Saturday, April 22, 2017

Hosting Multiple Websites on a Single IP Address

It is not too difficult to host multiple websites on a single server and a single IP address. Here is how to do it with Apache server, based on article1, article2, and article3. I am assuming that you already have a server setup with apache2. If not, please refer to this tutorial for instructions.

The first step is to create virtual hosts where each virtual host serves each different website. For example, assume you want to serve domain names: and You will need to create two virtual hosts, so that one will serve while the other will serve

On Ubuntu or Debian, apache default server configuration files are in /etc/apache2 directory, while web server directory is /var/www.

First, copy the default configuration file and create two virtual host config files:
$ sudo cp /etc/apache2/sites-available/000-default.conf /etc/apache2/sites-available/hi.conf
$ sudo cp /etc/apache2/sites-available/000-default.conf /etc/apache2/sites-available/hello.conf

Next, edit /etc/apache2/sites-available/hi.conf similar to below:
 <VirtualHost *:80>
    DocumentRoot /var/www/hi
    ErrorLog ${APACHE_LOG_DIR}/error.log
    CustomLog ${APACHE_LOG_DIR}/access.log combined

Note that you must create /var/www/hi directory that contains files to serve for

Similarly, edit /etc/apache2/sites-available/hello.conf in the same manner.
 <VirtualHost *:80>
    DocumentRoot /var/www/hello
    ErrorLog ${APACHE_LOG_DIR}/error.log
    CustomLog ${APACHE_LOG_DIR}/access.log combined

Again, you will need to create /var/www/hello directory that will serve visitors to

Next, enable the new virtual host configuration files:
$ sudo a2ensite hi.conf 
$ sudo a2ensite hello.conf 

Next, reload apache2 so that the change takes effect
$ sudo service apache2 reload

If you want to test these out, refer to this excellent article for more details.

These steps up to here will complete the setup for the server side. Now, it is time to setup your domain name configurations.

To direct any visitor who enters or to your virtual hosts, you will need to add A Record. Take a look at this article for more details.

Essentially, create A Record for and to direct to your server's IP address, and apache server will then take care of directing visitors of to your virtual host, and visitors of to the virtual host of that you have set up above.

*** Note: make sure not to enable forwarding of your domain name to your server. That was my first attempt, and it did not work. You will need to set up A Record instead in order for your apache server to point to appropriate virtual host.

Tuesday, April 18, 2017

Local Thresholding from Zxing Library (C++)

In this tutorial, I will go over how to call local threshold method in zxing library, ported to C++ (zxing-cpp) instead of Java. If you want to implement it in Java, refer to this tutorial. I will assume that OpenCV library is installed on the system.

First, one needs to download zxing-cpp
$ git clone
$ mkdir build && cd build

To integrate zxing-cpp with system's OpenCV, one needs to first search for OpenCVConfig.cmake file:
$ find / -iname opencvconfig.cmake 2>/dev/null

In my case, I installed OpenCV from Homebrew, and its config cmake file is in /usr/local/Cellar/opencv3/3.2.0/share/OpenCV/ directory. Export OpenCV_DIR environment variable to point to this path:
$ export OpenCV_DIR='/usr/local/Cellar/opencv3/3.2.0/share/OpenCV/'

Now, we are ready to compile and install:
$ cmake -G "Unix Makefiles" ..
$ make -j2
$ sudo make install

Next, change directory to your working directory and create zxing.cpp source file as below:

To compile, we first need to copy MatSource.h into zxing include directory. I am assuming that you have cloned the zxing-cpp repository into ~/zxing-cpp-master.
$ sudo cp ~/zxing-cpp-master/opencv/src/zxing/MatSource.h /usr/local/include/zxing/
where /usr/local/include/zxing is the folder that contains zxing-cpp library's header files.

We also need to locate where zxing-cpp library file is installed. You can look it up from the output after sudo make install command above, or search the file as shown below.
$ find / -name libzxing.a 2>/dev/null

Finally, we are ready to compile.
$ g++ zxing.cpp `pkg-config --libs opencv` -lzxing -L/usr/local/lib/

Make sure to replace /usr/local/lib/ above after -L option with the directory where libzxing.a file is installed in your system.

To execute, run the following:
$ ./a.out some_image.jpg

You should see local threshold binary image of your input image file.

Monday, April 17, 2017

Local Thresholding from Zxing Library (Java)

Although OpenCV has its own local threshold method, such as adaptiveThreshold, I was looking for something a bit sophisticated and better. After some search, I came across Zxing's HybridBinarizer class that does the job much better than simple adaptiveThreshold from OpenCV. So, below is a very rough code to make use of this excellent library in Java. If you want to do this in C++, refer to this tutorial.

I admit that the code above is a bit messy, but it is just for testing out to see this actually works. In writing the code, I would like to acknowledge the following especially helpful references.

To compile the file and run it, first, create with the code above. Then, download Zxing's core library file (most recent version at the time of writing this is core-3.3.0.jar) form here. Then, run the following, assuming the jar file is in the same directory.

$ javac -cp ".:core-3.3.0.jar"
$ java -cp ".:core-3.3.0.jar" Test image_to_test.jpg

This will output the local thresholded image binary_output.jpg to the same folder. If you want to know more about java and javac -cp option, refer to this tutorial.

Sunday, April 16, 2017

Java Compilation with Package and Jar Dependencies

In this tutorial, I will go over Java package and dependencies basics.

Let's go over the simplest java code with package declaration. Let's assume we are working on ~/java_package directory. Create file below in the current directory:

To compile and execute this, we will first create a directory whose name is equal to the package name. In this case, the directory name should be pkgtest. So, create this directory and move file into this. Run all of the following from ~/java_package directory:
$ mkdir pkgtest
$ mv pkgtest/

Next, we need to compile. This is very simple.
$ javac pkgtest/

Finally, we need to run it.
$ java pkgtest/Hello

Note that you must run this from ~/java_package directory; otherwise java will complain with the error below:
$ cd pkgtest
$ java Hello
Error: Could not find or load main class Hello

Next, we will see how to import the package. Modify pkgtest/ and create ./ as below:

To compile and run, run the following from ~/java_package directory:
$ javac
$ java JarTest

Next, we will create a jar library file and use this to compile and run. First, let's create the jar library. Run the following in ~/java_package directory
$ javac pkgtest/
$ jar cvf pkgtest.jar pkgtest/Hello.class
added manifest
adding: pkgtest/Hello.class(in = 648) (out= 372)(deflated 42%)

This creates pkgtest.jar file, which contains Hello class.

Next, we will rename the pkgtest directory so that we don't compile from the source.
$ mv pkgtest pkgtest_bk

Let's see what happens if don't specify the jar file as we compile.
$ javac error: package pkgtest does not exist
import pkgtest.Hello;
              ^ error: cannot find symbol
  symbol:   variable Hello
  location: class JarTest error: cannot find symbol
        Hello h = new Hello();
  symbol:   class Hello
  location: class JarTest error: cannot find symbol
        Hello h = new Hello();
  symbol:   class Hello
  location: class JarTest
4 errors

As you can see above, java compiler complains that it cannot find some symbols, since we renamed the package directory. To resolve this, let's link the jar file:
$ javac -cp ".:pkgtest.jar"
$ java -cp ".:pkgtest.jar" JarTest

Viola! Let's call it a day.

Friday, April 7, 2017

Useful Environment Variables for Mac OS X

Here is a list of some useful environment variables for Mac OS X:

C_INCLUDE_PATH: header search path for gcc

CPLUS_INCLUDE_PATH: header search path for g++

Thursday, April 6, 2017

Dynamic Programming: Maximizing Stock Profit Example

In this tutorial, I will go over a simple dynamic programming example. Dynamic programming simply refers to breaking down a complicated problem into simpler sub-problems and saving their results to refer back. I think of dynamic programming as an extension to recursion where we store its child recursive solutions, or more simply recursion + memoization.

Let's take a look at an example. Assume that you somehow have the ability to predict stock market for the next N days. Say you can trade stock exactly once a day, and the total number of trades you can make within the next N days is constrained to some number, say n. In addition, you can not own more than 1 stock at a time.

So, basically, given stock prices for next N consecutive days, [x1, x2, ..., xN], and given maximum of n total trades all on different days, when will you buy and sell each stock so that you can maximize the profit? Note that you must buy-sell-buy-sell and so forth with no consecutive buys or sells.

For example, if the next 5 day market price will be [5, 2, 2, 5, 3], and if I have at most 2 buy-sell trades to do, then I would buy on day 2 or 3 and sell on day 4, leaving me with +3 in profit. I will buy-sell only once, since this maximizes the profit.

It is easy when N is small, but it gets very difficult as N grows large. Given N = 50 with prices given by [8, 44, 39, 4, 41, 0, 19, 29, 44, 43, 44, 21, 34, 39, 23, 7, 1, 24, 30, 34, 40, 26, 28, 11, 44, 19, 48, 12, 5, 10, 2, 21, 0, 24, 8, 44, 36, 20, 6, 2, 4, 21, 14, 6, 47, 31, 23, 29, 4, 36] and you have at most 10 buy-sell trades allowed. What is your maximum profit?

To tackle this problem, I would first break down the problem. Here, it is asking for the maximum profit for at most n stock buy-sell stock trades. I will split it into sub-problems where I can find the maximum profit for exactly n buy-sell trades, and later I can search for the max among them.

So, the sub-problem is now to find the maximum profit given N-day forecast where I need to trade exactly n times. Since each trade is either buy or sell, I will once again split this sub-problem into sub-sub-problem as follows:

B(x, N, n): given N-day forecast of prices x, find the maximum cash after exactly n-1 buy-sell trades and another buy by the Nth day.

S(x, N, n): given N-day forecast of prices x, find the maximum cash after exactly n buy-sell trades by the Nth day.

With these functions, it is not too difficult to come up with a simple recursion relationship. Perhaps it is easy to see this from Python code below:

So, that is the recursive part of dynamic programming. Now, we need to deal with the memoization part. Note that with the recursion above, the performance will be very slow, since the number of recursive calls exponentially grows, where they are called with the same parameters multiple times. While we can store the results for all possible parameters, this will take up too much memory ~ O(N*N). A better way is to start from N = 1 and store all possible values for n, and increment N by 1 each time. This requires O(N) space.

It is probably better to explain in code below:

To run it:
$ python 10
[4, 1, 4, 6, 2, 7, 4, 9, 7, 4]
[0, 8, 12, 15, 12, 6]

That is, given 10-day stock price [4, 1, 4, 6, 2, 7, 4, 9, 7, 4], the most profit you can make by trading up to 5 buy-sell trades using aforementioned rule is 0, 8, 12, 15, 12, and 6. Now, to find the answer to the original question, we simply take the maximum of the array. Given 2 maximum buy-sell trades, the most profit one can make is 12.

By the way, the problem become trivial if maximum allowed trade is N/2, in which case one simply buys at local minimum and sells at local maximum. Hence, this algorithm shines when n is less than N/2.

Sunday, April 2, 2017

How to Deal with Argument List Too Long Error Message

In Mac OS X terminal,when you provide too many arguments to commands, you may get the error message below:

$ rm *.jpg
-bash: /bin/rm: Argument list too long

This is because there are too many jpg files in the directory, all of which are fed into rm arguments. Here are two ways to achieve it.

$ find . -name '*.jpg' -exec rm {} \;

$ find . -name '*.jpg' | xargs rm

Note that they will be slow, since we are basically calling the rm command as many times as the number of jpg files in the folder, but this works though!

Saturday, April 1, 2017

Run Jupyter Notebook in a Server

Jupyter notebook is quite useful as it displays python outputs within the same page. Here, I will show how to set up a server to run Jupyter notebook so that clients can connect to remotely over http.

First, I am going to assume that your server has python and jupyter installed. From the server, you will need to create jupyter config file.
$ jupyter notebook --generate-config

The command will create jupyter config file ~/.jupyter/

Open this file and look for keyword localhost. Replace localhost with * as shown below:
c.NotebookApp.ip = '*'

This will allow the jupyter app to broadcast to all IP addresses, not just to itself, so that any clients can connect to the notebook. If you want to set up password and so forth, please take a look at this documentation.

You may also want to turn off starting browser automatically by editing the line as c.NotebookApp.open_browser = 'False'

To change the port,  edit the following line
c.NotebookApp.port = 9999

How to Save Core Dumped FIle

Whenever you encounter the error message:
Segmentation fault (core dumped)

you may be wondering where is this core file?

Well, it is most likely that you need to increase the core file size. Try
$ ulmit -a
core file size          (blocks, -c) 0
data seg size           (kbytes, -d) unlimited
scheduling priority             (-e) 0
file size               (blocks, -f) unlimited
pending signals                 (-i) 63344
max locked memory       (kbytes, -l) 64
max memory size         (kbytes, -m) unlimited
open files                      (-n) 1024
pipe size            (512 bytes, -p) 8
POSIX message queues     (bytes, -q) 819200
real-time priority              (-r) 0
stack size              (kbytes, -s) 8192
cpu time               (seconds, -t) unlimited
max user processes              (-u) 63344
virtual memory          (kbytes, -v) unlimited
file locks                      (-x) unlimited

You see, the core file size is set as 0 by default in Ubuntu, and that's why you are not seeing the core dumped file. Increase this to, say, 50000:
$ ulimit -c 50000

Now, you should see a core file as core in the current directory whenever you encounter the above error message!

How to Deal with Grub-Efi-Amd64-Signed Failure Error

These days, computers come with UEFI, Unified Extensible Firmware Interface, which is an upgrade from traditional legacy BIOS. With UEFI, Linux installation may fail with the following error message:
grub-efi-amd64-signed failed installation /target/ Ubuntu 16.04 

If this message appears, here is what you need to do.

First, create EFI partition on your disk. That is, boot your system from the Linux installation media, which typically has gparted. Open up gparted, create a partition such that the first partition on the disk is FAT32 with 200MB of size.

Second, set esp and boot flags for this FAT32 partition from gparted. This will indicate that this partition is for EFI.

Now that you proceed to Linux installation, and select the disk with EFI partition for boot loader installation. Do not choose the partition itself; select the entire disk such as /dev/sda. Now it should work!