I've use $$\frac{y^Ts}{y^ty}I$$ for the initial guess in the BFGS. Now I've been looking for the reference for this guess. Can someone tell me which reference does this comes from?
In section of L-BFGS, Numerical Optimization from Jorge Npcedal, you can find what you want. Best
Related
I've encountered the raw_native_types in a tlb import statement and I'm unclear of what the benefit of this attribute is. I've read the MSDN description for this (http://msdn.microsoft.com/en-us/library/8etzzkb6%28v=vs.71%29.aspx#_predir_raw_native_types) but it hasn't helped clearing it up for me. Can someone help explain it to me?
Thanks.
The intention defines the benefit. If one really doesn't want to use _bstr_t or _variant_t because, who knows, one abominates them (not that uncommon), or one doesn't trust the compiler to actually do a good job at keeping them alive, or one already uses other smart wrappers, or one really likes to explicitly call SysFreeString and VariantClear, then one specifies raw_native_types.
Everyone:
I've just recently finished implementing a "Soft Deleting" mechanism for my database driven web product using NHibernate. I followed the tutorial here for creating a SoftDeleteListener and it works like a charm, Kudos to the Gabriel Schenker who wrote that very good tutorial.
So, what's the problem? Well, I updated his code for the SoftDeleteListener I've implemented, and I tested it and all seems well. However, when I use a code template from a third party, I ALWAYS want to understand completely what every line of code is doing. In this case, however, I've searched and searched but I cannot find any documentation on the functions of the CascadeBeforeDelete and the CascadeAfterDelete methods! I can pretty much infer that they are doing some sort of foreign key cascading deletion functions, but the documentation on what exactly they are doing is nowhere to be found.
Can someone please point me to the proper documentation for these methods? Or, if none exists, could you kindly explain them to me?
Thanks in advance.
There's no documentation for those methods.
What you can do is get the NH sources and see how it's used in the code.
After Diego's suggestion, I've taken a quick look at the NHibernate sources for these files and, from the comment blocks in the source code, have determined their functions:
CascadeBeforeDelete - Cascades the delete call to all collections of that object before the object is deleted.
CascadeAfterDelete - Cascades the delete call to all many-to-one relationships to the object after the parent has been deleted
This was just taken from the comment directly, and I really don't have the time to thoroughly investigate these methods functions (tight deadlines keep me awake at night). If anyone has any more information on these methods they would like to share it would be greatly appreciated. Thanks.
What is the best way to implement a constructor for a record? It seems like a function should be able to return a record object in the instantiation of the record in some later model higher up the tree, but I can't get that to work. For now I just use a bunch of parameters at the top of the record that populate the variables stored in the record, but it seems like that will only work in simple cases.
Can anyone shed a little light? Perhaps I shouldn't be using a record but a model. Also does anyone know how the PDE functionality is coming? The book only says that it is coming, but I have seen some other things around.
I don't seem to have the clout to add tags (which makes sense, since my "reputation" is lower than yours) so sorry about that. I thought I had actually added one at one point, but perhaps I am mistaken.
I think you need to be clear what you mean by constructor since it has a very specific meaning in Modelica. If I understand your question correctly, it sounds like what you want to do is create an instance of a record that has some fields that are specified in the constructor arguments and from those arguments a bunch of other fields in the record are computed. Is that correct?
If so, there is a mechanism to do this. You mention "the book" but it isn't clear which one you mean. If it is mine, it definitely has no mention of these so called "record constructors" because it is too old. I do not know if Peter Fritzson's book mentions them either. However, they do exist and are documented in Section 12.6 of the Modelica 3.2 specification.
As for PDEs, there has been work into this kind of thing but nothing has really been done within the design group on this topic. I would add that if you want to solve either elliptical or parabolic PDEs on regular grids, this isn't too hard even with the current language. The only real drawback is that most tools probably don't handle sparsity very efficiently. Irregular grids would also be possible, but then you get into complicated basis functions. Finally, hyperbolic PDEs are, in my opinion, quite tricky (in any environment) due to the implicit physical constraints between time and space which are difficult to express (i.e. the CFL condition).
I hope that answers your questions so far.
I can only comment on your question regarding the book of Peter Fritzson. He confirmed that he's working on an update and he hopes to get it ready 'in the course of 2011'.
Original post here:
http://openmodelica.org/index.php/forum/topic?id=50
And thanks for initiating the modelica tag, I might be useful in the near future for me too... :-)
regards,
Roel
I've always hated comments that fill half the screen with asterisks just to tell you that the function returns a string, I never read those comments.
However, I do read comments that describe why something is done and how it's done (usually the single line comments in the code); those come in really handy when trying to understand someone else's code.
But when it comes to writing comments, I don't write that, rather, I use comments only when writing algorithms in programming contests, I'd think of how the algorithm will do what it does then I'd write each one in a comment, then write the code that corresponds to that comment.
An example would be:
//loop though all the names from n to j - 1
Other than that I can't imagine why anyone would waste valuable time writing comments when he could be writing code.
Am I right or wrong? Am I missing something? What other good use cases of comments am I not aware of?
Comments should express why you are doing something not what you are doing
It's an old adage, but a good metric to use is:
Comment why you're doing something, not how you're doing it.
Saying "loop through all the names from n to j-1" should be immediately clear to even a novice programmer from the code alone. Giving the reason why you're doing that can help with readability.
If you use something like Doxygen, you can fully document your return types, arguments, etc. and generate a nice "source code manual." I often do this for clients so that the team that inherits my code isn't entirely lost (or forced to review every header).
Documentation blocks are often overdone, especially is strongly typed languages. It makes a lot more sense to be verbose with something like Python or PHP than C++ or Java. That said, it's still nice to do for methods & members that aren't self explanatory (not named update, for instance).
I've been saved many hours of thinking, simply by commenting what I'd want to tell myself if I were reading my code for the first time. More narrative and less observation. Comments should not only help others, but yourself as well... especially if you haven't touched it in five years. I have some ten year old Perl that I wrote and I still don't know what it does anymore.
Something very dirty, that I've done in PHP & Python, is use reflection to retrieve comment blocks and label elements in the user interface. It's a use case, albeit nasty.
If using a bug tracker, I'll also drop the bug ID near my changes, so that I have a reference back to the tracker. This is in addition to a brief description of the change (inline change logs).
I also violate the "only comment why not what" rule when I'm doing something that my colleagues rarely see... or when subtlety is important. For instance:
for (int i = 50; i--; ) cout << i; // looping from 49..0 in reverse
for (int i = 50; --i; ) cout << i; // looping from 49..1 in reverse
I use comments in the following situations:
High-level API documentation comments, i.e. what is this class or function for?
Commenting the "why".
A short, high-level summary of what a much longer block of code does. The key word here is summary. If someone wants more detail, the code should be clear enough that they can get it from the code. The point here is to make it easy for someone browsing the code to figure out where some piece of logic is without having to wade through the details of how it's performed. Ideally these cases should be factored out into separate functions instead, but sometimes it's just not do-able because the function would have 15 parameters and/or not be nameable.
Pointing out subtleties that are visible from reading the code if you're really paying attention, but don't stand out as much as they should given their importance.
When I have a good reason why I need to do something in a hackish way (performance, etc.) and can't write the code more clearly instead of using a comment.
Comment everything that you think is not straightforward and you won't be able to understand the next time you see your code.
It's not a bad idea to record what you think your code should be achieving (especially if the code is non-intuitive, if you want to keep comments down to a minimum) so that someone reading it a later date, has an easier time when debugging/bugfixing. Although one of the most frustrating things to encounter in reading someone else's code is cases where the code has been updated, but not the comments....
I've always hated comments that fill half the screen with asterisks just to tell you that the function returns a string, I never read those comments.
Some comments in that vein, not usually with formatting that extreme, actually exist to help tools like JavaDoc and Doxygen generate documentation for your code. This, I think, is a good form of comment, because it has both a human- and machine-readable format for documentation (so the machine can translate it to other, more useful formats like HTML), puts the documentation close to the code that it documents (so that if the code changes, the documentation is more likely to be updated to reflect these changes), and generally gives a good (and immediate) explanation to someone new to a large codebase of why a particular function exists.
Otherwise, I agree with everything else that's been stated. Comment why, and only comment when it's not obvious. Other than Doxygen comments, my code generally has very few comments.
Another type of comment that is generally useless is:
// Commented out by Lumpy Cheetosian on 1/17/2009
...uh, OK, the source control system would have told me that. What it won't tell me is WHY Lumpy commented out this seemingly necessary piece of code. Since Lumpy is located in Elbonia, I won't be able to find out until Monday when they all return from the Snerkrumph holiday festival.
Consider your audience, and keep the noise level down. If your comments include too much irrelevant crap, developers will just ignore them in practice.
BTW: Javadoc (or Doxygen, or equiv.) is a Good Thing(tm), IMHO.
I also use comments to document where a specific requirement came from. That way the developer later can look at the requirement that caused the code to be like it was and, if the new requirement conflicts with the other requirment get that resolved before breaking an existing process. Where I work requirments can often come from different groups of people who may not be aware of other requirements then system must meet. We also get frequently asked why we are doing a certain thing a certain way for a particular client and it helps to be able to research to know what requests in our tracking system caused the code to be the way it is. This can also be done on saving the code in the source contol system, but I consider those notes to be comments as well.
Reminds me of
Real programmers don't write documentation
I wrote this comment a while ago, and it's saved me hours since:
// NOTE: the close-bracket above is NOT the class Items.
// There are multiple classes in this file.
// I've already wasted lots of time wondering,
// "why does this new method I added at the end of the class not exist?".
I'm trying to come up with some clear and concise names for a Permission class that lets you check if a permission is, was, or will be, allowed/denied. I'm at a loss for what to call the future tense.
class Permission:
def can_read()
def could_read()
def will_read()?
def will_be_readable()?
I'm most partial to will_read(), but it sounds funny. will_be_readable() is clear, but its kinda long, and will_be_read() sounds misleading.
Since you are looking for a permission class, the code will be formulated as a question:
if (the_user.can_read()) ...
if (the_user.can_read_past()) ...
if (the_user.can_read_future()) ...
Apart from that I would try to maintain equal prefixes for the semantically equal function, and I would leave off the suffix for the default/most likely case. I have no real problem sacrificing grammatical correctness for a logical naming structure, even though I like to gear code towards natural language. So something like that is what I would be going for.
The will_read() sounds more like an event that is triggered before the action.
EDIT: I am aware that "can_read_past" may be percieved as "can read old stuff". Maybe Scott Evernden's suggestion is better?
was, is, and will_be is clearest to me..
or maybe readable_before, readable_now, readable_later
Yeah, English really has a problem here, with no future tense of "can"! What about switching to (say) readable_now, readable_past, readable_future? If past and future have specific meanings in your case better naming could be worked out here, I'm sure.
Since it is related to if a permission is allowed/denied, maybe there is a more descriptive word than read?
can_authorize
authorized
scheduled_to_authorize