15 Sep 2013
Raspberry Pie meetup this weekend. We had lot of fun
there playing with Pie. After coming home, i wanted to try all those
things on my Pie. But first step is to prepare a image for it.
I decided to use
pidora, the fedora remix for Raspberry Pie. Fedora
fedora-arm-installer package. After installing it on F19
machine and starting it, i inserted SD card and it didn’t get
Without SD card, i couldn’t do anything. After googling, i found out
that you have to install
kmod-staging package. It is available as
After that, next part was to find out your driver for SD card reader
lsusb | grep Reader
Bus 001 Device 005: ID 0bda:0139 Realtek Semiconductor Corp. RTS5139
Card Reader Controller
So it told me that my SD card driver is RTS5139.
Most of the google results also had same driver so i tried next step.
And i got error that rts5139 does not exist. The issue here is the
kernel version for which you install
So i needed to install the version of
kmod-version that matches with
my kernel version. Otherwise it won’t work.
yum whatprovides \*/rts5139.ko
provided list of all packages having
I found the appropriate version with my kernel
yum install kmod-staging-3.9.8-300.fc19.x86_64-3.9.2-2.fc19.7.x86_64
And it works and SD card gets mounted automatically. :)
pidora installation is going on :) I will post more about
my Raspberry Pie experiments soon.
Some links -
Arduino/Raspberry Pie meetup from Pune
IOT tutorial for the meetup
30 Jun 2013
rails-api is an awesome gem to design API for
One more interesting thing about
rails-api is, it doesn’t have
separate versions for
Rails 3 and
rails-api picks up
latest Rails release which is available. So with
the release of Rails 4, it will use Rails 4.
What if you are not yet ready for
What if you want to use
Thanks to @steveklanik for
answering my doubt.
You have to just specify in the Gemfile that you want
of gems that
rails-api depends on.
For example -
gem 'rails-api' # for Rails API
gem 'actionpack', '3.2.13' # Specifically say you want '3.2.13'
# rails-api depends on actionpack.
# only specifying actionpack dependency is enough.
rails-api will use
05 Jun 2013
As root user
cat >/etc/yum.repos.d/rpm-sphere.repo <<EOF
yum install racket
To run racket from Emacs, enable marmalade repo
'("marmalade" . "http://marmalade-repo.org/packages/"))
Then install geiser package
M-x package-install-RET geiser
It will ask for choice of language
Selecting racket will open the REPL to play with it.
20 Apr 2013
Fedora is quite easy.
$ sudo wget -O /etc/yum.repos.d/jenkins.repo http://pkg.jenkins-ci.org/redhat/jenkins.repo
$ sudo rpm --import http://pkg.jenkins-ci.org/redhat/jenkins-ci.org.key
$ sudo yum install jenkins
$ sudo service jenkins start/stop/restart
Start jenkins by
$ sudo service jenkins start
You will get a success message.
By default jenkins starts on port 8080. Visit
You will be greeted by jenkins index page.
Setting up System
shell for jenkins user
$ sudo usermod -s /bin/bash jenkins
home for jenkins user
$ sudo usermod -m /var/lib/jenkins jenkins
Now we have to perform next steps as
You can give superuser permissions to
$ sudo su - jenkins
$ curl -L https://get.rvm.io | bash -s stable --rails --autolibs=enabled
Complete script is here
$ vi /var/lib/jenkins/.bashrc
$ [ -s "/var/lib/jenkins/.rvm/scripts/rvm" ] && source "/var/lib/jenkins/.rvm/scripts/rvm" # This loads RVM into a shell session.
#### Configuring path
Make sure that you have configured your
bashrc properly so that
PATH has everything that is needed to run a Rails
project. I spent a lot of time with
error even if i had
Later i realized that my
bashrc was not containing
I am using Gitlab as the repository server.
Create new key with
From the jenkins UI, you can install gitlab plugin. More info can be found here.
Build now hook
Add this web hook on your Gitlab project:
Now whenever there will be commit, gitlab will send a request to
jenkins to trigger the build.
We can run rake tasks, shell scripts once the build is triggered. This
configuration can be done on the project page on jenkins server.
I prefer running a shell script which will do
all the steps required
to run specs. This script can be stored into version control system
like git so that it will be available to everyone in your project.
I have created a sample ci script for a Rails 3.2+ project with
Post Build configuration
It allows to add email ids of project members who will get email after
every build. It generates report for last build using various plugins.
Same steps can be used with CentOS to setup jenkins.
10 Mar 2013
A function f is defined by the rule that
f(n) = n if n < 3 and
f(n) = f(n-1) + 2 * f(n-2) + 3 * f(n-3) if n >= 3.
Write a procedure that computes f by means of a recursive process.
Write a procedure that computes f by means of an iterative process.
Recursive solutions is very simple.
scheme Recursive solution
(define (f n)
(if (< n 3)
(+ (f (- n 1)) (* 2 (f (- n 2))) (* 3 (f (- n 3))))))
Iterative solution is a bit different.
Base condition is same for iterative solution also. If n < 3, n is the
For n >= 3, we have to define a procedure which will evolve as a
For n >= 3,
f(n) = f(n-1) + 2 * f(n-2) + 3 * f(n-3)
For n = 3, we have to evaluate above rule once. For n = 4, we have to
evaluate it twice. First to find out f(3) which then will be used to
So, it is clear that for n >= 3, we have to evaluate this rule
n - 2
times. Lets name diff to (n - 2).
Now lets consider state variables which will hold all the information
to find solution at a particular instance in this process.
For n = 3, we need f(0), f(1) and f(2) to find out f(3).
They are respectively 0, 1 and 2.
Lets name them a = 0, b = 1 and c = 2.
Now answer in each iteration is
3 * a + 2 * b + c
This answer needs to be passed to recursive call till diff is 0.
So after each iteration, state variables are changed as follows:
b becomes a
c becomes b
3 * a + 2 * b + c becomes c
diff becomes diff - 1
For n = 3, initially, a = 0, b = 1, c = 2 and diff = 3 - 2 = 1,
After one iteration, a = 1, b = 2, c = 4 and diff = 0
Now, base condition is when diff = 0, c is the answer. So here for
f(3), 4 is the answer.
Lets repeat this for f(4)
n > 3
diff = 4 - 2 = 2
a = 0, b = 1, c = 2
After first iteration,
diff = 2 - 1 = 1
a = 1, b = 2, c = (3 * 0 + 2 * 1 + 2) = 4
diff > 0 So second iteration will be called
After second iteration,
diff = 1 - 1 = 0
a = 2, b = 4, c = (3 * 1 + 2 * 2 + 4) = 11
Now diff = 0, so c is the answer.
f(4) = 11
The implementation of this process is recursive as shown below. But
process which is evolved is iterative.
``` scheme iterative-solution
(define (f n)
(if (< n 3)
(f-iter 0 1 2 (- n 2))))
(define (f-iter a b c diff)
(if (= diff 0)
(f-iter b c (+ (* 3 a) (* 2 b) c) (- diff 1)))) ```
Iterative process evloved for f(4)
;; (f 4)
;; (f-iter 0 1 2 2)
;; (f-iter 1 2 4 1)
;; (f-iter 2 4 11 0)
16 Feb 2013
SICP Exercise 1.6 is very interesting. I found it difficult to understand the problem and then the solution. It was fun to actually understand and reason about the solution.
In section 1.1.7 we define a function to calculate square root of a number by Newton’s method of successive approximation.
``` scheme Newton’s method of successive approximation for finding square root of a number
(define (square x)
(* x x))
(define (average x y)
(/ (+ x y) 2))
(define (improve guess x)
(average guess (/ x guess)))
(define (good-enough? guess x)
(- x (square guess))) 0.0000000001))
(define (square-root-iter guess x)
(if (good-enough? guess x)
(square-root-iter (improve guess x) x)))
Exercise 1.6 defines a replacement for special form
if as follows
``` scheme ‘if’ as a ordinary procedure
(define (new-if predicate then-clause else-clause)
(cond (predicate then-clause)
square-root-iter function changed as follows
``` scheme New version of ‘square-root-iter’
(define (square-root-iter guess x)
(new-if (good-enough? guess x)
(square-root-iter (improve guess x) x)))
The problem asks us, what is the result of this version of
new-if is correct and it can be tested with some test
(new-if (= 5 0) 0 5)
(new-if (= 0 0) 0 0)
But still evaluation of
new-if results in
infinite loop. Why?
The answer is
order of evaluation.
In scheme, there is a general rule of evaluation. First value inside
parenthesis is considered as function and all other values are passed
as arguments to that function. Only
Special forms are evaluated
if is a special form which is evaluated in following way.
(if <predicate> <consequent> <alternative>)
First, <predicate> is evaluated. If it is true, then <consequent> is evaluated. Otherwise <alternative> is evaluated.
But both <consequent> and <alternative> are never evaluated at the same time.
new-if is not a special form. So it will be evaluated as any
other function following
applicative order of evaluation.
Applicative order of evaluation first evaluates all operands
completely and then passed them to operator. There is also
order evaluation in which operands are evaluated
when they are
Scheme interpreter uses applicative order. So in new version of
square-root-iter, while evaluating
new-if it will try to evaluate
it’s operands first. But the second operand of
new-if is recursive call to
square-root-iter. So this evaluation will never finish.
Problem is not in implementation of
new-if but in it’s
07 Feb 2013
Define a procedure that takes three numbers as arguments and returns the sum of squares of the two larger numbers
I broke this problem into 3 steps
square function which will take one argument and return
(define (square x) (* x x))
; (square 6) 36
sum-of-squares function which will take two arguments and calculate sum of their squares.
(define (sum-of-squares x y) (+ (square x) (square y)))
; (sum-of-squares 3 4) 25
max function which will take two numbers and return maximum of both numbers.
(define (max x y) (if (> x y) x y))
; (max 3 4) 4
Now the solution to the problem is just combine all the 3 subproblems
(define (sum-of-squares-of-2-larger x y z)
(sum-of-squares (max x y)
(if (= (max x y) x)
(max y z)
(max x z)
; (sum-of-squares-of-2-larger 1 2 3) 13
; (sum-of-squares-of-2-larger 5 4 3) 41
; (sum-of-squares-of-2-larger 1 3 2) 13
SICP chapter 1 tells about how problems can be solved by breaking them into smaller subproblems and then building from them and solving bigger problems.
05 Feb 2013
I am starting writing this blog again. Last year, i started this blog thinking that i will write about my coding adventures but soon sloth caught me. I just wrote two posts last year(one of them being just an image).
This year i aim to write more in
quantity and in
My new year resolution is to complete the SICP book. I have the hard copy of the book and also have the videos from MITOCW
I am doing exercises in MIT Scheme as the book also uses it. I have started with chapter 1 and completed upto
Exercise 1-3. I am keeping my solutions here. I am planning to write my next post about the chapter 1 of SICP.
22 Mar 2012
I was introduced to Octopress by Vedang Manerikar in his talk on emacs.
I wanted to have a blog for long time but wanted something new, fresh and Octopress is just that.
If you want to learn about basic rake tasks, markdown, git etc; Octopress is just for you :)
Octopress : Blogging using Git, Github, Ruby
Octopress is a framework designed for Jekyll, the blog aware static site generator powering Github Pages by Brandon Mathis.
Detail features are available @ here
This tutorial is only for *nix like systems :D
#### Before You Begin
You will need to install git and setup your ruby enviornment
Octopress requires Ruby 1.9.2 which you can easily install with RVM.
#### Get source and install initial dependancies
git clone git://github.com/imathis/octopress.git octopress
cd octopress # If you use RVM, You'll be asked if you trust the .rvmrc file (say yes).
ruby --version # Should report Ruby 1.9.2
Now install dependancies
gem install bundler
Install the default Octopress theme.
A Detailed explanation about bundler is found here
Configure your blog
You have to change following fields from _config.yml
url: # For rewriting urls for RSS, etc
title: # Used in the header and title tags
subtitle: # A description used in the header
author: # Your name, for RSS, Copyright, Metadata
simple_search: # Search engine for simple site search
description: # A default meta description for your site
subscribe_rss: # Url for your blog's feed, defauts to /atom.xml
subscribe_email: # Url to subscribe by email (service required)
email: # Email address for the RSS feed if you want it.
Spice it up with twitter and other 3rd party plugins
Simply fill in the configurations which are already generated like your twitter id , github handle etc;
Get started with blogging
Octopress provides some rake tasks for creating new posts
bundle exec rake new_post["title"]
The default file extension for new posts is markdown.
This is an example from my first post
date: 2012-03-21 03:07
Generate and Deploy
bundle exec rake generate # Generates posts and pages into the public directory
bundle exec rake watch # Watches source/ and sass/ for changes and regenerates
bundle exec rake preview # Watches, and mounts a webserver at http://localhost:4000
bundle exec rake deploy
I haven’t covered all points because i don’t know them :) .
You can get better help @ octopress home page
Octopress is great asset with good documentation available so it’s good tool to blog.
21 Mar 2012
I am attending RubyConfIndia from 24th to 25th March 2012 in Pune.
Are you coming ?