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  • On Ruby on Rails, <%= or <% should only matter whether it is show or no show, but why will it give

    - by Jian Lin
    The following code: <div id="vote_form"> <%= form_remote_tag :url => story_votes_path(@story) do %> <%= submit_tag 'shove it' %> <% end %> </div> gives compilation error while if the first <%= is replaced with <%, then everything works. I thought they only differ by "show" or "not show", but why will it actually cause a compile error? The error is: > SyntaxError in Stories#show > > Showing > app/views/stories/show.html.erb where > line #17 raised: > > compile error C:/Software > Projects/ror/shov12/app/views/stories/show.html.erb:17: > syntax error, unexpected ')' ... > story_votes_path(@story) do ).to_s); > @output_buffer.concat ... > ^ C:/Software > Projects/ror/shov12/app/views/stories/show.html.erb:23: > syntax error, unexpected kENSURE, > expecting ')' C:/Software > Projects/ror/shov12/app/views/stories/show.html.erb:25: > syntax error, unexpected kEND, > expecting ')'

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  • Cannot get a session with Facebook app? (using its Graph API)

    - by Jian Lin
    I have really simple few lines of Facebook app, using the new Facebook API: <pre> <?php require 'facebook.php'; // Create our Application instance. $facebook = new Facebook(array( 'appId' => '117676584930569', 'secret' => '**********', // hidden here on the post... 'cookie' => true, )); var_dump($facebook); ?> but it is giving me the following output: http://apps.facebook.com/woolaladev/i2.php would give out object(Facebook)#1 (6) { ["appId:protected"]=> string(15) "117676584930569" ["apiSecret:protected"]=> string(32) "**********" <--- just hidden on this post ["session:protected"]=> NULL <--- Session is NULL for some reason ["sessionLoaded:protected"]=> bool(false) ["cookieSupport:protected"]=> bool(true) ["baseDomain:protected"]=> string(0) "" } Session is NULL for some reason, but I am logged in and can access my home and profile and run other apps on Facebook (to see that I am logged on). I am following the sample on: http://github.com/facebook/php-sdk/blob/master/examples/example.php http://github.com/facebook/php-sdk/blob/master/src/facebook.php (download using raw URL: wget http://github.com/facebook/php-sdk/raw/master/src/facebook.php ) Trying on both hosting companies at dreamhost.com and netfirms.com, and the results are the same.

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  • When actually is a closure created?

    - by Jian Lin
    Is it true that a closure is created in the following cases for foo, but not for bar? Case 1: <script type="text/javascript"> function foo() { } </script> foo is a closure with a scope chain with only the global scope. Case 2: <script type="text/javascript"> var i = 1; function foo() { return i; } </script> same as Case 1. Case 3: <script type="text/javascript"> function Circle(r) { this.r = r; } Circle.prototype.foo = function() { return 3.1415 * this.r * this.r } </script> in this case, Circle.prototype.foo (which returns the circle's area) refers to a closure with only the global scope. (this closure is created). Case 4: <script type="text/javascript"> function foo() { function bar() { } } </script> here, foo is a closure with only the global scope, but bar is not a closure (yet), because the function foo is not invoked in the code, so no closure goo is ever created. It will only exist if foo is invoked , and the closure bar will exist until foo returns, and the closure bar will then be garbage collected, since there is no reference to it at all anywhere. So when the function doesn't exist, can't be invoked, can't be referenced, then the closure doesn't exist yet (never created yet). Only when the function can be invoked or can be referenced, then the closure is actually created?

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  • In SQL, if we rename INNER JOIN as INTERSECT JOIN, LEFT OUTER JOIN as LEFT UNION JOIN, and FULL OUTE

    - by Jian Lin
    In SQL, the name Join gives an idea of "merging" or a sense of "union", making something bigger. But in fact, as in the other post http://stackoverflow.com/questions/2706051/in-sql-a-join-is-actually-an-intersection-and-it-is-also-a-linkage-or-a-sidew it turns out that a Join (Inner Join) is actually an Intersection. So if we think of Join = Inner Join = Intersect Join Left Outer Join = Left Union Join Full Outer Join = Full Union Join = Union Join then we always get a feel of what's happening, and maybe never forget what they are easily. In a way, we can think of Intersect as "making it less", therefore it is excluding something. That's why the name "Join" won't go with the idea of "Intersect". But in fact, both Intersect and Union can be thought of as: Union: bringing something together and merge them unconditionally. Intersect: bringing something together and merge them based on some condition. so the "bringing something together" is probably what "Join" is all about. It is like, Intersection is a "half glass of water" -- we can thinking of it as "excluding something" or as "bringing something together and accepting the common ones". So if the word "Intersect Join" is used, maybe a clear picture is there, and "Union Join" can be a clear picture too. Maybe the word "Inner Join" and "Outer Join" is very clear when we use SQL a lot. Somehow, the word "Outer" tends to give a feeling that it is "outside" and excluding something rather than a "Union".

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  • In Ruby, what is good way to filter all the methods of an object that contain the word "time" in it?

    - by Jian Lin
    I tried the following and it was partly working: >> s.methods.map {|n| n if n =~ /time/} => [nil, nil, nil, nil, nil, nil, nil, nil, "skip_time_zone_conversion_for_attri butes", nil, nil, nil, nil, nil, nil, nil, nil, nil, nil, nil, nil, nil, nil, ni l, nil, nil, nil, nil, nil, nil, nil, nil, nil, nil, nil, nil, nil, nil, nil, ni l, "timestamped_migrations", nil, nil, nil, nil, nil, nil, nil, nil, nil, nil, n il, nil, nil, nil, nil, nil, nil, nil, nil, nil, nil, nil, nil, nil, nil, nil, n il, nil, nil, nil, nil, nil, nil, nil, nil, nil, nil, nil, nil, nil, nil, nil, n il, nil, nil, nil, nil, nil, nil, nil, nil, nil, nil, nil, nil, nil, nil, nil, n il, nil, nil, nil, nil, nil, nil, nil, nil, nil, nil, nil, nil, "time_zone_aware _attributes", nil, nil, nil, nil, nil, nil, nil, nil, nil, nil, nil, nil, nil, n il, nil, nil, nil, nil, nil, nil, nil, nil, nil, nil, "default_timezone", nil, n il, nil, nil, nil, nil, nil, nil, nil, nil, nil, nil, nil, nil, nil, nil, nil, n il, nil, nil, nil, nil, nil, nil, nil, nil, nil, nil, nil, nil, nil, nil, nil, n il, nil, nil, nil, nil, nil, nil, nil, nil, nil, nil, nil, nil, nil, nil, nil, n il, nil, nil, nil, nil, nil, nil, nil, nil, nil, nil, nil, nil, nil, nil, "recor d_timestamps", nil, nil, nil, nil, nil, nil, nil, nil, nil, nil, nil, nil, nil, nil, nil, nil, nil, nil, nil, nil] >> s.methods.each {|n| p n if n =~ /time/} "skip_time_zone_conversion_for_attributes" "timestamped_migrations" "time_zone_aware_attributes" "default_timezone" "record_timestamps" => ["extended_by", "before_create", "vote_ids=", "save_without_dirty", "_delete" , "touch", "daemonize", "after_destroy", "skip_time_zone_conversion_for_attribut es", "methods", "send", "to_query", "becomes", "after_validation", "store_full_s ti_class?", "save_with_transactions!", "autosave_associated_records_for_votes", "require_library_or_gem", "enum_for", "taint", "instance_variable_defined?", "ac [...] and the rest of the whole array >> s.methods.filter {|n| n =~ /time/} NoMethodError: undefined method `filter' for #<Array:0x4de6b00> from (irb):93

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  • differences between using wmode="transparent", "opaque", "window" for an embedded object on webpage

    - by Jian Lin
    when embedding a Flash object with the <object and <embed tag, there is an attribute called "wmode". It seems that most of the time, wmode="transparent" is the same as wmode="opaque" as the Flash doesn't actually have any transparent color so that the bottom HTML element is to be shown. As a result, "opaque" should be faster than "transparent" since it require less processing for transparency, yet most of the time i see Flash object embedded with "transparent" instead of "opaque". "opaque" is needed so that other HTML element won't be covered up by the Flash object. (such as a menu item that pops up an extra sub-menu won't be covered up by the Flash object). By the way, is there formal documentation for wmode's "opaque", "transparent", and "window"? I was only able to find blogs that describe it but not the formal documentation. thanks.

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  • How to write a recursive function that returns a linked list of nodes, when given a binary tree of n

    - by Jian Lin
    I was once asked of this in an interview: How to write a recursive function that returns a linked list of nodes, when given a binary tree of nodes? (flattening the data) For some reason, I tend to need more than 3 to 5 minutes to solve any recursive problem. Usually, 15 to 20 minutes will be more like it. How could we attack this problem, such as a very systematic way of reaching a solution, so that they can be solved in 3 to 5 minute time frame?

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  • In languages which create a new scope each time in a loop block, a new local copy of the local loop

    - by Jian Lin
    It seems that in language like C, Java, and Ruby (as opposed to Javascript), a new scope is created for each iteration of a loop block, and the local variable defined for the loop is actually made into a local variable every single time and recorded in this new scope? For example, in Ruby: p RUBY_VERSION $foo = [] (1..5).each do |i| $foo[i] = lambda { p i } end (1..5).each do |j| $foo[j].call() end the print out is: [MacBook01:~] $ ruby scope.rb "1.8.6" 1 2 3 4 5 [MacBook01:~] $ So, it looks like when a new scope is created, a new local copy of i is also created and recorded in this new scope, so that when the function is executed at a later time, the "i" is found in those scope chains as 1, 2, 3, 4, 5 respectively. Is this true? (It sounds like a heavy operation). Contrast that with p RUBY_VERSION $foo = [] i = 0 (1..5).each do |i| $foo[i] = lambda { p i } end (1..5).each do |j| $foo[j].call() end This time, the i is defined before entering the loop, so Ruby 1.8.6 will not put this i in the new scope created for the loop block, and therefore when the i is looked up in the scope chain, it always refer to the i that was in the outside scope, and give 5 every time: [MacBook01:~] $ ruby scope2.rb "1.8.6" 5 5 5 5 5 [MacBook01:~] $ I heard that in Ruby 1.9, i will be treated as a local defined for the loop even when there is an i defined earlier? The operation of creating a new scope, creating a new local copy of i each time through the loop seems heavy, as it seems it wouldn't have matter if we are not invoking the functions at a later time. So when the functions don't need to be invoked at a later time, could the interpreter and the compiler to C / Java try to optimize it so that there is not local copy of i each time?

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

    - 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 SQL / MySQL, can a Left Outer Join be used to find out the duplicates when there is no Primary ID

    - by Jian Lin
    I would like to try using Outer Join to find out duplicates in a table: If a table has Primary Index ID, then the following outer join can find out the duplicate names: mysql> select * from gifts; +--------+------------+-----------------+---------------------+ | giftID | name | filename | effectiveTime | +--------+------------+-----------------+---------------------+ | 2 | teddy bear | bear.jpg | 2010-04-24 04:36:03 | | 3 | coffee | coffee123.jpg | 2010-04-24 05:10:43 | | 6 | beer | beer_glass.png | 2010-04-24 05:18:12 | | 10 | heart | heart_shape.jpg | 2010-04-24 05:11:29 | | 11 | ice tea | icetea.jpg | 2010-04-24 05:19:53 | | 12 | cash | cash.png | 2010-04-24 05:27:44 | | 13 | chocolate | choco.jpg | 2010-04-25 04:04:31 | | 14 | coffee | latte.jpg | 2010-04-27 05:49:52 | | 15 | coffee | espresso.jpg | 2010-04-27 06:03:03 | +--------+------------+-----------------+---------------------+ 9 rows in set (0.00 sec) mysql> select * from gifts g1 LEFT JOIN (select * from gifts group by name) g2 on g1.giftID = g2.giftID where g2.giftID IS NULL; +--------+--------+--------------+---------------------+--------+------+----------+---------------+ | giftID | name | filename | effectiveTime | giftID | name | filename | effectiveTime | +--------+--------+--------------+---------------------+--------+------+----------+---------------+ | 14 | coffee | latte.jpg | 2010-04-27 05:49:52 | NULL | NULL | NULL | NULL | | 15 | coffee | espresso.jpg | 2010-04-27 06:03:03 | NULL | NULL | NULL | NULL | +--------+--------+--------------+---------------------+--------+------+----------+---------------+ 2 rows in set (0.00 sec) But what if the table doesn't have a Primary Index ID, then can an outer join still be used to find out duplicates?

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  • Can Git or Mercurial be set to bypass the local repository and go straight to the central one?

    - by Jian Lin
    Using Git or Mercurial, if the working directory is 1GB, then the local repository will be another 1GB (at least), residing normally in the same hard drive. And then when pushed to a central repository, there will be another 1GB. Can Git or Mercurial be set to use only a working directory and then a central repository, without having 3 copies of this 1GB data? (actually, when the central repository also update, then there are 4 copies of the same data... can it be reduced? In the SVN scenario, when there are 5 users, then there will be 6GB of data total. With Distributed Version Control, then there will be 12GB of data?)

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  • Can Ruby on Rails handle AJAX Response by Static Javascript code? by using RJS?

    - by Jian Lin
    So it looks like on RoR, when Ajax (using form_remote_tag) returns a success code, Javascript is also returned to handle the visual effects. (this is the RJS mechanism) using Fiddler, I do see the following response: try { Element.update("vote_score", "Score 58"); $("vote_score").visualEffect("highlight"); } catch (e) { alert('RJS error:\n\n' + e.toString()); alert('Element.update(\"vote_score\", \"Score 58\");\n$(\"vote_score\").visualEffect(\"highlight\");'); throw e } Will there be situation where the code is quite big (like 1 or 2k) that RJS can use some sort of static Javascript already loaded to handle the effect? Is there any other ways to use static Javascript already loaded with the page? Must it be just raw Javascript and use the Prototype's Ajax success response to call the static Javascript code, or can it be some Rails mechanism related to RJS?

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  • If Inner Join can be thought of as Cross Join but filtering out the records satisfying the condition

    - by Jian Lin
    If an Inner Join can be thought of as a cross join and then getting the records that satisfy the condition, then a LEFT OUTER JOIN can be thought of as that, plus ONE record on the left table that doesn't satisfy the condition. In other words, it is not a cross join that "goes easy" on the left records (even when the condition is not satisfied), because then the left record can appear many times (as many as how many records there are in the right table). So the LEFT OUTER JOIN is the Cross JOIN with the records satisfying the condition, plus ONE record from the LEFT TABLE that doesn't satisfy the condition.

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  • In Ruby on Rails, why will story.votes return an empty Array object, but story.votes.create will act

    - by Jian Lin
    In Ruby on Rails, say a Story object can "has_many" Vote objects (a story is voted "hot" by many users). So when we do a s = Story.find(:first) s is a Story object, and say s.votes returns [] and s.votes.class returns Array So clearly, s.votes is an empty Array object. At this time, when s.votes.create is called, it actually invokes a method of the Vote class? How come an Array class object can invoke a Vote class method?

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  • In Ruby, how to implement 20 - point and point - 20 using coerce() ?

    - by Jian Lin
    In Ruby, the operation of point - 20 20 - point are to be implemented. But the following code: class Point attr_accessor :x, :y def initialize(x,y) @x, @y = x, y end def -(q) if (q.is_a? Fixnum) return Point.new(@x - q, @y - q) end Point.new(@x - q.x, @y - q.y) end def -@ Point.new(-@x, -@y) end def *(c) Point.new(@x * c, @y * c) end def coerce(something) [self, something] end end p = Point.new(100,100) q = Point.new(80,80) p (-p) p p - q p q - p p p * 3 p 5 * p p p - 30 p 30 - p Output: #<Point:0x2424e54 @x=-100, @y=-100> #<Point:0x2424dc8 @x=20, @y=20> #<Point:0x2424d3c @x=-20, @y=-20> #<Point:0x2424cc4 @x=300, @y=300> #<Point:0x2424c38 @x=500, @y=500> #<Point:0x2424bc0 @x=70, @y=70> #<Point:0x2424b20 @x=70, @y=70> 30 - p will actually be taken as p - 30 by the coerce function. Can it be made to work? I am actually surprise that the - method won't coerce the argument this way: class Fixnum def -(something) if (/* something is unknown class */) a, b = something.coerce(self) return -(a - b) # because we are doing a - b but we wanted b - a, so it is negated end end end that is, the function returns a negated version of a - b instead of just returning a - b.

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  • Does jQuery have a plugin to display a "message bar" like the Twitter "wrong password" bar at the to

    - by Jian Lin
    Twitter will pop down a message bar at the top of the screen say "Wrong password" and after 10 seconds, it will slide up and disappear. Chrome also shows "Do you want to save the password" message box using such a way. Does jQuery have a plug in to do that already? Does it also work in IE 6? Because usually, the display of relative to the viewport (using position: fixed) will not work on IE 6. thanks.

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  • Can a binary tree or tree be always represented in a Database as 1 table and self-referencing?

    - by Jian Lin
    I didn't feel this rule before, but it seems that a binary tree or any tree (each node can have many children but children cannot point back to any parent), then this data structure can be represented as 1 table in a database, with each row having an ID for itself and a parentID that points back to the parent node. That is in fact the classical Employee - Manager diagram: one boss can have many people under him... and each person can have n people under him, etc. This is a tree structure and is represented in database books as a common example as a single table Employee.

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  • Can MySQL / SQL's short hand of "Using" be used without saying "Inner Join" ?

    - by Jian Lin
    The following 2 statements are to join using gifts.giftID = sentgifts.giftID: mysql> select * from gifts, sentgifts using (giftID); ERROR 1064 (42000): You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near 'using (giftID)' at line 1 and the second one: mysql> select * from gifts INNER JOIN sentgifts using (giftID); +--------+------------+----------------+---------------------+--------+------------+--------+------+---------------------+ | giftID | name | filename | effectiveTime | sentID | whenSent | fromID | toID | trytryWhen | +--------+------------+----------------+---------------------+--------+------------+--------+------+---------------------+ | 2 | teddy bear | bear.jpg | 2010-04-24 04:36:03 | 4 | 2010-04-24 | NULL | 111 | 2010-04-24 03:10:42 | | 6 | beer | beer_glass.png | 2010-04-24 05:18:12 | 5 | 2010-03-03 | 11 | 22 | 2010-03-03 00:00:00 | | 6 | beer | beer_glass.png | 2010-04-24 05:18:12 | 6 | 2010-04-24 | 11 | 222 | 2010-04-24 03:54:49 | | 6 | beer | beer_glass.png | 2010-04-24 05:18:12 | 7 | 2010-04-24 | 1 | 2 | 2010-04-24 03:58:45 | +--------+------------+----------------+---------------------+--------+------------+--------+------+---------------------+ 4 rows in set (0.00 sec) Can the first statement also use the "using" shorthand? It seems that when it is used then the word "Inner Join" must be specified... but the first statement is actually an inner join?

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  • Is using Natural Join or Implicit column names not a good practice when writing SQL in a programming

    - by Jian Lin
    When we use Natural Join, we are joining the tables when both table have the same column names. But what if we write it in PHP and then the DBA add some more fields to both tables, then the Natural Join can break? The same goes for Insert, if we do a insert into gifts values (NULL, "chocolate", "choco.jpg", now()); then it will break the code as well as contaminating the table when the DBA adds some fields to the table (example as column 2 or 3). So it is always best to spell out the column names when the SQL statements are written inside a programming language and stored in a file in a big project.

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