profile for Kells1986 at Stack Overflow, Q&A for professional and enthusiast programmers

Sunday, 22 December 2013

New Android App To Control Classroom Chatter

I've not blogged for quite a while now, despite saying I would make an effort to blog more often. Anyway I decided to write a post to let people know about a new app that I've made.

My sister works as a Math teacher in a secondary school, teaching kids aged 11 - 16. She sometimes struggles to keep the class quiet, so she had the idea of sounding an alarm on her phone when the noise level got too high.

I set to work and a week or so later I had put together an app that did exactly what she wanted. The app is called Quiet Classroom, you can download it from the play store. So far it's been fairly well received. I linked to the app on reddit where it seemed to have a fairly decent response, 90% of people liked it, so I was pretty happy with that.

This was the first app that I'd tried charging people for. I decided to charge £0.69 for the app to recoup the $25 developer signup fee and also to try and make a little profit as I'd spent a good chunk of time working on it.

This was the first app I'd released in a couple of years, and I was disappointed with the changes to the Android market. When I released my first ever app there was at least a "new releases" section that showcased some apps, my first app got 250 downloads in the first day because of this. However, now there is no "showcasing" and it seems really difficult to get any downloads at all.

For all of the 90% likes and positive reviews to-date, the app has only had 14 downloads. I don't know how this stacks up to other apps, or apps for iOS etc, but I was really disappointed by this. I was wondering what techniques other Android developers use? It would be good to perhaps start a discussion about this in the comments section.

Tuesday, 8 October 2013

A Ruby Gem for Parsing Finktank Soccer Predictions

I've recently been learning Ruby, as I have an idea for a web app I'd like to make which I think would be well suited to the Rails framework. In order to become familiar with Ruby on Rails I've completed the Rails for Zombies course, and enrolled on an Edx course.

I also did a short tutorial on using APIs with Ruby. This gave me the idea of generating an API for a Soccer prediction website run by a group known as the Finktank. The predictions of the Finktank have been consistently more accurate than the bookmakers' odds, allowing the opportunity for profit.

Deciding which games to bet on based on the Finktank's odds is a cumbersome process as they are listed as percentage probabilities and don't link to bookmakers, so you have to do some tedious calculations. That's why I wanted to do something to automate the system.

To do this I decided to take a look at the site's source code. In there they call a perl script which runs on the server and generates a json of game probabilities. The script can be passed various division Ids, so you can control which games you get predictions for.

Now to the code. This is really simple to do, all we have to do is send a request to the dectech server and parse the json that they send us. So here it is:

require 'open-uri'
require 'json'

def get_all_predictions()

  game = open("")
  body =
  hashes = JSON.parse(body)
  predictions = hashes["games"]
  return predictions

def get_league_predictions(league)

  game = open("{league}")
  body =
  hashes = JSON.parse(body)
  predictions = hashes["games"]
  return predictions


def get_prem_predictions()

  return get_league_predictions(0)


def get_champ_predictions()

  return get_league_predictions(12)


def get_l1_predictions()

  return get_league_predictions(13)


def get_l2_predictions()

  return get_league_predictions(14)


def get_spl_predictions()

  return get_league_predictions(1)


There you go, you can now get an array of hashes that contains all the predictions for the upcoming two weeks. Each hash has the following format:

["home":<home team>, "away": <away team>, "homeID":<home integer>, "awayID":<away integer>, "date":<match date>, "expGoalDiff": <expected goal difference (positive in favour of home, negative in favour of away)>, "hpct":<home percentage win probability>, "dpct":<draw probability>, "apct":<away win probability>]

If you want to use this code luckily it's really easy as I posted it on, so you can simply do gem install finktank. Once you've done that you can very quickly test it by running this file:

require 'finktank'

puts get_l2_predictions

And if you run this some time before October 12th, you'll see that the might Morecambe FC have only a 33% chance of a win away at Bury this weekend!

Tuesday, 24 September 2013

Write Your First Rails 4 App

I've recently been trying to learn Ruby on Rails, as I've got an idea for an app that I'd like to develop. I completed the excellent Rails for Zombies series of tutorials, this gave me a good grasp of models, controllers, views and routes, however I wasn't sure how to stick all these things together from scratch.

After a lot of Googling I managed to put together the steps required to make a "Hello World" type app. I think a lot of tutorials have been made obsolete by updates to Rails, but this one will work as long as you're using Rails version 4.

The first step is to install Rails. I used Ruby Version Manager to install Rails on my Mac, there are pretty good instructions on that website to get up and running quickly.

We're going to make a really simple website where anyone can come along and write a message, and all the messages that have been left can be read.

To generate a new app - which we're calling message_list - run the following command: rails new message_list. This will create a directory called message_list, next you should cd message_list, and then rails server.

If you point your browser to localhost:3000 you should see a Rails test page if everything has worked properly. If there are any problems at this stage it's probably best to got back to the RVM page and check your installation.

Now we make our model, it's going to be called a message. We do this by typing rails g model message. This will make us the following files: app/models/message.rb and db/migrate/<date>_create_messages.rb.

Next, edit db/migrate/*_create_messages.rb file, to look like this:

class CreateMessages < ActiveRecord::Migration
  def change
    create_table :messages do |t|

        t.string :name
        t.string :content


Now we need to build the database, cd to the top directory of the app, and type:

rake db:create
rake db:migrate

Run rails console and then run these commands:

mes =
mes.attributes={ :name => "James", :content => "This is a test"}

Now edit app/models/message.rb so it looks like this:

class Message < ActiveRecord::Base

        validates :name, :presence => true;
        validates :content, :presence => true;


Now you're done with the model we need view and controllers. To get these we run this command from the top directory of the app:

rails g controller messages

Next, edit app/controllers/messages_controller.rb. We're going to add functions index - for showing all the messages and new and create for writing new messages. These functions should of course have corresponding view files, which we'll get to later.

Here is what messages_controller.rb should look like:

class MessagesController < ApplicationController

        def index
                @posts = Message.all

        def new
                @message = 

        def create
                @message =
                redirect_to '/'

       def show

                respond_to do |format|


 def mess_params
    params.require(:message).permit(:name, :content)

We also have to add an additional function to permit the the create function to access the parameters necessary to formulate a message. Apparently this is new to Rails 4.

Now the Model and Controller is in place, let's write the views.

We need two files, app/views/messages/index.html.erb and app/views/messages/new.html.erb. In index.html.erb we need the following:

<h1>Leave A Message!</h1>
<%= link_to "New Message", new_message_path %>
<h1>All Posts:</h1>

<% @posts.each do |post| %>
        <td><%= %></td>
        <td><%= post.content %></td>
        <% end %>

And in new.html.erb we need this:

<%= form_for :message, url: messages_path do |f| %>
    <%= f.label :name %><br>
    <%= f.text_field :name %>
    <%= f.label :Message %><br>
    <%= f.text_area :content %>
    <%= f.submit %>
<% end %>

So we have Models, Controllers and Views, now we just need Routes to complete the app. In config/routes.rb we need to add the following:

root :to => "messages#index"
resources :messages

Now you have a complete Rails app, run rails server and go to localhost:3000. You should see the message that we wrote before on the console. There will also be a link to write a new message. Click that link, write out a message and click submit, and you should go back to the list of messages and see your message!

Hopefully that's brought a few things together and you can see how to pull together the theory you learnt to build an app from scratch.

Tuesday, 17 September 2013

Makefile Tutorial


I thought I'd write a brief tutorial about Makefiles, as this was a subject I had little understanding of while at university, and it wasn't until I began my current job that I realised how powerful they were.

The main use of a Makefile is to simplify the process of compiling large multi-file projects. However, this is not their only use. They can also be used in a similar way to shell scripts, to perform other tasks for you.

Makefiles generally consist of targets, dependencies, commands and variables. A Makefile is generally in the following format


target: dependency1 dependency2...
        command to make target

Note: There must be a tab before the command! Spaces are not adequate, you will get mysterious errors if you don't include a tab.

Create a fresh directory somewhere and create a file called "Makefile", then copy in the following commands:


all: hello myname

echo "Hello"

echo "My Name Is "$(NAME)

mynamedep: hello myname

echo "This was written using dependencies"

Now, if you run make you will see the commands in the Makefile and the result of their execution. There are three rules here, one for the target all, one to target hello, and one to target myname. You can run make and supply a specific target to just run that command, for instance running make myname will execute the rule to the target myname. If you don't specify a target then it will run for target all. If you don't specify a target called all, it will work on the target you specify at the top of the Makefile. 

There are two targets that make use of dependencies, these are all,and mynamedep. You can list a dependency next to a target and make will check if the dependency target needs to be built before it can continue. This comes in useful for compiling later, the above example is a little contrived.

You will notice that there is also a variable being used, much like you might see in a shell script, however in Makefiles these variables are braced in parentheses when they are referenced. I'll talk more about variables later.


Suppose that you had two files, main.c and functions.c. The first file main.c contains only the "main" function, but inside main, functions from functions.c are called. How would we write a Makefile for that?

Well we need to compile functions.c to get our object file functions.o, and then compile main.c to generate main.o, and finally we need to link functions.o and main.o to create ourprogram.

A really simple file would look like this:

#ourprogram needs main.o and functions.o
ourprogram: main.o functions.o
        gcc main.o functions.o -o ourprogram

#main.o is the target and needs main.c and functions.o
main.o: main.c functions.o
        gcc -c main.c

#functions.o is the target and depends on functions.c
functions.o: functions.c
        gcc -c functions.c

The above Makefile would work just fine, when we call "make" our code would build, assuming there were no compiler errors. However, if we wanted to tweak this Makefile it would be kind of laborious - say we wanted to use a different compiler we would have to find and replace the instances of gcc.

We could use variables to make this easier to maintain:


#ourprogram needs main.o and functions.o
$(EXEC): main.o functions.o
        $(CC) main.o functions.o -o $(EXEC)

#main.o is the target and needs main.c and functions.o
main.o: main.c functions.o
        $(CC) -c main.c

#functions.o is the target and depends on functions.c
functions.o: functions.c
        $(CC) -c functions.c 

Now if we want to change the name of our program we can change the variable EXEC, or we can change the compiler by just editing the CC variable. We could also have written the line $(CC) main.o functions.o -o $(EXEC) as $(CC) main.o functions.o -o $@. The $@ variable is a special variable that references the target name.

We can also store more than one thing in a variable, and make will iterate through. For example, we could store all of the objects we need in a variable:

OBJECTS=main.o functions.o

#ourprogram needs main.o and functions.o
        $(CC) $(OBJECTS) -o $@

#main.o is the target and needs main.c and functions.o
main.o: main.c functions.o
        $(CC) -c main.c

#functions.o is the target and depends on functions.c
functions.o: functions.c
        $(CC) -c functions.c 

Now we are beginning to see how Makefiles are so powerful. They can do other really cool stuff like working out which file to compile to generate the correct object. This means that we can replace the final two rules with a single rule, if we add another variable containing our source files, and one more containing compiler flags:

OBJECTS=main.o functions.o
SOURCES=main.c functions.c

        $(CC) $(OBJECTS) -o $(@)

    $(CC) $(CFLAGS) $*.c -o $@

There we have a complete Makefile that can easily be extended to incorporate more files, change the name of the executable, change the compiler, or change the compiler flags.

I hope that this has been a comprehendible tutorial and that you've managed to follow along. I remember when I was new to Linux that tutorials could quickly go out of my depth and look a little daunting. There are plenty more resources for learning about Makefiles, but there is one book in particular I'd recommend if you are keen to learn more, 21st Century C

Thursday, 12 September 2013

Use Dialog to Add a Simple GUI to a Bash Script

I've been looking for a tool to create a GUI for my scripts recently. I wanted to find something that wouldn't introduce too much more coding but add a professional front-end.

I came across dialog, it's a tool that you can use in your scripts to do just what I wanted to do. Here is a simple "Hello World!" code that you can use:

dialog --title 'Message' --msgbox 'Hello, world!' 5 20

You can create menu pages, checklists, yes-no boxes, input-boxes and much more. 

To install it, all you need to do on Ubuntu is sudo apt-get install dialog. You can also install it on Mac OSX - if you install home brew - using brew install dialog.

There are a couple of things you need to know about dialog to get a simple app running. You get data back from the GUI by either reading stderr, or from dialog's return value. For instance the dialog --menu page will return 0 if the user presses Ok, 1 if the user presses Cancel, or 255 is Esc is pressed. If the user selects Ok, the number of the selected item will be written to stderr.

This is best illustrated by a very simple example app. This script doesn't have much functionality but should illustrate the principles adequately:



while [[ 1 ]]; do
dialog --cancel-label "Quit" --menu "Choose one:" 10 30 5 1 "Enter Name" 2 "Something Else" 2>$reader

if [[ $retval != 0 ]]; then
exit 0

choice=$(cat $reader)

if [[ $choice == 1 ]]; then

dialog --inputbox "Enter your name:" 8 40 2>$reader
name=$(cat $reader)
dialog --infobox "Your name is ${#name} characters long" 10 50
sleep 2


dialog --infobox "Unfortunately I can't think of another example." 10 50
sleep 2


If you run this script you will see a menu page like this:

Here you can see that stderr - file descriptor number 2 - is redirected to a file in /tmp. That is how the menu and input boxes are read. The return value of the menu page is checked to see if the user wants to quit. On the main page it should be possible to cancel or press escape to quit the script.

Another compatible library for developing GUIs is Xdialog. It is syntactically similar to dialog, but shows X windows instead.

Sunday, 1 September 2013

Playing with an Arduino

I'm making a conscious effort to try and blog more regularly, so I thought I'd write up the stuff I learnt playing around with an Arduino Uno this weekend. My boss bought me this some time ago and I've kind of ignored it until I thought it might be useful for something work related. I was really impressed, these things are cool, so I just thought I'd add to the massive amount of resources out there with my experiences.

There are tons of projects out there that people have shared online. If you need some ideas about what can be done then check this website out. Also there's an interesting TED talk from the inventor of the Arduino board who talks about some of the stuff people have done with them.

Getting started was really easy, I just needed a USB cable that I got from an old external hard-drive, and a 70 MB download. Within about 5 minutes I had one of the arduino's LEDs blinking. I can't provide a better guide than the one on the official site.

Anyway, I am interested in using the digital reading/writing capabilities for my work project, so I just thought I'd experiment with these, along with the analog reading feature. The simplest test I could think of was writing a digital signal out, converting this to analog, and then reading this back in through the analog reading port.

To make a digital to analog converter you just need some resistors and a breadboard. You can use something called an R-2R ladder to convert the signal to analog, here's the circuit diagram:
 I only had a few resistors handy, so I made a 2 bit DAC. I used pins 2 and 3 of the digital connectors to output the digital signal, and pin 0 of the analog connector to read the analog input.

The code looks like this:

int analogPin = 0;

int val =0;

int lsb = 2;
int msb = 3;

void setup()
  pinMode(lsb, OUTPUT);
  pinMode(msb, OUTPUT);

void loop()
 int num = 0;
 for (num =0; num < 4; num++){
   val = analogRead(analogPin);

void flash(int num)
  if (num == 0){
    digitalWrite(lsb, LOW);
    digitalWrite(msb, LOW);
  if(num == 1){
    digitalWrite(lsb, HIGH);
    digitalWrite(msb, LOW);
  if(num == 2){
    digitalWrite(lsb, LOW);
    digitalWrite(msb, HIGH);
  if(num == 3){
    digitalWrite(lsb, HIGH);
    digitalWrite(msb, HIGH);
  if ( (num < 0) || (num > 3) ){
    digitalWrite(lsb, LOW);
    digitalWrite(msb, LOW);

To test this code, you simply need to copy the above into an Arduino sketch window, connect up the circuit properly, and then upload the code. When it starts running if you click "Serial Monitor" and you should see output that looks like this:


The code will keep looping through, and your values may be different to mine depending on the tolerance of your resistors, but it seems to do a reasonable job of converting the values.

Since playing around with this I've actually been able to use its functions to accomplish my task. Using and working with the arduino has been quite refreshing. As an embedded software engineer I find myself googling obscure stuff, and any information I find is generally unreliable. 

In contrast there is so much reliable information out there for the arduino, and it's really quick to get things to work. I can see why it's taken off and been commercially successful, and I will certainly be extolling it's virtues to anyone with a passing interest.  

That's all I've got to share today, hope you enjoy playing with your Arduino!

Tuesday, 27 August 2013

Free Python XML Soccer Live Scores

I recently had the problem, described in my previous blog, of inexact string matching between an XML feed and the Betfair API. This led to me creating a phoentic baby name generator which you should check out, if only to see the worst matches for yourself :)

Anyway, finding an XML feed that I could use for free and that was in any way reliable was proving a challenge. Eventually I found a site that provided the data for free and coded a Python wrapper to get the data with.

Here is the code you need to use it:

#Written by James Kelly on 28/06/13 

import feedparser
import time
import datetime

def get_feed():
    url = ""
    feed = feedparser.parse( url )
    return feed

def extract_info(game):

    start = game['title'].find('#',0)
    end = game['title'].find('vs',0)

    start = game['title'].find('#', start+1)
    end = game['title'].find(':', start)

    start = game['title'].find(' ',end)
    end = game['title'].find('-',start)

    end = len(game['title'])

    return game_info

def process_feed(feed):
    for game in feed.entries:
    return nicely_processed_feed

def get_feed_processed():

    return process_feed(get_feed())
def print_in_play_games():

    for info in get_feed_processed():
        if (info['summary'] != "Game Finished"):
            print info['home'] + " : "+ info['home_goals']
            print info['away'] + " : "+ info['away_goals']
            print info['summary'] 
            print ""

So far the feed has proved pretty reliable for my purposes. You will also need another Python module called Feedparser for the above code to work.

I hope this is helpful to some of you guys. Please leave a comment if you put this code to use!