Four men in hats puzzle

Question

As shown in picture above there are 4 men looking forward. None of them can see back. There is a opaque wall between man number 3 and man 4 (1,2,3 cannot see pass the wall). Two of the men are wearing a black hat and two of them are wearing a white hat. Each man can see the color of the hat wore by the men in front of him. (1 can see 2,3 and 2 can see 3) but each person does not know the color of the hat he is wearing.

Now one of the man needs to call out the color of his hat else they all die in 10 mins. Which man will callout the color of his hat correctly and why?

Solution

Answer is Man no 2.

Reasoning

 Lets start by eliminating men. Man number 4 is at the other end of opaque wall facing other side. There is no way he can see any men or the color of their hat. So he is eliminated. Now man no 3 also cannot see anyone else - he cannot look back and he cannot see beyond wall. So he is eliminated too. Now man number 1 knows the color of man 2 and 3. Now lets say they (2 and 3) were wearing same color hat then man no 1 would know the color of his hat since there are 2 white and 2 black hat. But he keeps mum which means man 2 and 3 are wearing different hat. S0 man number 2 waits for sometime if he does not hear man 1 calling out that means man 2 and 3 are wearing different color hats. Since man 2 knows the color of hat wore by man 3 he know the color of his hat and calls it out.

Enabling Eclipse key map/shortcuts in Android Studio

Background

If you are from a developer using Eclipse then when you start using Android Studio then it becomes difficult to learn new shortcuts. For such cases Android Studio provides an option to use Eclipse shortcuts and in this post we will see how.

Enabling Eclipse key map/shortcuts in Android Studio

Go to 

• Android Studio -> Preferences

Type in keymap in the searchbox. You should be able to see a keymap section -

Next select Eclipse or Eclipse(Mac OS X) whichever you are more comfortable with. Then click Apply and Ok.

You should be good to use Eclipse shortcuts now. No need to restart.

Related Links

• How to add Eclipse Code Formatter file in Android Studio(OSFG)
• Android : URI is not registered (Settings | Project Settings | Schemas and DTDs) (OSFG)  

Select top N records from each category in PL/SQL

Background

Lets say you have 2 tables -

1. EMPLOYEE
2. EMPLOYEE_SALARY

EMPLOYEE table has employee id which is a primary key and his name.  EMPLOYEE_SALARY has employee id which is foreign key to id in EMPLOYEE table. This table has employee department and salary. You need to write a query that returns top 2 employees from each department that has highest salary.

Tables creation and data insertion

Table Queries :

create table schema8.EMPLOYEE(ID int, name varchar2(255));
alter table schema8.EMPLOYEE ADD CONSTRAINT PRIMARY_KEY PRIMARY KEY (ID);
create table schema8.EMPLOYEE_SALARY(EMPLOYEE_ID int, department varchar2(255), salary int);
alter table schema8.EMPLOYEE_SALARY ADD CONSTRAINT FK_EMP_ID FOREIGN KEY (EMPLOYEE_ID) REFERENCES schema8.EMPLOYEE(ID);

Data Queries for  EMPLOYEE table:

insert into schema8.EMPLOYEE values(1,'Aniket');
insert into schema8.EMPLOYEE values(2,'John');
insert into schema8.EMPLOYEE values(3,'Sam');
insert into schema8.EMPLOYEE values(4,'Ron');
insert into schema8.EMPLOYEE values(5,'Sky');
insert into schema8.EMPLOYEE values(6,'Paul');
insert into schema8.EMPLOYEE values(7,'Dan');
insert into schema8.EMPLOYEE values(8,'Jess');
insert into schema8.EMPLOYEE values(9,'Troy');
insert into schema8.EMPLOYEE values(10,'Mike');

 Data Queries for EMPLOYEE_SALARY table:

insert into schema8.EMPLOYEE_SALARY values(1,'IT',10000);
insert into schema8.EMPLOYEE_SALARY values(2,'Admin',500);
insert into schema8.EMPLOYEE_SALARY values(3,'Sales',1200);
insert into schema8.EMPLOYEE_SALARY values(4,'Sales',1500);
insert into schema8.EMPLOYEE_SALARY values(5,'IT',9000);
insert into schema8.EMPLOYEE_SALARY values(6,'Admin',4000);
insert into schema8.EMPLOYEE_SALARY values(7,'Admin',5000);
insert into schema8.EMPLOYEE_SALARY values(8,'IT',9500);
insert into schema8.EMPLOYEE_SALARY values(9,'Sales',1000);
insert into schema8.EMPLOYEE_SALARY values(10,'Admin',6000);

Final data :

select * from schema8.EMPLOYEE;
select * from schema8.EMPLOYEE_SALARY;

Solution

We are going to use RANK to partition by department and order by salary - 

select * from (
select id , name, department, salary, RANK() over (partition by department order by salary desc) as rank from(
select e.id, e.name, es.department,es.salary from schema8.EMPLOYEE e left OUTER join schema8.EMPLOYEE_SALARY es on (e.id=es.employee_id))
) where rank <= 2;

First we have done a left outer join so that we capture all employee records with their respective salaries and departments. In outer query we have ranked it based on their salaries in respective department. Finally we select all records that have rank <=2 i.e top 2 records.

Related Links

• Remove duplicate rows from table in Oracle(OSFG) 
• Basic PL/SQL syntax and query executions on Oracle 11g R2 database(OSFG) 
• How does database indexing work? (OSFG) 

Install Oracle instant client and sqlplus using Homebrew

Background

 In one of the previous posts we say how to install and run sql plus  and Oracle instant client  on Ubuntu operating system -

• Installing Oracle instant database client in Ubuntu

In this post we will see the same for a Mac.

This post expects you have homebrew installed. If not please refer -

•  Tips and Tricks for your MacBook

 Install Oracle instant client and sqlplus using Homebrew

For this you need to download following two files -

1. instantclient-basic-macos.x64-12.1.0.2.0.zip 
2. instantclient-sqlplus-macos.x64-12.1.0.2.0.zip 

You can download these files from oracle site - http://www.oracle.com/technetwork/topics/intel-macsoft-096467.html

Once download copy these files into following folder-

•  ~/Library/Caches/Homebrew 

 Once done run following commands -

• brew tap InstantClientTap/instantclient
• brew install instantclient-basic
• brew install instantclient-sqlplus

 This should install sqlplus for you.

Related Links

• Installing Oracle instant database client in Ubuntu(OSFG)
• Tips and Tricks for your MacBook(OSFG) 

How to Disable a MacBook’s Built-In TrackPad When Using a Mouse or Wireless Trackpad

Background

For me this became a requirement as my trackpad started malfunctioning. But it can be a useful feature as well. When we connect a mouse we would not want trackpad to work. For eg. lets say you are playing counter strike with a mouse you definitely don't want trackpad to change your aim in game. So it would be better if you disable in build trackpad while mouse is connected. In this post I will show you how you can do this.

Disable Trackpad in OS X Lion and Above

For this you need to go to -

• System Preferences > Accessibility > Mouse & Trackpad

Here you will find a checkbox saying "Ignore built-in trackpad when mouse or wireless trackpad is present". Click it and make sure you have checked it. Setting should take effect immediately.

Disable Trackpad in OS X Snow Leopard

Setting is the same here too. It's just at different path. So head to 

• System Preferences > Universal Access > Mouse & Trackpad

Here again you will find the same checkbox. Check it and you should be good to go.