Doors in the cloud

3.1 Register Our App on Twitter

Twitter API interface

Cloud applications are characterized by an increased focus on user participation and content creation, but also by a deep interaction and interconnection of applications sharing content from different types of services in order to integrate multiple systems together. This scenario is, doubtlessly, possible thanks to the rise of the “Application Programming Interfaces” (API).

An API, or Application Programming Interface, provides a way for computer systems to interact with each other. There are many types of APIs. Every programming language has a built-in API that is used to write programs. For instance, you studied in previous courses that operating systems themselves have APIs used by programs to open files or draw text on the screen. Due to this course is centered in the Cloud, we are going to focus on API that is built with web technologies as HTTP. We will refer to this type of API as web API, an interface to either a web server or a web browser. These APIs are used extensively for the development of web applications and work at either the server end or the client end. Web APIs are a key component of today’s cloud era. Many cloud applications provide an API that allows developers to integrate their own code with these applications, taking advantage of the services’ functionality in their own apps.

One example amount the vast number of available ones is Twitter API. Twitter API allows to access all tweets made by any user, the tweets containing a particular term or even a combination of terms, tweets have done on the topic in a particular date range, etc.

In order to set up this hands-on to access Twitter data, there are some preliminary steps. Twitter implements OAuth (called Open Authorization) as its standard authentication mechanism, and in order to have access to Twitter data programmatically, we need to create an app that interacts with the Twitter API. There are four primary identifiers we will need to note for an OAuth workflow: consumer key, consumer secret, access token, and access token secret. The good news from the developer’s perspective is that the Python ecosystem has already well-established libraries for most social media platforms, which come with an implementation of the authentication process.

Register your app

The first step in this hands-on is the registration of your app. In particular, you need to find by your own how to register a new application from the official doc web site (https://developer.twitter.com/en/portal/petition/essential/basic-info | https://developer.twitter.com/en/docs). After doing it, you will receive a Consumer Key and a Consumer Secret. From the configuration page “Keys and Access Token” of your app, you can also obtain the Access Token and an Access Token Secret. Save this information to perform the following Lab session.

Before proceeding to the previous steps, please do a kick review of all this hands-on in order to know the type of data you will collect because we should answer questions about that during the registration process.    It is important to use English for the answers. 

Warning: these are application settings that should always be kept private.

Note that you will need a Twitter account in order to log in, create an app, and get these credentials.

Note: For the following code lines, beware at COPY&PASTE!. Some symbols are “converted” by the HTML tranlator into non-standard. If a command does not work properly, repeat it by typing it.

3.2 Lab Tasks: Get Started with NLTK

One of the most popular packages in Python for NLP is Natural Language Toolkit (NLTK). The toolkit provides a friendly interface for many of the common NLP tasks, as well as lexical resources and linguistic data.

Tokenisation is one of the most basic, yet most important, steps in text analysis required in the following task. The purpose of tokenisation is to split a stream of text into smaller units called tokens, usually words or phrases. For this purpose we will use the NLTK Python Natural Language Processing Toolkit:

import nltk

A difference between NLTK and many other packages is that this framework also comes with linguistic data for specific tasks. Given their size, such data is not included in the default installation but has to be downloaded separately. For this reason, after importing NLTK, we need to download NLTK Data which includes a lot of corpora, grammars, models and etc. You can find the complete nltk data list here. You can download all nltk resources by nltk.download(‘all’) but it takes ~3.5G. For English text we could use nltk.download(‘punkt’) to download the NLTK data package that includes a pre-trained tokenizer for English.

Let’s see the example using the NLTK to tokenise the open book First Contact with TensorFlow (ISBN 978-1-326-56933-4 Barcelona, April 2016 [Content deprecated]). The text of this book could be downloaded as FirstContactWithTensorFlow.txt  from this GitHub . Then, we will output the 10 most common words in the book.

Task 3.1: Update the docker image, create a new container and run Jupyter Notebook

#update image 
docker pull jorditorresbcn/dl 
#Create a container 
docker run -it -p 8888:8888 --name CC-MEI-2022 jorditorresbcn/dl:latest 
#Clone the course repository 
git clone https://github.com/jorditorresBCN/CC-MEI-2018.git 
cd CC-MEI-2018 
#Ignore the error-> No web browser found: could not locate runnable browser. 
jupyter notebook --ip=0.0.0.0 --allow-root

If you close the container and you need to re-open it, run:

docker start -i CC-MEI-2022

On your computer, open your browser and go to http://localhost:8888, the password is dl.

If you are on windows and you are experiencing connectivity issues, please check THIS.

Task 3.2: On your browser, create a new notebook. Run the following code:

import nltk
nltk.download('punkt') 
import string

from collections import Counter

def get_tokens():
   with open('FirstContactWithTensorFlow.txt', 'r') as tf:
    text = tf.read()
    tokens = nltk.word_tokenize(text)
    return tokens, text

tokens, text = get_tokens()
count = Counter(tokens)
print(count.most_common(10))

Task 3.3: Create a notebook with the name Lab2.WordCountWithNLTK.ipynb, that computes and prints the 10 most common words in the book.

Task 3.4: Add a new code cell into the same notebook with the code that computes and prints the total number of words of this book.

Task 3.5: We can see that punctuation is many of the most common words. We can remove the punctuation using the character deletion step of the translate method as:

lowers = text.lower()
no_punctuation = lowers.translate(str.maketrans( '' , '' , string.punctuation))
tokens = nltk.word_tokenize(no_punctuation)

Then, theget_tokens()  function will be:

def get_tokens():
   with open('FirstContactWithTensorFlow.txt', 'r') as tf:
    text = tf.read()
    lowers = text.lower()
    no_punctuation = lowers.translate(str.maketrans( '' , '' , string.punctuation))
    tokens = nltk.word_tokenize(no_punctuation)
    return tokens

tokens = get_tokens()
count = Counter(tokens)
print(count.most_common(10))

Add a new code cell to the same notebook with the code (and the comments with markdown cells if you consider interesting) that computes and prints the 10 most common words without punctuation characters.

Note: The first contact with Markdown syntax can be found in my Github.

Task 3.6: Is not “Tensorflow” the most common word? Why? What are Stop Words? Include your answer in a markdown cell in the same notebook.

When we are working with text mining applications, we often hear of the term “Stop Word Removal”. We can do it using the same nltkpackage:

from nltk.corpus import stopwords
tokens = get_tokens()
nltk.download('stopwords')
filtered = [w for w in tokens if not w in stopwords.words('english')]
count = Counter(filtered)

print (count.most_common(10))

In these tasks, we will use tweepy package as a tool to access Twitter data in a fairly easy way with Python. There are different types of data we can collect, however, we will focus on the “tweet” object.

As homework, we already register Our App on Twitter in order to set up our project to access Twitter data. However, the Twitter API limits access to applications. You can find more detail in the official documentation. It’s also important to consider that different APIs have different rate limits. The implication of hitting the API limits is that Twitter will return an error message rather than the data we are asking for. Moreover, if we continue performing more requests to the API, the time required to obtain regular access again will increase as Twitter could flag us as potential abusers. If our application needs many API requests we can use the time module (time.sleep() function).

Another important thing before starting is to know that we have two classes of API: REST APIs and Streaming API. All the REST APIs only allow you to go back in time (tweets that have already been published). Often these APIs limit the number of tweets you can retrieve, not just in terms of rate limits as we mentioned, but also in terms of time span. In fact, it’s usually possible to go back in time up to approximately one week. Also, another aspect to consider about the REST API is that they are not guaranteed to provide all the tweets published on Twitter.

On the other hand, the Streaming API looks into the future, we can retrieve all the tweets that match our filter criteria, as they are published. The Streaming API is useful when we want to filter a particular keyword and download a massive amount of tweets about it, While the REST APIs are useful when we want to search for tweets authored by a specific user or we want to access our own timeline.

Task 3.7: Tweepy

In order to interact with the Twitter APIs, we need a Python client that implements the different calls to the API itself. There are several options as we can see from the official documentation. We will choose for this lab Tweepy.

Create a notebook with the name Lab2.TweepyAPI.ipynb to keep track of all your work.

In order to authorize our app to access Twitter on our behalf, we need to use the OAuth interface:

import tweepy
from tweepy import OAuthHandler

consumer_key = 'YOUR-CONSUMER-KEY'
consumer_secret = 'YOUR-CONSUMER-SECRET'
access_token = 'YOUR-ACCESS-TOKEN'
access_secret = 'YOUR-ACCESS-SECRET'

auth = OAuthHandler(consumer_key, consumer_secret)
auth.set_access_token(access_token, access_secret)

api = tweepy.API(auth)

The api variable is now our entry point for most of the operations with Twitter.

Tweepy provides access to the well-documented Twitter API. With tweepy, it’s possible to get an object and use any method that the official Twitter API offers. For example, a User object has its documentation at this link and following those guidelines, tweepy can get the appropriate information.

Main Model classes in the Twitter API are: Tweets, Users, Entities and Places.

In order to be sure that everything is correctly installed print the main information of your Twitter account. After creating the user object, the me()method returns the user whose authentication keys were used:

user = api.me()

print('Name: ' + user.name)
print('Location: ' + user.location)
print('Friends: ' + str(user.followers_count))
print('Created: ' + str(user.created_at))
print('Description: ' + str(user.description))

Is the data printed correctly? Is it yours?

Task 3.8: Accessing Tweets

Tweepy provides a convenient Cursor interface to iterate through different types of objects. For example, we can read our own Twitter home timeline with (we are using 1 to limit the number of tweets we are reading and only reading the text of the tweet):

for status in tweepy.Cursor(api.home_timeline).items(1):
    print(status.text) 

The status variable is an instance of the Status() class, a nice wrapper to access the data. The JSON response from the Twitter API is available in the attribute _json (with a leading underscore), which is not the raw JSON string, but a dictionary.

import json

for status in tweepy.Cursor(api.home_timeline).items(1):
    print(json.dumps(status._json, indent=2))

What if we want to have a list of 10 of our friends?

for friend in tweepy.Cursor(api.friends).items(1):
    print(json.dumps(friend._json, indent=2))

And how about a list of some of our tweets? 

for tweet in tweepy.Cursor(api.user_timeline).items(1):
    print(json.dumps(tweet._json, indent=2))

As a conclusion, you can notice that with tweepy we can easily collect all the information and store them in the original JSON format, fairly easy to convert into different data models (many storage systems provide import features).

Use the previous API presented for obtaining information about your tweets. Keep track of your executions and comments in the Lab2.TweepyAPI.ipynb notebook.

Task 3.9: Tweet pre-processing

In these tasks, we’ll enter in more detail the overall structure of a tweet and discuss how to pre-process its text before we can get into some more interesting analysis in the next Lab. In particular, we will see how tokenisation, despite being a well-understood problem, can get tricky with Twitter data. After that, we’ll discuss the analysis of term frequencies to extract meaningful terms from our tweets.

The code used in this Lab is using part of the work done by Marco Bonzanini). As Marco indicates, it is far from perfect but it’s a good starting point to become aware of the complexity of the problem, and fairly easy to extend.

Let’s have a look at the structure of the previous tweet that you printed. The main attributes are the following:

• text: the text of the tweet itself
• created_at: the date of creation
• favorite_count, retweet_count: the number of favourites and retweets
• favorited, retweeted: boolean stating whether the authenticated user (you) have favourited or retweeted this tweet
• lang: acronym for the language (e.g. “en” for English)
• id: the tweet identifier
• place, coordinates, geo: geo-location information if available
• user: the author’s full profile
• entities: list of entities like URLs, @-mentions, hashtags and symbols
• in_reply_to_user_id: user identifier if the tweet is a reply to a specific user
• in_reply_to_status_id: status identifier id the tweet is a reply to a specific status
• _json: This is a dictionary with the JSON response of the status
• author: The tweet author

As you can see there is a lot of information we can play with. All the *_id fields also have a *_id_str counterpart, where the same information is stored as a string rather than a big int (to avoid overflow problems).

We will focus our task on looking for the text of a tweet, breaking it down into words. While tokenisation is a well-understood problem with several out-of-the-box solutions from popular libraries, Twitter data pose some challenges because of the nature of the language. Let’s see the example using the NLTK package previously used to tokenise a fictitious tweet:

tweet = 'RT @JordiTorresAI: just an example! :D https://torres.ia #masterMEI'
print(nltk.word_tokenize(tweet))

You will notice some peculiarities of twitter that are not captured by a general-purpose English tokeniser like the one from NLTK: @-mentions, emoticons, URLs and #hash-tags are not recognized as single tokens. Right?

Using some code borrowed from Marco Bonzanini we could consider these aspects of the language (A former student, Cédric Bhihe, suggested this alternative code).

import re

emoticons_str = r"""

    (?:
        [:=;] # Eyes
        [oO\-]? # Nose (optional)
        [D\)\]\(\]/\\OpP] # Mouth

    )"""

regex_str = [
    emoticons_str,
    r'<[^>]+>', # HTML tags
    r'(?:@[\w_]+)', # @-mentions
    r"(?:\#+[\w_]+[\w\'_\-]*[\w_]+)", # hash-tags
    r'http[s]?://(?:[a-z]|[0-9]|[$-_@.&+]|[!*\(\),]|(?:%[0-9a-f][0-9a-f]))+', # URLs
    r'(?:(?:\d+,?)+(?:\.?\d+)?)', # numbers
    r"(?:[a-z][a-z'\-_]+[a-z])", # words with - and '
    r'(?:[\w_]+)', # other words
    r'(?:\S)' # anything else
]

tokens_re = re.compile(r'('+'|'.join(regex_str)+')', re.VERBOSE | re.IGNORECASE)
emoticon_re = re.compile(r'^'+emoticons_str+'$', re.VERBOSE | re.IGNORECASE)

def tokenize(s):
    return tokens_re.findall(s)

def preprocess(s, lowercase=False):
    tokens = tokenize(s)
    if lowercase:
        tokens = [token if emoticon_re.search(token) else token.lower() for token in tokens]
    return tokens

tweet = 'RT @JordiTorresAI: just an example! :D https://torres.ia #masterMEI'
print(preprocess(tweet))

As you can see, @-mentions, URLs, and #hash-tags are now preserved as individual tokens. This tokeniser gives you the general idea of how you can do it for twitter text based on regular expressions (regexp), which is a common choice for this type of problem.

With the previous basic tokenizer code, some particular types of tokens will not be captured and will be probably broken into several tokens. To overcome this problem you can improve the regular expressions, or employ more sophisticated techniques like Named Entity Recognition.

In this example, the regular expressions are compiled with the flags re.VERBOSE, to allow spaces in the regexp to be ignored (see the multi-line emoticons regexp), and re.IGNORECASE to catch both upper and lower cases. The tokenize() function simply catches all the tokens in a string and returns them as a list. This function is used within preprocess(), which is used as a pre-processing chain: in this case, we simply add a lowercasing feature for all the tokens that are not emoticons (e.g. 😀 doesn’t become :d).

Keep track of your executions with different fictitious tweets and comments in the Lab2.TokenizeTweetText.ipynb notebook.

Now, we are ready for the next Lab, where we will mining streaming twitter data.

Task 3.10: Streaming API of Twitter

In case we want to “keep the connection open”, and gather all the upcoming tweets about a particular event, the Streaming API is what we need. The Streaming APIs give developers low latency access to Twitter’s global stream of Tweet data. Proper implementation of a streaming client will be pushed messages indicating Tweets and other events have occurred. Connecting to the streaming API requires keeping a persistent HTTP connection open. In many cases, this involves thinking about your application differently than if you were interacting with the REST API. Visit the Streaming API for more details about the differences between Streaming and REST. The Streaming API is one of the favorite ways of getting a massive amount of data without exceeding the rate limits. If your intention is to conduct singular searches, read user profile information, or post Tweets, consider using the REST APIs instead.

We need to extend the StreamListener() class to customise the way we process the incoming data. We will base our explanation with a working example (from Marco Bonzanini) that gathers all the new tweets with the “ArtificialIntelligence” content:

import tweepy
from tweepy import OAuthHandler

consumer_key = 'YOUR-CONSUMER-KEY'
consumer_secret = 'YOUR-CONSUMER-SECRET'
access_token = 'YOUR-ACCESS-TOKEN'
access_secret = 'YOUR-ACCESS-SECRET'

auth = OAuthHandler(consumer_key, consumer_secret)
auth.set_access_token(access_token, access_secret)

api = tweepy.API(auth)

from tweepy import Stream
from tweepy.streaming import StreamListener

class MyListener(StreamListener):

    def on_data(self, data):
        try:
            with open('ArtificialIntelligenceTweets.json', 'a') as f:
                f.write(data)
                return True
        except BaseException as e:
            print("Error on_data: %s" % str(e))
        return True

    def on_error(self, status):
        print(status)
        return True

twitter_stream = Stream(auth, MyListener())
twitter_stream.filter(track=['ArtificialIntelligence'])

Warning: The previous cell should be stopped after a while (it is bloked  by the API , its just adding more text to the ArtificialIntelligenceTweets.json).

The core of the streaming logic is implemented in the CustomListener class, which extends StreamListener and overrides two methods: on_data() and on_error(). These are handlers that are triggered when data is coming through and an error is given by the API. if the error is that we have been rate limited by the Twitter API, we need to wait before we can use the service again. The on_data() method is called when data is coming through. This function simply stores the data as it is received in the  ArtificialIntelligenceTweets.json file. Each line of this file will then contain a single tweet, in the JSON format. You can use the command wc -l ArtificialIntelligenceTweets.json from a Unix shell to know how many tweets you’ve gathered.

Before continuing the hands-on, be sure that you generated correctly the .json file. now try with another term of your interest.

Task 3.11: Analyzing tweets – Counting terms

The first exploratory analysis that we can perform is a simple word count. In this way, we can observe what are the terms most commonly used in the data set.

Let’s go to read the file with all tweets in order to be sure that everything is fine:

import json

with open('ArtificialIntelligenceTweets.json','r') as json_file:
         for line in json_file:
             tweet = json.loads(line)
             print(tweet["text"])

Now we are ready to start to tokenize all these tweets:

import json

with open('ArtificialIntelligenceTweets.json', 'r') as f:
    line = f.readline() 
    tweet = json.loads(line)
    print(json.dumps(tweet, indent=4)) 

Now, if we want to process all our tweets, previously saved on file:

with open('ArtificialIntelligenceTweets.json', 'r') as f:
        for line in f:
            tweet = json.loads(line)
            tokens = preprocess(tweet['text'])
            print(tokens)

Remember that preprocess have been already defined in order to capture Twitter-specific aspects of the text, such as #hashtags, @-mentions, and URLs.:

import re

emoticons_str = r"""
    (?:
        [:=;] # Eyes
        [oO\-]? # Nose (optional)
        [D\)\]\(\]/\\OpP] # Mouth
    )"""

regex_str = [
    emoticons_str,
    r'<[^>]+>', # HTML tags
    r'(?:@[\w_]+)', # @-mentions
    r"(?:\#+[\w_]+[\w\'_\-]*[\w_]+)", # hash-tags
    r'http[s]?://(?:[a-z]|[0-9]|[$-_@.&+]|[!*\(\),]|(?:%[0-9a-f][0-9a-f]))+', # URLs
    r'(?:(?:\d+,?)+(?:\.?\d+)?)', # numbers
    r"(?:[a-z][a-z'\-_]+[a-z])", # words with - and '
    r'(?:[\w_]+)', # other words
    r'(?:\S)' # anything else
]

tokens_re = re.compile(r'('+'|'.join(regex_str)+')', re.VERBOSE | re.IGNORECASE)
emoticon_re = re.compile(r'^'+emoticons_str+'$', re.VERBOSE | re.IGNORECASE)

def tokenize(s):
    return tokens_re.findall(s) 

def preprocess(s, lowercase=False):
    tokens = tokenize(s)
    if lowercase:
        tokens = [token if emoticon_re.search(token) else token.lower() for token in tokens]
    return tokens

In order to keep track of the frequencies while we are processing the tweets, we can use collections.Counter() which internally is a dictionary (term: count) with some useful methods like most_common():

import operator
import json
from collections import Counter

fname = 'ArtificialIntelligenceTweets.json'
with open(fname, 'r') as f:
   count_all = Counter()
   for line in f:
       tweet = json.loads(line)
       # Create a list with all the terms
       terms_all = [term for term in preprocess(tweet['text'])]
       # Update the counter
       count_all.update(terms_all)
   print(count_all.most_common(5))

As you can see, the above code produces words (or tokens) that are stop words. Given the nature of our data and our tokenisation, we should also be careful with all the punctuation marks and with terms like RT (used for re-tweets) and via (used to mention the original author), which are not in the default stop-word list.

import nltk

from nltk.corpus import stopwords
nltk.download("stopwords") # download the stopword corpus on our computer
import string

punctuation = list(string.punctuation)
stop = stopwords.words('english') + punctuation + ['rt', 'via', 'RT']

We can now substitute the variable terms_all in the first example with something like:

import operator
import json
from collections import Counter

fname = 'ArtificialIntelligenceTweets.json'
with open(fname, 'r') as f:
    count_all = Counter()
    for line in f:
        tweet = json.loads(line)
        # Create a list with all the terms
        terms_stop = [term for term in preprocess(tweet['text']) if term not in stop]
        count_all.update(terms_stop)
    for word, index in count_all.most_common(5):
        print( '{} : {}'.format (word, index))

Besides stop-word removal, we can further customize the list of terms/tokens we are interested in. For instance, if we want to count hashtag sonly:

terms_hash = [term for term in preprocess(tweet['text'])
              if term.startswith('#')]

In this case, we are interested to count terms only, no hashtags and no mentions:

terms_only = [term for term in preprocess(tweet['text'])
              if term not in stop and
              not term.startswith(('#', '@'))]

NOTE: Mind the double brackets (( )) startswith() takes a tuple (not a list) if we pass a list of inputs.

Although we do not consider it in this Lab, there are other functions from NLTK very useful. For instance, to put things in context, some analysis considers sequences of two terms. In this case, we can use bigrams() function that will take a list of tokens and produce a list of tuples using adjacent tokens.

Do the same analysis with the .json file generated by you in the previous task.

3.4 Case study and Student proposal

Task 3.12: Case study

At Racó (course intranet)” you can find a small dataset as an example (please do not distribute due to Twitter licensing). This dataset contains 1060 tweets downloaded from  18:05 to 18:15 on January 13, 2018. We used “Barcelona” as a track parameter at twitter_stream.filterfunction.

In order to copy the file from your local storage to the docker, you can use this command:

docker cp Lab2.CaseStudy.json CC-MEI-2022:/app/CC-MEI-2018/.

You can add the file to Jupyter Notebook using the upload button on the main page of Jupyter.

We would like to get a rough idea of what was telling people about Barcelona. For example, we can count and sort the most commonly used hashtags:

import operator
import json
from collections import Counter

fname = 'Lab2.CaseStudy.json'
with open(fname, 'r') as f:
    count_all = Counter()
    for line in f:
        tweet = json.loads(line)
        # Create a list with all the terms
        terms_hash = [term for term in preprocess(tweet['text']) if term.startswith('#') and term not in stop]
        count_all.update(terms_hash)
# Print the first 15 most frequent words
print(count_all.most_common(15))

The output is: [(u'#Barcelona', 68), (u'#Messi', 30), (u'#FCBLive', 17), (u'#UDLasPalmas', 13), (u'#VamosUD', 13), (u'#barcelona', 10), (u'#CopaDelRey', 8), (u'#empleo', 6), (u'#BCN', 6), (u'#riesgoimpago', 6), (u'#news', 5), (u'#LaLiga', 5), (u'#SportsCenter', 4), (u'#LionelMessi', 4), (u'#Informe', 4)]

In order to see a more visual description, we can plot it. There are different options to create plots in Python using libraries like matplotlib, ggplot, etc. We decided to use matplotlib With the following code:

%matplotlib inline  
import matplotlib as mpl
mpl.rcParams['figure.figsize'] = (15,10)
import matplotlib.pyplot as plt

sorted_x, sorted_y = zip(*count_all.most_common(15))
#print(sorted_x, sorted_y)

plt.bar(range(len(sorted_x)), sorted_y, width=0.75, align='center');
plt.xticks(range(len(sorted_x)), sorted_x, rotation=80);
plt.axis('tight');

that uses the function zip(). We obtain the following plot:

We can see that people were talking about football, more than other things! And it seems that they were mostly talking about the football league match that was played the next day.

Create a “matplot” with your dataset generated by you in the previous task.

Task 2.13: Student proposal

We are asking the student to create an example that will allow us to find some interesting insight from Twitter, using some realistic data taken by the student. Using what we have learned in the previous sections, you can download some data using the streaming API, pre-process the data in JSON format and extract some interesting terms and hashtags from the tweets.

Create a .pynb file with markdown cells describing the program steps and the characteristics of the dataset created (e.g. the time frame for the download, etc.).

3.5 Lab report

Please, follow the indications of your teacher about how to create your lab report and how to submit it.

Acknowledgments

To Francesc Sastre, Jordi Sala, Juan L. Dominguez, David García and Raúl García for their reviews and suggerences.