## Thursday, April 11, 2013

### Odd-Even sort visualized

The Odd/Even sort is a sorting algorithm which uses the concept of the Bubble Sort to move elements around. Unlike Bubble sort, the Odd/Even sort compares disjointed pairs by using alternating odd and even index values splitting the sorting in different phases. We have that in the odd phase all the element with an odd index i are compared with the adjacent even index i+1 and in the even phase all the element with an even index i are compared with the adjacent odd index i+1. These two phases are repeated until no exchanges are required. This is a Python snippet that make us able to visualize the behavior of the algorithm:
```def oddeven_anim(a):
imgidx = 0
x = range(len(a))
sort = False;
while not sort:
sort = True;
for i in range(1,len(a)-1,2): # odd phase
if a[i] > a[i+1]:
a[i+1], a[i] = a[i], a[i+1]
sort = False;
for i in range(0,len(a)-1,2): # even phase
if a[i] > a[i+1]:
a[i+1], a[i] = a[i], a[i+1]
sort = False;
pylab.plot(x,a,'k.',markersize=6)
pylab.savefig("oddevensort/img" + '%04d' % imgidx + ".png")
pylab.clf()
imgidx =  imgidx + 1

# run the algorithm
a = range(300)
shuffle(a)
oddeven_anim(a)
```
As in the other examples of sorting visualization in this blog, we have an image for each step of the algorithm. The following video have been produced joining all the images (here is explained how):

## Monday, April 1, 2013

The twitter API is a great tool for analyze tweets by code. In particular, the streaming API gives real time access to the global stream of tweets and, unlike a conventional REST API, it is used through a continuous connection to the Twitter servers. So it requires a persistent HTTP connection open as long as you need to collect tweets. The typical workflow of an application which uses this API is the following:

The easiest way to handle an HTTP streaming in Python is to use PyCurl, the Python bindings for the famous Curl network library. PyCurl allows you to provide a callback function that will be executed every time a new block of data is available. The following code is a simple demonstration of how we can use HTTP streaming with PyCurl in order to analyzie a stream of tweet:
```from __future__ import division
from collections import defaultdict
from pylab import barh,show,yticks
import pycurl
import simplejson
import sys
import nltk
import re

def plot_histogram(freq, mean):
# using dict comprehensions to remove not frequent words
topwords = {word : count
for word,count in freq.items()
if count > round(2*mean)}
# plotting
y = topwords.values()
x = range(len(y))
labels = topwords.keys()
barh(x,y,align='center')
yticks(x,labels)
show()

def __init__(self):
self.freq = defaultdict(int)
self.cnt = 0
self.mean = 0.0
# composing the twitter stream url
nyc_area = 'locations=-74,40,-73,41'

def begin(self,usr,pws):
"""
init and start the connection with twitter using pycurl
usr and pws must be valid twitter credentials
"""
self.conn = pycurl.Curl()
# we use the basic authentication,
# in future oauth2 could be required
self.conn.setopt(pycurl.USERPWD, "%s:%s" % (usr, pws))
self.conn.setopt(pycurl.URL, self.url)
self.conn.perform()

""" Handles the arrive of a single tweet """
self.cnt += 1
# a little bit of natural language processing
tokens = nltk.word_tokenize(tweet['text']) # tokenize
tagged_sent = nltk.pos_tag(tokens) # Part Of Speech tagging
for word,tag in tagged_sent:
# filter sigle chars words and symbols
if len(word) > 1 and re.match('[A-Za-z0-9 ]+',word):
# consider only adjectives and nouns
if tag == 'JJ' or tag == 'NN':
self.freq[word] += 1 # keep the count
# print the statistics every 50 tweets
if self.cnt % 50 == 0:
self.print_stats()

def print_stats(self):
maxc = 0
sumc = 0
for word,count in self.freq.items():
if maxc < count:
maxc = count
sumc += count
self.mean = sumc/len(self.freq)
print '-------------------------------'
print ' tweets analyzed:', self.cnt
print ' words extracted:', len(self.freq)
print '   max frequency:', maxc
print '  mean frequency:', self.mean

def close_and_plot(self,signal,frame):
print ' Plotting...'
plot_histogram(self.freq,self.mean)
sys.exit(0)
```
In the constructor of this class we initialize a dictionary that will contain the frequency of each word, a string that contains the url of the service we need to call (composed in order to query twitter for the tweets in NYC) and the variables cnt and mean to keep track of the number of tweets analyzed and of the mean frequency over all the words.
In the method begin, we use the PyCurl library for the authentication to Twitter and start the connection. In particular, we set that the method on_receive is the callback function demanded to processing of the incoming. In this method the actual analysis is done, every tweet is split in tokens and a part of speech tagging is performed. Then, the frequency of all the words that are adjective or nouns is updated.
The method print_stats is used to print the our statistics on the console while close_and_plot plots an histogram using the frequencies in the dictionary and closes the program.
Let's use this class:
```import signal
pws = "yessosexyiam"

# invoke the close_and_plot() method when a Ctrl-D arrives
signal.signal(signal.SIGINT, ta.close_and_plot)
ta.begin(usr,pws) # run the analysis
```
In the code above, a TwitterAnalyzer object is instantiated, its method close_and_plot is registered as handler for the SIGINT signal and, finally, begin is invoked.
This code starts a program which analyzes all the tweet of the New York Area in real time and prints the statistics every 50 tweets, just like follows:
```-------------------------------
tweets analyzed: 50
words extracted: 110
max frequency: 8
mean frequency: 1.12727272727
-------------------------------
tweets analyzed: 100
words extracted: 200
max frequency: 22
mean frequency: 1.235
-------------------------------
tweets analyzed: 150
words extracted: 286
max frequency: 29
mean frequency: 1.26573426573
-------------------------------
tweets analyzed: 200
words extracted: 407
max frequency: 39
mean frequency: 1.31203931204
-------------------------------
tweets analyzed: 250
words extracted: 495
max frequency: 49
mean frequency: 1.37575757576
```
Pressing Cntrl-D we can stop the program and plot a bar chart of adjectives and nouns detected. This is what I got in the morning of March 21, 2013:

We see that is very common to post a link in a tweet (turned out that http is considered as a noun most of the time) and that the words day, today, good and morning were the most used during the analysis.