Category Archives: General

Using hosts file to redirect URLs locally

There are times when you want to access machines on LAN or internet using the IP address. Hosts files in various systems can help you auto redirect logical names to the IP address. For example

127.0.0.1 mysite.com

Now whenever you will call mysite.com, you will be pointing to localhost. Similarly you can use any IP address on LAN or Internet and redirect logical names to them.

More on usage of hosts file : en.wikipedia.org/wiki/Hosts_(file)

Infosys launches Big Data Edge

Infosys has launched “Big Data Edge” tool for analyzing large amount of data and extracting information. The data analysis tool claims to find required information in 40 % lesser time compared to existing solutions. It is backed by 50 customizable dashboards and 250 built-in algorithms to provide it a world class performance.

It is good to see some innovative products coming out of India. More info available here

http://www.infosys.com/newsroom/press-releases/Pages/bigdataedge-rapid-business-insights.aspx

http://www.infosys.com/industries/financial-services/features-opinions/Pages/change-game-big-data.aspx

Android app: out of memory exception

An android app has some limitations when it comes to memory availability. Unlike a normal Java app, you do not have luxury of expanding memory your application uses, as it will be completely dependent on the device on which the app is running, memory available in the phone, number of applications running simultaneously. We will not have control over these external factors so we will need to optimize memory usage on our end.

Some of the techniques I used in one of my last projects which used a lot of images/ bitmaps and hence ran into many memory issues.

  • Use of flags: Set android:hardwareAccelerated=”true” and android:largeHeap=”true” flags in manifest file. Downside is that these flags are only available after android 3.0. More than 50% of android devices still run on gingerbread (2.3+), so this is not a very reliable solution if you are targeting a larger audience. Though worth a try for high end (3.0+) devices.
  • Memory leakage: DVM grabage collection works n principle of JVM garbage collection, that is it will reclaim the memory for an object if there is no reference to it. So make sure when you are done with an object and do not need it anymore, mark the object reference as null.
  • Use minimum space for an object: Design your classes intelligently, for example if you are handling student data and you address details will not be used frequently, create a separate class and initialize the object only when it is required. Remember, if a char can work, don;t use int or long.
  • Objects are always placed on heap: One of my colleagues was wondering that how can an object occupy space, after the method exits, if the object is declared inside a method. Remember, only references are stored in the stack for method, the actual object is always stored in the heap. So the memory will be freed only when next cycle of garbage collection occurs
  • Use Garbage collection: You can call system.gc(); to initiate garbage collection, but this only suggests JVM/DVM to run a garbage collection, you cannot force it.
  • Playing with Bitmaps/ Images: Bitmaps and images take highest amount of memory, so make sure you recycle() bitmaps as soon as you are done using it. Make sure you have removed all references (imageviews/ objects) to bitmap before calling recycle().
  • Handle Activity Stack: If the app can be used back and forth multiple times, it is better to handle the back-front flow in the code, as default back button handling in android needs to keep a tack of activity stack and uses memory for that. Simplest will be to call previous activity on back button through code and always finish current activity before moving to next.

100 Door puzzle

Puzzle: There are various versions of the puzzle, there are 100 doors (or some other object), and 100 cops (prisoners). All the doors are closed initially. Cop 1 goes and opens up all the the doors, Cop 2 goes and closes door at even number 2,4,6 etc. Cop 3 goes and alters the state of doors divisible by 3 (3,6,9), so if the door was closed it is opened and if it was open, it is closed by cop 3. Cop 4 will alter state of all the doors divisible by 4 and so on. At the end we have to find out how many doors are closed now.

Solution: Before getting to a generic solution, lets take case of few doors 1 by one. We can easily see that a door’s state is same if it was visited even number of time (initially closed than (open + close) (open + close).. ), and it is changed if the door is visited odd number of times.
Case of Door 1: Only visited once by Cop1, so the state changes to open
Case of Door 2: Visited twice by cop1 and cop2, so finally it is closed, no change in state (see it is visited even number of times)
Case of Door 3: Visited by 1 and 3- Closed
Case of Door 4: Visited by 1, 2 and 4- Open
Case of Door 5: Visited by 1 and 5- Closed
Case of Door 6: by 1, 2, 3 and 6- Closed
Case 9: 1, 3 and 9- Open
Case 10: 1,2, 5 and 10- Open

So on careful examination, figure out doors which have even number of divisors are not changing state. When will a number have odd number of divisors? A divisor pair of a number z is x*y (always 2, and hence even)

for 50: 1*50, 2*25, 5*10- Even
for 100: 1 * 100, 2*50, 4*25, 5*20, 10*10- Odd (As 10 appears twice)

So for all the perfect squares, we can see we have odd number of divisors and hence the state of the door will be changed.

Installing postgres 9

I was trying to install postgres 9 on my Ubunto 11.04 machine, but the GUI installer simply hanged. A little bit of googling provided the workaround.

Follow the steps

1. sudo apt-get install python-software-properties
2. sudo add-apt-repository ppa:pitti/postgresql
3. sudo apt-get update
4. sudo apt-get install postgresql-9.0 libpq-dev

Source
http://www.dctrwatson.com/2010/09/installing-postgresql-9-0-on-ubuntu-10-04/

Forgot MySql password?

Forgot the root password for mysql database. Here are easy steps to recover

1. Restart MySql database without password in safe mode
/etc/init.d/mysql stop
mysqld_safe --skip-grant-tables &

2. Change root password 
mysql -u root
mysql> use mysql;
mysql> update user set password="mypassword" where User='root';
mysql> flush privileges;
mysql> quit

**please note that the password might be in encrypted form. use PASSWORD('some_pass') method 
mysql> update user set password=PASSWORD('mypassword') where User='root';

3. Restart database
/etc/init.d/mysql stop
/etc/init.d/mysql start

References: http://www.cyberciti.biz/tips/recover-mysql-root-password.html

Insertion Sorting

Insertion sorting, as the name suggest, is about inserting the elements of an unsorted array to their correct position one by one. To make it clearer, lets say we have an unsorted array. We take another array, which is empty initially and then we will keep adding numbers one by one into this array at their correct positions.

Say the original array A is

5, 3, 7, 2, 6

Now we take first element and add to zeroth position of a new array B, so B is now containing 5

next we pick 3 from original array and add to first position of B, it becomes 3, 5

After step 3 B is, 3, 5, 7 and then 2, 3, 5, 7 and finally 2, 3, 5, 6, 7

We have taken 2 arrays just to make things clear, the sorting functionality can be achieved with 1 array only. Instead of taking an additional array, we will think of original array as 2 sub-parts, at any given time first N-x elements are sorted and remaining x are not sorted.

Again taking the original array A

5, 3, 7, 2, 6

Step 1: 5 (sorted till here), 3, 7, 2, 6
Step 2: 3, 5 (sorted till here), 7, 2, 6
Step 3: 3, 5, 7 (S), 2, 6
Step 4: 2, 3, 5, 7, (S) 6
Step 5: 2, 3, 5, 6, 7 – Final

Complexity:

If there are N elements in array, at Nth point, we know we have N-x sorted and x unsorted. We pick up next element (N-x+1)st, and need to insert it into proper position in for N-x+1 elements. So in worst case we need to make N-x comparisons (in best case 1 comparison). The process has to be repeated N times for N elements.

So in worst case we have comparisons like

1+2+3+.. +N= N*(N-1)/2 which is quadratic i.e. N*N.

In best case (sorted array) we have 1+1+1+.. N times comparison which is order of N (linear).

We generally take worst case complexity so we will say Insertion sort has complexity of N^2.

Antivirus Marketing strategy?

It has happened to me twice now. Days before my anti-virus subscription is about to finish, I start getting messages that some virus or worm has infected my system which was successfully cleaned up by the anti-virus. Just wondering if this is some marketing strategy by these anti-virus companies to tell us about the importance of their software or a sheer coincidence?

Have you paid a bribe?

Recently came to know about ‘I paid a bribe‘ initiative form a Bangalore based NGO Janaagrah. Here you can report incidents where you had to pay a bribe and blow the whistle. I like the idea of reporting the incidents where you actually did not have to pay the bribe. So these guys are not only pointing fingers but also appreciating the individuals/ offices which say no to bribe and do their jobs honestly.

I am not sure if this will help actually solving the problem, but it is definitely a good first step for a country like ours where corruption has become a part of life.