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  • Does Ruby on Rails "has_many" array provide data on a "need to know" basis?

    - by Jian Lin
    On Ruby on Rails, say, if the Actor model object is Tom Hanks, and the "has_many" fans is 20,000 Fan objects, then actor.fans gives an Array with 20,000 elements. Probably, the elements are not pre-populated with values? Otherwise, getting each Actor object from the DB can be extremely time consuming. So it is on a "need to know" basis? So does it pull data when I access actor.fans[500], and pull data when I access actor.fans[0]? If it jumps from each record to record, then it won't be able to optimize performance by doing sequential read, which can be faster on the hard disk because those records could be in the nearby sector / platter layer -- for example, if the program touches 2 random elements, then it will be faster just to read those 2 records, but what if it touches all elements in random order, then it may be faster just to read all records in a sequential way, and then process the random elements. But how will RoR know whether I am doing only a few random elements or all elements in random?

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  • Thinking of an Inner Join as a Cross Join and then satisfying some condition(s)?

    - by Jian Lin
    It seems like the safest way to think of an Inner Join is to think of it as a Cross Join and then satisfying some condition(s)? Because the equi-join can be obvious, but the non-equi-join can be a bit confusing. But if we always use the Cross Join, and then filter out the ones satisfying the condition, then we get the resulting table. In other words, we can always analyze it by using the first record on the left table, and then go through every single records on the right, and then repeat that for 2nd record on the left, and for the 3rd, 4th, ... etc. So in our mind, we can analyze it using this way, and it is like O(n^2), although what happens in the DBMS maybe that it is a lot faster (when an index is present). Is there another good way to think of it besides this method?

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  • Why is the ( ) mandatory in the SQL statement select * from gifts INNER JOIN sentgifts using (giftID

    - by Jian Lin
    Why is the ( ) mandatory in the SQL statement select * from gifts INNER JOIN sentgifts using (giftID); ? The ( ) usually is for specifying grouping of something. But in this case, are we supposed to be able to use 2 or more field names... in the example above, it can be all clear that it is 1 field, is it just that the parser is not made to bypass the ( ) when it is all clear? (such as in the language Ruby).

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  • Can GIT, Mercurial, SVN, or other version control tools work well when project tree has binary files

    - by Jian Lin
    Sometimes our project tree can have binary files, such as jpg, png, doc, xls, or pdf. Can GIT, Mercurial, SVN, or other tools do a good job when only part of a binary file is changed? For example, if the spec is written in .doc and it is part of the repository, then if it is 4MB, and edited 100 times but just for 1 or 2 lines, and checked in 100 times during the year, then it is 400MB. If it is 100 different .doc and .xls files, then it is 40GB... not a size that is easy to manage. I have tried GIT and Mercurial and see that they both seem to add a big size of data even when 1 line is changed in a .doc or .pdf. Is there other way inside of GIT or Mercurial or SVN that can do the job?

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  • Strange behaviour with mediaplayer and seekTo

    - by Mathias Lin
    I'm implementing a custom video player because I need custom video controls. I have an app with only one activity, which on startup shall start playing a video right away. Now, the problem I have is: I don't want the video to start from the beginning, but from a later position. Therefore I do a seekTo(16867). Since seekTo is asynchronous, I place the start call of the mediaplayer (player.start()) in the onSeekComplete of the onSeekCompleteListener. The strange behaviour I experience though is that I can see/hear the video playing from the beginning for a few millisecs before it actually plays from/jumps to the position I seeked to. But - on the other hand - the Log output I call before the player.start returns the correct position 16867, where I seeked to. Below is the relevant code section, the complete class is at http://pastebin.com/jqAAFsuX (I'm on Nexus One / 2.2 StageFright) private void playVideo(String url) { try { btnVideoPause.setEnabled(false); if (player==null) { player=new MediaPlayer(); player.setScreenOnWhilePlaying(true); } else { player.stop(); player.reset(); } url = "/sdcard/myapp/main/videos/main.mp4"; // <--- just for test purposes hardcoded here now player.setDataSource(url); player.setDisplay(holder); player.setAudioStreamType(AudioManager.STREAM_MUSIC); player.setOnCompletionListener(this); player.setOnPreparedListener(this); player.setOnSeekCompleteListener(new MediaPlayer.OnSeekCompleteListener() { public void onSeekComplete(MediaPlayer mediaPlayer) { Log.d("APP", "current pos... "+ player.getCurrentPosition() ); player.start(); // <------------------ start video on seek completed player.setOnSeekCompleteListener(null); } }); player.prepareAsync(); } catch (Throwable t) { Log.e(TAG, "Exception in btnVideoPause prep", t); } } public void onPrepared(MediaPlayer mediaplayer) { width=player.getVideoWidth(); height=player.getVideoHeight(); if (width!=0 && height!=0) { holder.setFixedSize(width, height); progressBar.setProgress(0); progressBar.setMax(player.getDuration()); player.seekTo(16867); // <------------------ seeking to position } btnVideoPause.setEnabled(true); }

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  • It seems like the safest way to think of an Inner Join is to think of it as a Cross Join and then sa

    - by Jian Lin
    It seems like the safest way to think of an Inner Join is to think of it as a Cross Join and then satisfying some condition(s)? Because the equi-join can be obvious, but the non-equi-join can be a bit confusing. But if we always use the Cross Join, and then filter out the ones satisfying the condition, then we get the resulting table. In other words, we can always analyze it by using the first record on the left table, and then go through every single records on the right, and then repeat that for 2nd record on the left, and for the 3rd, 4th, ... etc. So in our mind, we can analyze it using this way, and it is like O(n^2), although what happens in the DBMS maybe that it is a lot faster (when an index is present). Is there another good way to think of it besides this method?

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  • In CSS, how to not float a 300px wide Div to the next line?

    - by Jian Lin
    Say, there is a bar that is styled at the bottom of the viewport, using position: fixed; bottom: 0; left: 0; width: 100%; height 50px; overflow: hidden and then there are 4 Divs inside it, each one floated to the left. Each Div is about 300px wide or can be more (depending on the content) Now, when the window is 1200 pixel wide, and we see all 4 Divs, but when the window is resize to be 1180 pixel wide (just 20 pixels less), then the whole 300px wide Div will disappear, because it is "floated" to the next line. So how can this be made so that, the Div will stay there and showing 280px of itself, rather than totally disappear? By the way, white-space: nowrap won't work as that probably has to do with not wrapping inline content. I was thinking of putting another Div inside this Div, having a fixed width of 1200px or 2000px, so that all Divs will float on the same level in this inner Div, and the outer Div will cut it off with the overflow: hidden. But this seems more like a hack... since the wide of all those Divs can be dynamic, and setting a fixed width of 1200px or 2000px seems like too much of a hack.

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  • Using jQuery's animate(), if the clicked on element is "<a href="#" ...> </a>", the fucntion should

    - by Jian Lin
    I was reading jQuery's page for animate() http://api.jquery.com/animate/ Its examples don't mention about if using <a href="#" id="clickme">click me</a> ... $('#clickme').click(function() { $('#someDiv').animate({left: "+=60"}); }) we actually still have to return false like in the old days? $('#clickme').click(function() { $('#someDiv').animate({left: "+=60"}); return false; }) (but then, those examples didn't use a <a> for the "click me"... but used something else. Otherwise the page will jump back to the beginning of the page? Does jQuery have a more elegant or magical way of doing it?

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  • In Ruby, why is a method invocation not be able to be treated as a unit when "do" and "end" is used?

    - by Jian Lin
    The following question is related to the question "Ruby Print Inject Do Syntax". My question is, can we insist on using do and end and make it work with puts or p? This works: a = [1,2,3,4] b = a.inject do |sum, x| sum + x end puts b # prints out 10 so, is it correct to say, inject is a class method of the Array class, which takes a block of code, and then returns a number. If so, then it should be no different from calling a function and getting back a return value: b = foo(3) puts b or b = circle.getRadius() puts b In the above two cases, we can directly say puts foo(3) puts circle.getRadius() so, there is no way to make it work directly by using the following 2 ways: a = [1,2,3,4] puts a.inject do |sum, x| sum + x end but it gives ch01q2.rb:7:in `inject': no block given (LocalJumpError) from ch01q2.rb:4:in `each' from ch01q2.rb:4:in `inject' from ch01q2.rb:4 grouping the method call using ( ) doesn't work either: a = [1,2,3,4] puts (a.inject do |sum, x| sum + x end) and this gives: ch01q3.rb:4: syntax error, unexpected kDO_BLOCK, expecting ')' puts (a.inject do |sum, x| ^ ch01q3.rb:4: syntax error, unexpected '|', expecting '=' puts (a.inject do |sum, x| ^ ch01q3.rb:6: syntax error, unexpected kEND, expecting $end end) ^ finally, the following version works: a = [1,2,3,4] puts a.inject { |sum, x| sum + x } but why doesn't the grouping of the method invocation using ( ) work in the earlier example? What if a programmer insist that he uses do and end, can it be made to work?

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  • An exception to avoid copy and paste code?

    - by Jian Lin
    There are many files in our project that would call a Facebook api. And the call is complicated, spanning usually 8 lines or more, just for the argument values. In this case, we can make it into a function, and place that function in a common_library.php, but doing so would just change the name of the function call from the Facebook API function to our name, and we still need to repeat the 8 lines of arguments. Each time, the call is very similar, but with slight variations to the argument values. In that case, would copy and paste be needed no matter what we do?

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  • Using Mercurial (hg), how to push just one file or one directory out?

    - by Jian Lin
    Using Mercurial, we can commit one file by using hg commit file.rb or 1 folder hg commit foldername But how can we push just 1 file or 1 folder out? The whole project can be pushed using hg push ssh://[email protected]//project/code/preliminary but there seems to be no way to push out just 1 file or 1 folder? I tried the following and they don't work: hg push ssh://[email protected]//project/code/preliminary app/views/index.html.erb or hg push ssh://[email protected]//project/code/preliminary/app/views/index.html.erb

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  • is there a rule in CSS that says class="one two" vs class="two one" and the second will override the

    - by Jian Lin
    Right now, there seems to be no such effect. Example: both divs are orange in color on Firefox <!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN" "http://www.w3.org/TR/html4/strict.dtd"> <html> <head> <style> .one { border: 6px dashed green } .two { border: 6px dashed orange } </style> </head> <body> <div class="one two"> hello world </div> <div class="two one"> hello world </div>

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  • How to skip all the column names in MySQL when the table has auto increment primary key?

    - by Jian Lin
    A table is: mysql> desc gifts; +---------------+-------------+------+-----+---------+----------------+ | Field | Type | Null | Key | Default | Extra | +---------------+-------------+------+-----+---------+----------------+ | giftID | int(11) | NO | PRI | NULL | auto_increment | | name | varchar(80) | YES | | NULL | | | filename | varchar(80) | YES | | NULL | | | effectiveTime | datetime | YES | | NULL | | +---------------+-------------+------+-----+---------+----------------+ the following is ok: mysql> insert into gifts -> values (10, "heart", "heart_shape.jpg", now()); Query OK, 1 row affected (0.05 sec) but is there a way to not specify the "10"... and just let each one be 11, 12, 13... ? I can do it using mysql> insert into gifts (name, filename, effectiveTime) -> values ("coffee", "coffee123.jpg", now()); Query OK, 1 row affected (0.00 sec) but the column names need to be all specified. Is there a way that they don't have to be specified and the auto increment of primary key still works? thanks.

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  • In Ruby, why does a method invocation not be able to be treated as a unit when "do" and "end" is use

    - by Jian Lin
    The following question is related to http://stackoverflow.com/questions/2127836/ruby-print-inject-do-syntax The question is, can we insist on using DO and END and make it work with puts or p? This works: a = [1,2,3,4] b = a.inject do |sum, x| sum + x end puts b # prints out 10 so, is it correct to say, inject is a class method of the Array class, which takes a block of code, and then returns a number. If so, then it should be no different from calling a function and getting back a return value: b = foo(3) puts b or b = circle.getRadius() puts b In the above two cases, we can directly say puts foo(3) puts circle.getRadius() so, there is no way to make it work directly by using the following 2 ways: a = [1,2,3,4] puts a.inject do |sum, x| sum + x end but it gives ch01q2.rb:7:in `inject': no block given (LocalJumpError) from ch01q2.rb:4:in `each' from ch01q2.rb:4:in `inject' from ch01q2.rb:4 grouping the method call using ( ) doesn't work either: a = [1,2,3,4] puts (a.inject do |sum, x| sum + x end) and this gives: ch01q3.rb:4: syntax error, unexpected kDO_BLOCK, expecting ')' puts (a.inject do |sum, x| ^ ch01q3.rb:4: syntax error, unexpected '|', expecting '=' puts (a.inject do |sum, x| ^ ch01q3.rb:6: syntax error, unexpected kEND, expecting $end end) ^ finally, the following version works: a = [1,2,3,4] puts a.inject { |sum, x| sum + x } but why doesn't the grouping of the method invocation using ( ) work? What if a programmer insists that he uses do and end, can it be made to work directly with p or puts, without an extra temporary variable?

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  • What is a SQL statement that can tally up the counts even including the Zeros? (all in 1 statement)

    - by Jian Lin
    A SQL statement can give a list of the most popular gifts that are sent in a Social application, all the way to the ones that are sent 1, or 2 times, but it won't include the Zeros. I think the same goes for getting the list of the most popular Classes that students are registering for, when the registration process for all students is 10 days and now it is the 3rd day. Again, we get the count but the Zeros are not there. Is there a simple SQL statement that can show the whole list, including the zeros?

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  • Firebug's "net" tab is not showing anything?

    - by Jian Lin
    I usually run Fiddler for net traffic monitoring and now am using a Mac machine. I thought Firebug's net tab can show the traffic that is fetched through AJAX (the net tab is enabled). But if I try google.com, and type in something, its "google suggest" will show a bunch of suggestions, but the Firebug's "net" tab is not showing anything?

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  • In SQL, can we always write an inner join statement as a main query and subquery if we only want to

    - by Jian Lin
    In SQL, can we always write an inner join statement as a main query and subquery or vice versa if we only want to find the intersection? For example, select * from gifts g where g.giftID in (select giftID from sentGifts); can do a join and show the gifts sent in the sentGifts table, but it won't be able to show the sentTime because that is inside the subquery. But if all we care is to find the intersection, without caring what is being displayed, then we can always convert one to the other?

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  • What is the exact rule of jQuery's animate() parameters?

    - by Jian Lin
    jQuery 1.4.2's animate() API spec is .animate( properties, [ duration ], [ easing ], [ callback ] ) but it seems that we can supply duration, callback, and no easing .animate({left: '+= 100'}, 600, doThis) and it will work. But if we supply easing and callback and no duration .animate({left: '+=100'}, 'swing', doThis) then the easing won't be taken into effect. So what exactly is the API supposed to be?

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  • Recreation of DB using "mysql mydb < mydb.sql" is really slow when the table has tens of millions of

    - by Jian Lin
    It seems that a MySQL database that has a table with tens of millions of records will get a big INSERT INTO statement when the following mysqldump some_db > some_db.sql is done to back up the database. (is it 1 insert statement that handles all the records?) So when reconstructing the DB using mysql some_db < some_db.sql then the CPU is hardly busy (about 1.8% usage by the mysql process... I don't see a mysqld either?) and also the hard disk doesn't seem to be too busy... Last time, the whole restore process took 5 hours. Is there a way to make it faster? Such as, when doing mysqldump, can it break the INSERT statement into shorter ones, so that the mysql doesn't need to parse the line so hard when restoring the DB?

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  • In Javascript, is it true that function aliasing works as long as the function being aliased doesn't

    - by Jian Lin
    In Javascript, if we are aliasing a function, such as in: f = g; f = obj.display; obj.f = foo; all the 3 lines above, they will work as long as the function / method on the right hand side doesn't touch this? Since we are passing in all the arguments, the only way it can mess up is when the function / method on the right uses this? Actually, line 1 is probably ok if g is also a property of window? If g is referencing obj.display, then the same problem is there. In line 2, when obj.display touches this, it is to mean the obj, but when f() is invoked, the this is window, so they are different. In line 3, it is the same: when f() is invoked inside of obj's code, then the this is obj, while foo might be using this to refer to window if it were a property of window. (global function). So line 2 can be written as f = function() { obj.display.apply(obj, arguments) } and line 3: obj.f = function() { foo.apply(window, arguments) } Is this the correct method, and are there there other methods besides this?

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  • How to install python package without copy everything into lib/site-packages?

    - by Victor Lin
    I want to develop a common python package, I got other packages depends on it. For example: packageA/ packageB/ packageC/ commonPackage/ packageA, packageB and packageC can all be executed directly, but they are all depend on commonPackage. I want to install the commonPackage into lib/site-packages, but I don't want it copys the source code. Instead, I want it creates a commonPackage.pth in lib/site-packages with the path of where the commonPackage at. So that when I modify commonPackage or update it from SVN, I don't need to install it again. Here comes the problem, how can I write the setup.py or use the options of python setup.py install so that it would do what I want?

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  • In CSS, can "#footer #caption" coexist with "#content # caption"?

    - by Jian Lin
    I was going to "nest" the CSS ids like this #content #caption { color: teal } ... #footer #caption { margin: 2em 1em } because that's the way SASS (a CSS generator) can do nesting for... but then in one HTML document, we cannot have two ids with the same name, isn't that true, so the above nesting won't work or won't work well. (esp if document.getElementById() or $('#caption') or $('caption') is needed to select the element). We can use #content #content_caption { color: teal } ... #footer #footer_caption { margin: 2em 1em } but then why 1 more level of nesting? why not just #content_caption { color: teal } ... #footer_caption { margin: 2em 1em } ?

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