Dimensionality Reduction and Scattered Data Visualization with MNIST

We live in 3D world, and we can only view scattered data in 1D, 2D, or 3D. Yet, we deal with data that have very large dimension. Consider MNIST dataset, which is considered to be a toy example in deep learning field, consists of 28 X 28 gray images; that is 784 dimensions.

How would this MNIST data look like in 2D or 3D after dimensionality reduction? Let's figure it out! I am going to write the code in Pytorch. I have to say, Pytorch is so much better than other deep learning libraries, such as Theano or Tensorflow. Of course, it is just my personal opinion, so let's not get into this argument here.

What I want to do is to take Pytorch's MNIST example found here, and make some modifications to reduce the data dimension to 2D and plot scattered data. This will be a very good example that shows how to do all the following in Pytorch:
1. Create a custom network
2. Create a custom layer
3. Transfer learning from an existing model
4. Save and load a model

Here is the plot I get from running the code below.

This code is tested on Pytorch 0.3.1.

Three Minutes Daily Vim Tip: Highlight Current Cursor Line

In Vim, it is often difficult to locate the current cursor, especially when you jump around. Here is a tip to easily locate the cursor.

Simply add the following line to ~/.vimrc
set cursorline

That's it!

Python IDE with YouCompleteMe

OK, I have tried YouCompleteMe, but then I stopped using it after some time. Well, I am going to give it a try once more. The reason is that I am usually developing on a server through ssh. I could launch GUI-based IDE, such as PyCharm through X-forwarding, but the response is too slow. I realized that editing with Vim on a remote server is probably the best option here.

In any case, I explained how to install YouCompleteMe plugin in my previous post. Here, I will talk about a few more useful tips.

One of the must-have feature in IDE is perhaps GoTo feature, where one can go to the definition of certain method of variable. YouCompleteMe also provides this feature. Formally, this is mapped with
:YcmCompleter GoTo

We can create a shortcut. Add the following in the ~/.vimrc file
nnoremap <C-P> :YcmCompleter GoTo<CR>

In the source file that YouCompleteMe understands, you can simply move the cursor to the symbol and press <Control> + p key to go to the definition. To go back, you can press <Control> + o key.

Also, add the following to ~/.vimrc file
let g:ycm_python_binary_path = 'python'

This will load the default python first found on PATH environment.

Happy coding!

Android Camera2 Bare Minimum Code Sample

Google has replaced camera API with camera2 API a few years ago. I guess it is a good thing that Google is working hard to improve Android, personally I think camera2 API is just so difficult. Compared to the deprecated camera API, it requires so much more code.

I am trying to understand this new camera2 API, and it is just not easy. I looked at Google's Camera2Basic sample code, it is still daunting for Android beginners like myself. Well, here is my attempt to trim down this fat sample code into the bare minimum so that it is easier to understand the fundamentals of the camera2 API.

All it does is to connect to a camera and display its preview to the screen. No capture, no manual focus, no error checks, etc. I am not happy that even this bare-bone app requires so many lines. Anyways, I will add more functionalities in the upcoming post.

OpenCV on PyCharm

PyCharm is such a nice IDE for developing in Python. I really love it. However, with OpenCV module, I realized that PyCharm didn't recognize cv2 module.

After some search, here is a very simple solution! Credit goes to here.
When you import cv2, simply copy paste the whole thing below:

import cv2

# this is just to unconfuse pycharm
try:
    from cv2 import cv2
except ImportError:
    pass

That's it!

Redirecting Outputs from Parallel Subprocess Runs

Say you are call some executable from python in parallel. You don't want to clog the stdout, so you want to save each stdout from executable as a file. Below is what you can do.

Solution to Android CountDownTimer Skipping the Last Tick

Google's implementation of CountDownTimer in Android is a bit strange; it often skips the very last tick. I can see why they did it this way: they want to make sure that onFinish() function will be called with no delay in the case onTick() takes so much time. However, this implantation causes the last tick to skip pretty much.

Well, here is my modified version of CountDownTimer which will not skip the last tick, unless onTick() function takes too long, which it shouldn't. So, make sure that onTick() will return quickly first.

The code modification starts at line 114 below:
The original source code is taken from here.

Prevent Process Termination over SSH Connection

When running heavy long-time running processes on a remote server through ssh, it is quite annoying that your processes are terminated when you are disconnected. Here is a tip just for this.

Let's assume you ssh into your server
$ ssh USERNAME@SERVER_IP

You want to run some program, which will run for quite some time.
$ ./a.out arg1 arg2 ...

What happens if your ssh connection is lost, perhaps due to unstable network connection, etc. Well, your program a.out will terminate itself, so you will have to re-run it. In fact, you will need to keep your ssh connection until the program completes, which is quite annoying.

One solution is using nohup
$ nohup ./a.out arg1 arg2 ... &

This will redirect the output to nohup.out file. With this, even if you disconnect from ssh connection, your program will keep running in the background.

Another solution, which is probably much more sophisticated, is to use screen. It is your work session where you can run processes, and you can easily detach from it to do other things while your processes running inside the session are intact and not terminated. You can always re-attach to the screen session and resume your processes still running.

To use this, you need to first create a new screen session
$ ssh USERNAME@SERVER_IP
$ screen -S SESSION_NAME

This will create a new screen. Here, you can run your program
$ ./a.out arg1 arg2 ...

Next, you can always detach from this current session with <CTRL + a> d. That is, press <CTRL> and <a> keys together, release the keys and then press <d> key and release. This will detach from the session. At this point, your program will keep running regardless of whether you disconnect from your ssh connection to the server.

To list running screen sessions, run
$ screen -list

To re-attach to the session, you simply run
$ screen -r SESSION_NAME

To exit the screen, you run
$ exit

By the way, you may notice that within screen session, your scroll will register as up/down arrow keys. If you want your scroll to work as scroll terminal output, you can run the following (credit to pistos)
$ echo 'termcapinfo xterm* ti@:te@' >> ~/.screenrc

You can also press <CTRL> <a> and then <ESC> to enter copy mode and do scroll freely. To exit copy mode, simply press <ESC>

Happy hacking!

Three Minutes Daily Vim Tip: Display Indentation Levels

It could be just me, but it is difficult for me to sometimes tell the indentation level of multiple indented lines. So here is a tip for someone like me:

IndentLine is an excellent vim plugin that does it for you. For installation, simply use Vundle; insert the following line in your .vimrc
Plugin 'Yggdroot/indentLine'

If you are not too familiar with Vundle, refer to this post where I explained how to use Vundle to install a different plugin.

Debugging Bash Scripts

Bash scripts are extremely handy when mostly dealing with Linux commands, such as find, grep, sed, and so on. One can write functions, just like in any other languages. However, I have to admit that I haven't really dealt with Bash scripting much so far. One of the reasons, I suppose, is that it is difficult to debug Bash scripts. I didn't think there was any debugging tool for Bash. Well, I just realized that I was wrong! There is in fact a very nice debugger for Bash: bashdb

To install, you can run the following on macOS
$ brew install bashdb

or the following for Ubuntu
$ sudo apt-get install bashdb

To start debugging, run
$ bashdb bash_script.sh argument1 argument2 ...

Many of the commands are similar to that of gdb, but for more info, type in help. Also, this documentation can be of great reference.

Happy bashing!