Recently I had the opportunity to give a short 10 min presentation on Memoization Decorator at our local UtahPython Users Group meeting. 

Memoization: 

• Everytime a function is called, save the results in a cache (map).
• Next time the function is called with the exact same args, return the value from the cache instead of running the function.

The code for memoization decorator for python is here: http://wiki.python.org/moin/PythonDecoratorLibrary#Memoize

Example:

The typical recursive implementation of fibonacci calculation is pretty inefficient O(2^n).   

def fibonacci(num):
        print 'fibonacci(%d)'%num
        if num in (0,1):
            return num
        return fibonacci(num-1) + fibonacci(num-2)

>>> math_funcs.fibonacci(4)   # 9 function calls
    fibonacci(4)
    fibonacci(3)
    fibonacci(2)
    fibonacci(1)
    fibonacci(0)
    fibonacci(1)
    fibonacci(2)
    fibonacci(1)
    fibonacci(0)
    3

But the memoized version makes it ridiculously efficient O(n) with very little effort.

import memoized
@memoized
def fibonacci(num):
    print 'fibonacci(%d)'%num
    if num in (0,1):
        return num
    return fibonacci(num-1) + fibonacci(num-2)
    
>>> math_funcs.mfibonacci(4)  # 5 function calls
    fibonacci(4)
    fibonacci(3)
    fibonacci(2)
    fibonacci(1)
    fibonacci(0)
    3

We just converted an algorithm from Exponential Complexity to Linear Complexity by simply adding the memoization decorator.

Slides:

memoization_decorator.pdf Download this file

Presentation:

I generated the slides using LaTeX Beamer. But instead of writing raw LaTeX code I used reStructured Text (rst) and used rst2beamer script to generate the .tex file. 

Source:

The rst file and tex files are available in Github.

https://github.com/amjith/User-Group-Presentations/tree/master/memoization_de...

 

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A few weeks ago I decided that I should suck it up and start learning how to develop for the web. After asking around, my faithful community brethren, I decided to learn Django from its docs. 

::Django documentation is awesome::

Around this time I came across this post about Waking up at 5am to code. I tried it a few times and it worked wonders. I've been working on a small project that can keep track of my productivity on the computer. The concept is really simple, just log the window that is on top and find a way to display that data in a meaningful way. 

Today's 5am session got me to a milestone on my project. I am finally able to visaulize the time I spend using a decent looking graph. Which is a huge milestone for someone who learned how to display html tables 3 weeks ago.

Tools:

• Django for backend
• Sqlite
• Haystack/Solr - search backend for Django
• FancyBox - jquery plugin
• flot - jquery plotting lib
• Bootstrap - html/css

A huge thanks to my irc friends and random geeks who wrote awesome blog posts and SO answers on every problem I encountered.

I will be open-sourcing the app pretty soon. Stay tuned.

See the full gallery on Posterous

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I'm taking a couple of the free online classes offered by Standford. One on Artifical Intelligence and one on Machine Learning. 

I haven't had so much fun since kindergarten. Actually that's not fair, I didn't enjoy kindergarten this much. I'm listening to the classes during my lunch, after work, during weekends. I'm working on my assignment with so much enthusiasm, I dread the day when this class ends. 

Stanford just announced a slew of new online classes offered starting in Jan 2012. I was way too excited when I first read the description on them. Now I'm a little sad, becasue I want to take 8 out of the 11 courses that are being offered and I don't have enough time. :(

Woe is me. 

ps: If you are not taking any of these classes you are missing out big time. Please do yourself a favor and sign up. 

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Let me preface this post by saying I suck at recursion. But it never stopped me from trying to master it. Here is my latest (successful) attempt at an algorithm that required recursion. 

Background: 

You can safely skip this section if you're not interested in the back story behind why I decided to code this up. 

I was listening to KhanAcademy videos on probability. I was particularly intrigued by the combinatorics video. The formula to calculate the number of combinations of nCr was simple, but I wanted to print all the possible combinations of nCr. 

Problem Statement:

Given 'ABCD' what are the possible outcomes if you pick 3 letters from it to form a combination without repetition (i.e. 'ABC' is the same as 'BAC'). 

At first I tried to solve this using an iterative method and gave up pretty quickly. It was clearly designed to be a recursive problem. After 4 hours of breaking my head I finally got a working algorithm using recursion. I was pretty adamant about not looking it up online but I seeked some help from IRC (Thanks jtolds). 

Code: 

def combo(w, l):
        lst = []
        if l < 1:
            return lst
        for i in range(len(w)):
            if l == 1:
                lst.append(w[i])
            for c in combo(w[i+1:], l-1):
                lst.append(w[i] + c)
        return lst

Output:

>>> combinations.combo('abcde',3)
    ['abc', 'abd', 'abe', 'acd', 'ace', 'ade', 'bcd', 'bce', 'bde', 'cde']

Thoughts:

• It helps to think about recursion with the assumption that an answer for step n-1 already exists.
• If you are getting partial answers check the condition surrounding the return statement.
• Recursion is still not clear (or easy). 

I have confirmed that this works for bigger data sets and am quite happy with this small victory.

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I did a presentation at our local Python User Group meeting tonight. It was well received, but shorter than I had expected. I should've added a lot more code examples. 

We talked about usage of cProfile, pstats, runsnakerun and timeit. 

Here are the slides from the presentations: 

profiling.pdf Download this file

The slides were done using latex-beamer, but I wrote the slides in reStructuredText and used rst2beamer to create the tex file which was then converted to pdf using pdflatex. 

The source code for the slides are available on github.

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The probability of getting 2 heads while tossing a coin 5 times is:

But then computingby hand (or a calculator) was painfully slow, let alone do it 4 times.

So I wrote two little functions in Python that will calculate factorial (yes I reinvented the wheel) and

Bonus: It also teaches you corner cases like 0! = 1 and  that you wouldn't think of otherwise.

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When was the last time I vented about C++? The answer for that is always:

"TOO LONG AGO". 

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I was recently assigned to a new project at work. Like any good software engineer I started writing the pseudocode for the modules. We use C++ at work to write our programs.

I quickly realized it's not easy to translate programming ideas to English statements without a syntactic structure. When I was whining about it to Vijay, he told me to try prototyping it in Python instead of writing pseudocode. Intrigued by this, I decided to write a prototype in Python to test how various modules will come together.

Surprisingly it took me a mere 2 hours to code up the prototype. I can't emphasize enough, how effortless it was in Python.

What makes Python an ideal choice for prototyping:

Dynamically typed language:

Python doesn't require you to declare the datatype of a variable. This lets you write a function that is generic enough to handle any kind of data. For eg:

def max_val(a,b):
    return a if a >b else b

This function can take integers, floats, strings, a combination of any of those, or lists, dictionaries, tuples, whatever.

A list in Python need not be homogenous. This is a perfectly good list:

[1, 'abc', [1,2,3]]

This lets you pack data in unique ways on the fly which can later be translated to a class or a struct in a statically typed language like C++.

class newDataType
{
    int i;
    String str;
    Vector vInts;
};

Rich Set to Data-Structures:

Built-in support for lists, dictionaries, sets, etc reduces the time involved in hunting for a library that provides you those basic data-structures.

Expressive and Succinct:

The algorithms that operate on the data-structures are intuitive and simple to use. The final code is more readable than a pseudocode.

For example: Lets check if a list has an element

>>> lst = [1,2,3]    # Create a list
>>> res = 2 in lst   # Check if 2 is in 'lst'
True

If we have to do it in C++.

list lst;
lst.push_back(3);
lst.push_back(1);
lst.push_back(7);
list::iterator result = find(lst.begin(), lst.end(), 7); 
bool res = (result != lst.end())

Python Interpreter and Help System:

This is a huge plus. The presence of interpreter not only aids you in testing snippets of code, but it acts as an help system. Lets say we want to look up the functions that operate on a List.

>>> dir([])
['__add__', '__class__', '__contains__', '__delattr__', '__delitem__',
'__delslice__', '__doc__', '__eq__', '__format__', '__ge__', 
'__getattribute__', '__getitem__', '__getslice__', '__gt__', '__hash__',
'__iadd__', '__imul__', '__init__', '__iter__', '__le__', '__len__',
'__lt__', '__mul__', '__ne__', '__new__', '__reduce__', '__reduce_ex__',
'__repr__', '__reversed__', '__rmul__', '__setattr__', '__setitem__',
'__setslice__', '__sizeof__', '__str__', '__subclasshook__', 'append',
'count', 'extend', 'index', 'insert', 'pop', 'remove', 'reverse', 'sort']

>>> help([].sort)
Help on built-in function sort:
     
sort(...)
    L.sort(cmp=None, key=None, reverse=False) -- stable sort *IN PLACE*;
    cmp(x, y) -> -1, 0, 1

Advantages of prototyping instead of pseudocode:

• The type definition of the datastructures emerge as we code.
• The edge cases start to emerge when you prototype.
• A set of required supporting routines.
• A better estimation of the time required to complete a task.

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A typical conversation between my wife and I: 

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Tmux is an awesome replacement for Screen. I have a couple of standard terminal layouts for programming. One of them is show below.

• Vim editor on the left.
• Top right pane has the bpython interpreter. 
• Bottom right pane has the bash prompt. 

I have a small tmux script in my ~/.tmux/pdev file that has the following lines

selectp -t 0              # Select pane 0
splitw -h -p 50 'bpython' # Split pane 0 vertically by 50%
selectp -t 1              # Select pane 1
splitw -v -p 25           # Split pane 1 horizontally by 25%
selectp -t 0              # Select pane 0

In my tmux.conf file I have bound <prefix>+P to sourcing this file. So now anytime I want to launch my python dev layout, I hit <prefix>+<shift>+p. 

bind P source-file ~/.tmux/pdev

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I decided to prepare myself for the Intro to AI, a free online course offered by Stanford. I found the course website: http://www.stanford.edu/class/cs221/schedule.html and wanted to download all the slides. 

It's time to pull all the ppt files from that page.

wget -r -A.ppt http://www.stanford.edu/class/cs221/notes/ 

This created a tree of empty directories with one of them that had all the ppt files. Time to clean up the empty folders:

find -depth -type d -empty -exec rmdir {} \;

Ta-da! Empty folders are gone. 

Remember if you are typing a command more than once in succession there is a way to automate it. 

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Looking for tech jobs can be daunting. Networking is touted as the magic bullet for job seekers. But where do you start?

Here are some robust ways to build your network.

Users Group:

User groups are typically monthly meetings for geeks who get together to talk about their favorite programming language or operating system. Usually they are accompanied with a mailing list which is used to announce the meetings, ask questions and post job openings. So sign up to the mailing list and start attending the meetups. They are full of really nice people who are willing to help. 

• Utah Python - Utah Python Users Group
• URUG - Utah Ruby Users Group
• SLLUG - Salt Lake Linux Users Group
  • SLLUG-JOBS - Mailing list to announce job postings
• PLUG - Provo Linux Users Group

Local Conferences:

Most cities have some tech conferences that are a great source for networking. I found out about a lot of the user group by going to one of the following conference.

UTOSC - Utah Open Source Conference.

HackUTOS  - Utah Open Source Project Day - Geeks, snacks and open source.

LaunchUp - A local entreneurship clinic. A great way to learn about the local start-up scene. You can meet new CEOs and fresh companies looking to hire tech talent. A must for job-seekers.

I hope this helps someone.

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Last week I successfully submitted my first patch to an open source project and it was accepted. 

I like the bpython interpreter for all my python needs. It is quite handy for a python newbie like me. A few weeks ago I was in the middle of building an elaborate datastructure to learn list comprehension in python, when bpython crashed and took all the history with it. I whined about it on twitter and one of the developers of the project prompted me to submit a bug report. I was quite impressed by the fact that a core developer of bpython replied to my bitching on twitter.

After I filed the bug report, I decided to get the source code and poke around. I finally implemented a feature that saved the history after each command instead of waiting till the end of a session. 

The following factors were the main impetus that led me to contribute to the project. 

Project Hosting: 

The project was hosted on bit bucket which is a Github equivalent for mercurial. This makes it so easy to fork a project and issue pull requests, compared to the traditional source forge model of submitting patches in a mailing list. The social coding sites like Github and BitBucket have reduced much of the initial friction in starting an open source project.

Project Size:

This one has a huge impact when I decide to dive into the code. Traditional C projects tend to have a ton of files that are too big which is daunting for a beginner. The bpython project was written in python and had a total of 13 .py files. This makes it dead simple to make a quick change and run the project without compiling it. Again the choice of language has a lot to do with this. 

IRC:

The welcoming nature of the community around a project does a lot to encourage a new comer. The IRC channels are a great way to interact with the developers compared to a passive form of communication such as emails. I jumped on #bpython irc channel and started asking questions when I ran into an issue with bpython source code. People on that channel are really helpful and prompt in answering questions.

Persistence:

My first pull request was scrutinized by the core developers and some suggestions for improvements were given. During that process I learned a lot about code review and how to check for corner cases. Finally after I made all those improvements the pull request was accepted and merged with the main repo. So having a beginners mind (no ego) is an absolute must when getting started on any project. Don't be discouraged if your first attempt is unsuccessful. 

Now I'm proud to say my name is listed in the AUTHORS file of bpython project.

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Tmux is an alternative for screen. For anyone who doesn't know screen, it is a terminal multiplexer which means, it allow multiple windows in terminal. It can split your window into multiple panes (vertical/horizontal), detach a session which can be attached at a later time. Detach/Attach is very useful for running a job in a remote server without having to keep the ssh open the whole time. 

tmux.avi Watch on Posterous

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I went to attend the URUG (Utah Ruby Users Group) meeting today because a little birdy said there will be some Javascript related presentations. 

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Just got back from Snowboarding at Sundance Ski resort. I fell down a lot less than I did during my previous times. I'm also doing a lot more toe turns which has always been an issue. I decided to try the blue slopes today which was good and bad. I got over my fear of going down the steep sections, but I also got too tired to enjoy the green sections of the run. I can tell that I'm getting a lot faster these days because the runs seem shorted (or quicker). One more season and I'll be a pro :). 

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After wasting about 3 hours watching old episodes of The Office, I had a sinking feeling last night that I'm not working towards any of my life goals. 

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I had a chance to watch one of my role models in action today. He is one of the young, intelligent managers who rose to the top of the ladder pretty quickly. He managed to do that without creating enemies along the way which is pretty hard to do in big corporations. Everyone who has worked with him will attest to the fact that he deserves to be in the top and has nothing bad to say about him. 

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If you think your users are idiots, only idiots will use it.

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