed with it. Today I created this code that should random trought 01 to 60 (but is running from 01 to 69) import random dez = ['0', '1', '2', '3', '4', '5', '6'] uni = ['0', '1', '2', '3', '4', '5', '6', '7', '8', '9'] sort = [] while len(sort) <= 5: random.shuffle(dez) random.shuffle(uni) w = random.choice(dez) z = random.choice(uni) chosen = str(w) + str(z) if chosen != "00" and chosen not in sort: sort.append(chosen) print chosen I'm also in doubt how to make the code stop at "60".

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• syntax error in python:

  - by pecker

  Hello, I'm learning python. It gives syntax error in this script. I'm unable to figure out. import exceptions class FOUND(Exception): pass x = [1,2,3,4,56,73,29,35,12,32,63,12,76,75,89] while True: test = int(raw_input('Enter integer to be checked in list:')) try: count = -1 for y in x: count += 1 if y == test: raise FOUND except ValueError,e: print "Not a valid integer (%d)"%(e) except FOUND: print "Found (%d) at (%d)"%(test,count) else: print "Not found ,Appending (%d) to list at location (%d)"%(test,count+1) x.append(test) finally: print "The List:" print x print " " Invalid syntax & it highlights comma in this line: 'except ValueError,e:'

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• The next step & technology towards web services.

  - by webzide

  Hi, My name is Dennis and I am pretty ambitious of creating the next big thing for the web. I have ideas but now I gotta get to work to learn the tools. I am learning Javascript. I don't know if next step for me to take is PHP or some suggests RUBY on Rails. What are the pros and cons. With your experience, I wish you could hint me to the right direction. Thanks. I don't mind working hard. My goal is to create a comprehensive and secure web service platform as well as a fast and user friendly UI.

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• Understanding run time code interpretation and execution

  - by Bob

  I'm creating a game in XNA and was thinking of creating my own scripting language (extremely simple mind you). I know there's better ways to go about this (and that I'm reinventing the wheel), but I want the learning experience more than to be productive and fast. When confronted with code at run time, from what I understand, the usual approach is to parse into a machine code or byte code or something else that is actually executable and then execute that, right? But, for instance, when Chrome first came out they said their JavaScript engine was fast because it compiles the JavaScript into machine code. This implies other engines weren't compiling into machine code. I'd prefer not compiling to a lower language, so are there any known modern techniques for parsing and executing code without compiling to low level? Perhaps something like parsing the code into some sort of tree, branching through the tree, and comparing each symbol and calling some function that handles that symbol? (Wild guessing and stabbing in the dark)

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• Using the value of a variable as another variables name in Ruby

  - by hrickards

  Hi, I'm just starting out in learning Ruby and I've written a program that generates some numbers and assigns them to variables @one, @two, @three etc. The user can then specify a variable to change by inputting it's name (e.g one). I then need to do something like '@[valueofinout] = asd'. How would I do this, and is there a better way as the way I'm thinking of seems to be discouraged? I've found x = "myvar" myvar = "hi" eval(x) -> "hi" but I don't completely understand why the second line is needed. In my case would I use something like @one = "21" input = "one" input = "@" + input changeto = "22" eval(input) -> changeto Thanks

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• Memory Allocation - Arduino

  - by Joey Arnold Andres

  I'm new to this low level stuff. I'm currently learning arduino. I'm currently using an Arduino Mega 2560 and in our course we are practicing memory management. I'm a pro at memory management in pc but somehow I'm having crazy problems here in arduino. For instance: The arduino have 8192B, I'm trying to overflow it with uint_16 so I made an array of 8192/16 which is 512. so I did uint16_t A[512+1]; Well I expected that to cause an overflow. What is wrong with my concept?

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• How to select the nth row in a SQL database table?

  - by Charles Roper

  I'm interested in learning some (ideally) database agnostic ways of selecting the nth row from a database table. It would also be interesting to see how this can be achieved using the native functionality of the following databases: SQL Server MySQL PostgreSQL SQLite Oracle I am currently doing something like the following in SQL Server 2005, but I'd be interested in seeing other's more agnostic approaches: WITH Ordered AS ( SELECT ROW_NUMBER() OVER (ORDER BY OrderID) AS RowNumber, OrderID, OrderDate FROM Orders) SELECT * FROM Ordered WHERE RowNumber = 1000000 Credit for the above SQL: Firoz Ansari's Weblog Update: See Troels Arvin's answer regarding the SQL standard. Troels, have you got any links we can cite?

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• Why are the contents of my UITableViewCell disappearing?

  - by barfoon

  Hey everyone, I am learning about using the checkmark cell accessory and am wondering why the contents of my cell is disappearing when toggling it. Here is my code: + (void) toggleCheckmarkedCell:(UITableViewCell *)cell { if (cell.accessoryType == UITableViewCellAccessoryNone) cell.accessoryType = UITableViewCellAccessoryCheckmark; else cell.accessoryType = UITableViewCellAccessoryNone; } And here is didSelectRowAtIndexPath: UITableViewCell *cell = [tableView cellForRowAtIndexPath:indexPath]; [cell setSelectionStyle:UITableViewCellSelectionStyleNone]; [RootViewController toggleCheckmarkedCell:cell]; The checkmark is toggling on/off and is visible but the main contents of the cell disappears. Can anyone explain why this is happening? Thank you,

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• abstract method signature, inheritance, and "Do" naming convention

  - by T. Webster

  I'm learning about design patterns and in examples of code I've seen a convention where the abstract class declares a method, for example: public abstract class ServiceBase { ... public virtual object GetSomething(); and then protected abstract object DoGetSomething(); My question is on why these two methods exist, since they appear to serve the same purpose. Is this so that the base class GetSomething() method logic cannot be overridden by inherited classes? But then again, the method is marked virtual, so it can be overridden anyway. What is the usefulness here in requiring derived class implementers to implement the abstract method when the virtual method can be called anyway?

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• What's the point of lambda in scheme?

  - by incrediman

  I am learning scheme. I know how to use both lambda and let expressions. However I'm struggling to figure out what the point is of using lambda. Can't you do everything with let that you can with lambda? It would be especially helpful to see an example of a situation where a lambda expression is a better choice than let. One other thing - are there also situations where let is more useful than lambda? If so such an example would be nice as well. Thanks!

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• How is Java Platform independent when it needs JVM to run ?

  - by happysoul

  Just started learning Java and I am confused about this whole independent platform thingy. Doesn't independent means that Java code should be able to run on any machine and would need no special software to be installed (JVM in this case has to be present in the machine)? Like, for example, we need to have Turbo C Compiler in order to compile C/C++ source code and then execute it.. The machine has to have the C compiler. guess I am confused..Somebody please explain in simple language or may be direct me to a tutorial that explain things in simple language ? that would be great I am just not getting the concept.

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