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• What are some practical uses of generating all permutations of a list, such as ['a', 'b', 'c'] ?

  - by Jian Lin

  I was asked by somebody in an interview for web front end job, to write a function that generates all permutation of a string, such as "abc" (or consider it ['a', 'b', 'c']). so the expected result from the function, when given ['a', 'b', 'c'], is abc acb bac bca cab cba Actually in my past 20 years of career, I have never needed to do something like that, especially when doing front end work for web programming. What are some practical use of this problem nowadays, in web programming, front end or back end, I wonder? As a side note, I kind of feel that expecting a result in 3 minutes might be "either he gets it or he doesn't", especially I was thinking of doing it by a procedural, non-recursive way at first. After the interview, I spent another 10 minutes and thought of how to do it using recursion, but expecting it to be solved within 3 minutes... may not be a good test of how qualified he is, especially for front end work.

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• In Javascript, a function starts a new scope, but we have to be careful that the function must be in

  - by Jian Lin

  In Javascript, I am sometimes too immerged in the idea that a function creates a new scope, that sometimes I even think the following anonymous function will create a new scope when it is being defined and assigned to onclick: <a href="#" id="link1">ha link 1</a> <a href="#" id="link2">ha link 2</a> <a href="#" id="link3">ha link 3</a> <a href="#" id="link4">ha link 4</a> <a href="#" id="link5">ha link 5</a> <script type="text/javascript"> for (i = 1; i <= 5; i++) { document.getElementById('link' + i).onclick = function() { var x = i; alert(x); return false; } } </script> but in fact, the anonymous function will create a new scope, that's right, but ONLY when it is being invoked, is that so? So the x inside the anonymous function is not created, no new scope is created. When the function was later invoked, there is a new scope alright, but the i is in the outside scope, and the x gets its value, and it is all 6 anyways. The following code will actually invoke a function and create a new scope and that's why the x is a new local variable x in the brand new scope each time, and the invocation of the function when the link is clicked on will use the different x in the different scopes. <a href="#" id="link1">ha link 1</a> <a href="#" id="link2">ha link 2</a> <a href="#" id="link3">ha link 3</a> <a href="#" id="link4">ha link 4</a> <a href="#" id="link5">ha link 5</a> <script type="text/javascript"> for (var i = 1; i <= 5; i++) { (function() { var x = i; document.getElementById('link' + i).onclick = function() { alert(x); return false; } })(); // invoking it now! } </script> If we take away the var in front of x, then it is a global x and so no local variable x is created in the new scope, and therefore, clicking on the links get all the same number, which is the value of the global x.

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• TextMate can show definition of a function easily?

  - by Jian Lin

  I am using TextMate on a Ruby on Rails project and wonder if you can put the mouse on link_to, and then press a key and it will show the definition of link_to, or does this for any other helper functions? Or, click open a box and type in a function name and it will show you the definition?

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• In MySQL, if the movies table has a description field, how to do search on this description?

  - by Jian Lin

  Say, using MySQL, if the movies table has 20,000 records, and each record has a field that is the description of the movie, up to 2k byte long. How can we search for movies with the word "nature" in its description? If possible, it is to be fast, instead of going through all the 20,000 records. (if in some other situations, like for books, where n can be 200,000 or more).

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• How to create a function and pass in variable length argument list?

  - by Jian Lin

  We can create a function p in the following code: var p = function() { }; if (typeof(console) != 'undefined' && console.log) { p = function() { console.log(arguments); }; } but the arguments are passed like an array to console.log, instead of passed one by one as in console.log(arguments[0], arguments[1], arguments[2], ... Is there a way to expand the arguments and pass to console.log like the way above? Note that if the original code were var p = function() { }; if (typeof(console) != 'undefined' && console.log) { p = console.log; } then it works well on Firefox and IE 8 but not on Chrome.

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• Is it true that in most Object Oriented Programming Languages, an "i" in an instance method always r

  - by Jian Lin

  In the following code: <script type="text/javascript"> var i = 10; function Circle(radius) { this.r = radius; this.i = radius; } Circle.i = 123; Circle.prototype.area = function() { alert(i); } var c = new Circle(1); var a = c.area(); </script> What is being alerted? The answer is at the end of this question. I found that the i in the alert call either refers to any local (if any), or the global variable. There is no way that it can be the instance variable or the class variable even when there is no local and no global defined. To refer to the instance variable i, we need this.i, and to the class variable i, we need Circle.i. Is this actually true for almost all Object oriented programming languages? Any exception? Are there cases that when there is no local and no global, it will look up the instance variable and then the class variable scope? (or in this case, are those called scope?) the answer is: 10 is being alerted.

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• In SQL, why is "Distinct" not used in a subquery, when looking for some items "not showing up" in th

  - by Jian Lin

  Usually when looking for some items not showing up in the other table, we can use: select * from gifts where giftID not in (select giftID from sentgifts); or select * from gifts where giftID not in (select distinct giftID from sentgifts); the second line is with "distinct" added, so that the resulting table is smaller, and probably let the search for "not in" faster too. So, won't using "distinct" be desirable? Often than not, I don't see it being used in the subquery in such a case. Is there advantage or disadvantage of using it? thanks.

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• In Javascript, if there is an object with a lot of properties that can functions, how do you convert

  - by Jian Lin

  In Javascript, if an object has lots of properties that are functions: var obj = { foo: function() { ... }, bar: function() { ... }, ... } then how can you get an array of names of those functions? That is, an array ["foo", "bar", ... ] thanks.

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• In CSS, does it make sense or is it legal to nest an id in another id -- such as "#main #display img

  - by Jian Lin

  In CSS, if it is #main #display img { height: 80px } that means all images within an element with id display that is within another element with id main. But does it make sense or is it legal since id seems to be just global names. It is because SASS actually allow nesting and some code may nest it like #main width: 700px #display img height: 80px which is "id within id".

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• Can TextMate be changed to "shift text to right" by highlighting text and pressing Tab?

  - by Jian Lin

  Can TextMate be changed to "shift text to right" by highlighting text and pressing Tab? Right now it actually replace the whole selected text with a tab character but I almost never want to do something like that. I think some other editors like Notepad++ will indent to the next level.

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• In Ruby on Rails, how is view code able to use instance variables that are set inside the controller

  - by Jian Lin

  In Ruby on Rails, how is an ActionView object able to access the instance variables of ActionController object (how is view code able to use instance variables that are set inside the controller code)?

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• why adding a simple div to HAML will break the code?

  - by Jian Lin

  it is quite strange that the following works: - 1.upto(30) do |i| = i %br but the following won't: %div this line - 1.upto(30) do |i| = i %br Isn't the second part just to add a div and let the numbers go into that div?

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• In MVC framworks (such as Ruby on Rails), do usually Model spell as singular and controller and view

  - by Jian Lin

  I usually see Ruby on Rails books using script/generate model Story name:string link:string which is a singular Story, while when it is controller script/generate controller Stories index then the Story now is Stories, which is plural. Is this a standard on Ruby on Rails? Is it true in other MVC frameworks too, like CakePHP, Symfony, Django, or TurboGears? I see that in the book Rails Space, the controller is also called User, which is the same as the model name, and it is the only exception I see.

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• In Ruby, why is a method invocation not 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 an instance method of the Array object, and this instance method takes a block of code, and then returns a number. If so, then it should be no different from calling a function or method 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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• With database table design, Is there an advantage of using special naming convention for a Many-to-m

  - by Jian Lin

  Whenever there is a many-to-many relationship, I think there will be immediately a many-to-many table needed (also called a junction table). Is there any advantage of using special naming convention for these tables, as opposed to the other 1 to many tables? Or are there popular special naming conventions used by companies designing database tables?

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• In SQL, a Join is actually an Intersection? And it is also a linkage or a "Sideway Union"?

  - by Jian Lin

  I always thought of a Join in SQL as some kind of linkage between two tables. For example, select e.name, d.name from employees e, departments d where employees.deptID = departments.deptID In this case, it is linking two tables, to show each employee with a department name instead of a department ID. And kind of like a "linkage" or "Union" sideway". But, after learning about inner join vs outer join, it shows that a Join (Inner join) is actually an intersection. For example, when one table has the ID 1, 2, 7, 8, while another table has the ID 7 and 8 only, the way we get the intersection is: select * from t1, t2 where t1.ID = t2.ID to get the two records of "7 and 8". So it is actually an intersection. So we have the "Intersection" of 2 tables. Compare this with the "Union" operation on 2 tables. Can a Join be thought of as an "Intersection"? But what about the "linking" or "sideway union" aspect of it?

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• Using sass, how do you style "#div1 img, #div2 img { height: 80px }" ?

  - by Jian Lin

  Using .sass, how do you style #box1 img, #anotherbox img { height: 80px } ? Is using mixins the only way?

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• In Ruby on Rails, the :id is only optional in routes.rb? How to tell optional vs not optional?

  - by Jian Lin

  In routes.rb map.connect ':controller/:action/:id' map.connect ':controller/:action/:id.:format' but then http://localhost:3000/foobars/alt will work too. The params are: {"action"=>"alt", "controller"=>"foobars"} so looks like :id is optional here. How can you tell it is optional vs not optional? Can you make it so that it is not optional in routes.rb?

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• In Mercurial, what is the command to say "make all my files as of 2 months ago! (Revision 328)" or "

  - by Jian Lin

  In Mercurial, what is the command to say 1) make all my files as of two months ago (say, April 16, 6:03pm) (Revision 328 as seen from the hg log) 2) or, make my files back to the most current repository version?

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• In SQL, can we always write an inner join statement as a main query and subquery?

  - by Jian Lin

  In SQL, can we always write an inner join statement as a main query and subquery? 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?

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• In SQL, why is "select *, count(*) from sentGifts group by whenSent;" ok, but when "*" and "count(*)

  - by Jian Lin

  In SQL, using the table: mysql> select * from sentGifts; +--------+------------+--------+------+---------------------+--------+ | sentID | whenSent | fromID | toID | trytryWhen | giftID | +--------+------------+--------+------+---------------------+--------+ | 1 | 2010-04-24 | 123 | 456 | 2010-04-24 01:52:20 | 100 | | 2 | 2010-04-24 | 123 | 4568 | 2010-04-24 01:56:04 | 100 | | 3 | 2010-04-24 | 123 | NULL | NULL | 1 | | 4 | 2010-04-24 | NULL | 111 | 2010-04-24 03:10:42 | 2 | | 5 | 2010-03-03 | 11 | 22 | 2010-03-03 00:00:00 | 6 | | 6 | 2010-04-24 | 11 | 222 | 2010-04-24 03:54:49 | 6 | | 7 | 2010-04-24 | 1 | 2 | 2010-04-24 03:58:45 | 6 | +--------+------------+--------+------+---------------------+--------+ 7 rows in set (0.00 sec) The following is OK: mysql> select *, count(*) from sentGifts group by whenSent; +--------+------------+--------+------+---------------------+--------+----------+ | sentID | whenSent | fromID | toID | trytryWhen | giftID | count(*) | +--------+------------+--------+------+---------------------+--------+----------+ | 5 | 2010-03-03 | 11 | 22 | 2010-03-03 00:00:00 | 6 | 1 | | 1 | 2010-04-24 | 123 | 456 | 2010-04-24 01:52:20 | 100 | 6 | +--------+------------+--------+------+---------------------+--------+----------+ 2 rows in set (0.00 sec) But suppose we want the count(*) to appear as the first column: mysql> select count(*), * from sentGifts group by whenSent; 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 '* from sentGifts group by whenSent' at line 1 it gave an error. Why is it so and what is a way to fix it? I realized that this is ok: mysql> select count(*), whenSent from sentGifts group by whenSent; +----------+------------+ | count(*) | whenSent | +----------+------------+ | 1 | 2010-03-03 | | 6 | 2010-04-24 | +----------+------------+ 2 rows in set (0.00 sec) but what about the one above that gave an error? thanks.

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• If the "with" statement in Javascript creates a new scope, why does the following code not work as e

  - by Jian Lin

  If the "with" statement in Javascript creates a new scope, shouldn't clicking on the links show a different x which are in different scopes? It doesn't. <a href="#" id="link1">ha link 1</a> <a href="#" id="link2">ha link 2</a> <a href="#" id="link3">ha link 3</a> <a href="#" id="link4">ha link 4</a> <a href="#" id="link5">ha link 5</a> <script type="text/javascript"> for (i = 1; i <= 5; i++) { with({foo:"bar"}) { var x = i; document.getElementById('link' + i).onclick = function() { alert(x); return false; } } } </script>

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• why assert_equal() in Ruby on Rails sometimes seem to compare by Identity and sometimes by value?

  - by Jian Lin

  it was very weird that yesterday, I was do an integration test in Rails and assert_equal array_of_obj1, array_of_obj2 # obj1 from db, obj2 created in test and it failed. The values shown inside the array and objects were identical. If I change the test to assert array_of_obj1 == array_of_obj2 Then it will pass. But today, the first test actually passed. What reason could it be? Is assert_equal always using == or .equal? in Rails 2.2 or 2.3.5?

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• Is there a way to use a Ruby loop inside of HAML's :javascript region?

  - by Jian Lin

  Inside of HAML, can we have a loop inside the :javascript region? This will work: - 10.upto(20) do |i| :javascript document.getElementById('aDiv').innerHTML += '#{i}'; and this will not: :javascript - 10.upto(20) do |i| document.getElementById('aDiv').innerHTML += '#{i}'; can the code above also be made to work as well?

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• In Ruby, how to write a method to display any object's instance variable names and its values

  - by Jian Lin

  Given any object in Ruby (on Rails), how can I write a method so that it will display that object's instance variable names and its values, like this: @x: 1 @y: 2 @link_to_point: #<Point:0x10031b298 @y=20, @x=38> I also want to be able to print <br> to the end of each instance variable's value so as to print them out nicely on a webpage. the difficulty now seems to be that not every instance variable has an accessor, so it can't be called with obj.send(var_name) (the var_name has the "@" removed, so "@x" becomes "x")

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