Instant Datascience

When starting any course, you probably look at the syllabus to see what's covered and ask yourself if you can do it. In this lesson, we hope to give you a sense of how, with just a little bit of knowledge, you can make some real progress in using programs to answer questions with data.

To do so, we will be introducing the following topics:

  • Data types: working with text and data
  • Variables: storing data
  • Lists: working with data in an ordered collection
  • Dictionaries: representing data as a collection of attributes
  • Loops and iteration: repeating a sequence of instructions
  • Data visualization: using plots to display data
  • Functions: defining and running specific procedures in code

This lesson will offer a brief glimpse into each of these topics. In the rest of the course, we will provide lessons and labs on each of these topics to break down the material step by step. Let's get started!

Song Analysis

What makes a hit record? Does repetitiveness help? Is our music getting more repetitive over time? Questions like these were asked by the great computer scientist Donald Knuth in 1977, and then they were asked again and answered, by Colin Morris in this article.

Here is a chart that Colin Morris produced showing some of the most repetitive popular artists. How was something like this made and calculated?

After finishing the first section of our full-time data science curriculum, you will be able to produce the kind of data analysis you see in Colin Morris' article. Awesome, right?

Analyzing one song

Let's take the song Barbara Ann, the most repetitive song of the Beach Boys which was remastered by the cast of Saved by the Bell in 1990.

How did the cast learn all of the words so easily? Repetition. Here are some of the lyrics.

Ah, ba ba ba ba Barbara Ann
Ba ba ba ba Barbara Ann

Oh Barbara Ann take my hand
Barbara Ann
You got me rockin' and a-rollin'
Rockin' and a-reelin'
Barbara Ann ba ba
Ba Barbara Ann

...more lyrics...

Ba ba ba ba Barbara Ann
Ba ba ba ba Barbara Ann

It keeps going, but you get the point. Now let's say that we wanted to count up how many times each word in the above selection appears. Without a computer, we could do the following:

  • Place each of the words on a separate index card
  • Designate a spot for each unique word in our index cards
  • Then, flip through the index cards one by one
  • While flipping through each index card, find its designated pile and increase the size by one

Let's call these steps above our plan. At the end of this lesson and after completing each step of our above plan, we will have a chart, like the one below, giving us a visualization of the most repetitious words in the Beach Boys' song, Barbara Ann.

Great! Now let's translate our plan into code and make our chart!

Strings and Variables

To solve this problem with code, we do something similar. We start with our words in a string, which is a data structure that represents text. This is how it looks:

"Ah, Ba Ba Ba Ba Barbara Ann Ba Ba Ba Ba Barbara Ann Oh Barbara Ann Take My Hand Barbara Ann You Got Me Rockin' And A-Rollin' Rockin' And A-Reelin' Barbara Ann Ba Ba Ba Barbara Ann ...More Lyrics... Ba Ba Ba Ba Barbara Ann Ba Ba Ba Ba Barbara Ann"
"Ah, Ba Ba Ba Ba Barbara Ann Ba Ba Ba Ba Barbara Ann Oh Barbara Ann Take My Hand Barbara Ann You Got Me Rockin' And A-Rollin' Rockin' And A-Reelin' Barbara Ann Ba Ba Ba Barbara Ann ...More Lyrics... Ba Ba Ba Ba Barbara Ann Ba Ba Ba Ba Barbara Ann"

Note: What you see above in the first gray box is Python code. The content in this box is the code you would write. What comes below this first box is the output of running the code. So the output of creating a string, is just that same string - not very interesting.

To create a string in Python, notice that we place quotes at the start and end of text. If we don't do this, Python will give us an error.

So, to avoid errors, we need quotes around our text to make a string, but to hold on to this string and reference it later, we assign it to a variable. We call it lyrics.

lyrics = "Ah, Ba Ba Ba Ba Barbara Ann Ba Ba Ba Ba Barbara Ann Oh Barbara Ann Take My Hand Barbara Ann You Got Me Rockin' And A-Rollin' Rockin' And A-Reelin' Barbara Ann Ba Ba Ba Barbara Ann ...More Lyrics... Ba Ba Ba Ba Barbara Ann Ba Ba Ba Ba Barbara Ann"

Now whenever we type the word lyrics into Python we reference our string.

lyrics
"Ah, Ba Ba Ba Ba Barbara Ann Ba Ba Ba Ba Barbara Ann Oh Barbara Ann Take My Hand Barbara Ann You Got Me Rockin' And A-Rollin' Rockin' And A-Reelin' Barbara Ann Ba Ba Ba Barbara Ann ...More Lyrics... Ba Ba Ba Ba Barbara Ann Ba Ba Ba Ba Barbara Ann"

Okay, great. We now have our lyrics assigned to our variable, lyrics and we can use that to reference the string later in our code. If we remember our plan, the first step was to place each word on a separate index card, but strings are not good at separating our text into individual words. For that, we need a new data structure called a list.

Lists

To separate our string into a list of individual words, we need to change this continuous string into a Python list. Here is how we tell the computer to do this: split the string into a different entity every time you see a space. Here are those directions in code.

Note: we are going to be using methods like split and len throughout this lesson. Don't worry, we will explain what these methods do in later lessons. For now, focus on following the logic of what the inputs are doing.

list_of_lyrics = lyrics.split(' ')

Ok, let's see what list_of_lyrics looks like.

list_of_lyrics
['Ah,', 'Ba', 'Ba', 'Ba', 'Ba', 'Barbara', 'Ann', 'Ba', 'Ba', 'Ba', 'Ba', 'Barbara', 'Ann', 'Oh', 'Barbara', 'Ann', 'Take', 'My', 'Hand', 'Barbara', 'Ann', 'You', 'Got', 'Me', "Rockin'", 'And', "A-Rollin'", "Rockin'", 'And', "A-Reelin'", 'Barbara', 'Ann', 'Ba', 'Ba', 'Ba', 'Barbara', 'Ann', '...More', 'Lyrics...', 'Ba', 'Ba', 'Ba', 'Ba', 'Barbara', 'Ann', 'Ba', 'Ba', 'Ba', 'Ba', 'Barbara', 'Ann']

Ok, so this is a list. It's an ordered collection, and as you can see we are now treating each word as an individual entity. Each individual entity of a list is called an element. How many elements are there in this list?

len(list_of_lyrics)
51

Ok, but remember the second step of our plan was to allocate space for each unique word. Now that we have a list of all words, let's make a list of the unique words.

unique_words = set(list_of_lyrics)
{'...More', "A-Reelin'", "A-Rollin'", 'Ah,', 'And', 'Ann', 'Ba', 'Barbara', 'Got', 'Hand',
 'Lyrics...', 'Me', 'My', 'Oh', "Rockin'", 'Take', 'You'}

Ok, you may have noticed that our list of unique words is significantly smaller than our total list of words. How much smaller?

len(unique_words)
17

A lot.

So, there's a lot of repetition in Barbara Ann. It's time to keep track of each word and the number of occurrences of each word.

Using dictionaries

Our ultimate goal is to present our list of repetitions almost as a table, with a word to the left and the number of occurrences to the right.

Word Count
Ann 2
Barbara 3
Ba 8

In Python, this looks like a dictionary.

word_counts =  {'Ann': 2, 'Barbara': 3, 'Ba': 8}

A dictionary is a collection of key-value pairs which we use to store associated data. Here, each word is associated with its count, and we can use the key to access the associated value.

word_counts['Ann']
2

We can also use the key to reassign that value.

word_counts['Ann'] = 33
word_counts
{'Ann': 33, 'Ba': 8, 'Barbara': 3}

So that's the form we want. How do we get there with our list of lyrics?

Well we start by creating a dictionary with each key as a separate word, and then set the corresponding value to zero. Kind of like allocating a region on a table for each of our words. We do this with the unique_words list and the fromkeys method, whose second argument is the value we set for each key.

word_histogram = dict.fromkeys(unique_words, 0)
{'...More': 0, "A-Reelin'": 0, "A-Rollin'": 0, 'Ah,': 0, 'And': 0, 'Ann': 0, 'Ba': 0, 'Barbara': 0, 'Got': 0, 'Hand': 0, 'Lyrics...': 0, 'Me': 0, 'My': 0, 'Oh': 0, "Rockin'": 0, 'Take': 0, 'You': 0}

Loops

Ok, now we have two nice data structures. A list_of_lyrics of all of our words, and a word_histogram to keep track of the amount of words. It seems like we're making good progress. Let's look again at our plan.

  • Place each of the words on a separate index card.
    • Complete as list_of_lyrics
  • Allocate space for a small pile for each unique word
    • Complete as word_histogram
  • Then, go through the index cards one by one
  • And for each index card, increase the size of its related pile by one

So looking through the steps, all that's left are the last two steps.

In Python, to go through elements of a list one by one, we use a for loop.

for number in [1,2,3,4]:
    print(number + 10)
11
12
13
14

Ok, so here we want to go through the elements of our list_of_lyrics one by one. For each word in list_of_lyrics, we want to find the related key in the dictionary, word_histogram, and increase the value by one. So now, instead of looping through a list of numbers, we will loop through each of our words in list_of_lyrics, find the related value in the dictionary, and increase it by one.

Deep breath These next lines of code may look like magic. The lessons that follow will explain them.

word_histogram = dict.fromkeys(unique_words, 0)
for word in list_of_lyrics:
    word_histogram[word] = word_histogram[word]+ 1

We said it would be confusing. Good thing there are more lessons to explain it. Let's see if it worked.

word_histogram
{'...More': 1, "A-Reelin'": 1, "A-Rollin'": 1, 'Ah,': 1, 'And': 2, 'Ann': 8, 'Ba': 19, 'Barbara': 8, 'Got': 1,
 'Hand': 1, 'Lyrics...': 1, 'Me': 1, 'My': 1, 'Oh': 1, "Rockin'": 2, 'Take': 1, 'You': 1}

Ok, that's it. Now if we want to see how many times 'Rockin' appears, we can do so easily.

word_histogram["Rockin'"]
2

Visualizing the data

We've got our answer in code. The final step is to turn it into a chart. We'll use a library -- which is a collection of code we get from the Internet that does not come with Python -- called Plotly to make our charts.

In the first four lines we tell Python to get ready to use this library. And in the last line we tell Python to plot our trace.

The meat of the code is our trace. A trace points to a dictionary with a key of x that points to a list of x_values, and a key of y that points to y_values. And a type to indicate that this will be a bar chart.

import plotly
from plotly.offline import iplot, init_notebook_mode
from plotly import tools
import plotly.graph_objs as go
init_notebook_mode(connected=True)

trace = {'type': 'bar', 'x': list(unique_words), 'y': list(word_histogram.values())}

plotly.offline.iplot({'data': [trace]})

Above we can see that x points to a list of the unique_words, and y points to the list of values from our word_histogram, which represent the number of times each word appears.

We have now plotted our words! We can see that The Beach Boys say "Ba" 19 times, and remember we only copied over some of the lyrics! Repetitive indeed.

Summary

Hopefully, in this section you can see that even with just a bit of knowledge we can really put code to use. It may have seemed like a lot of work, but the work was in the learning, not the code.

All of the code written so far was really just six lines of code plus another 8 lines to plot our chart, giving us a grand total of a mere 14 lines of code!

In the following sections, we will cover the topics we introduced in this lesson and more, so that we can begin use the tools above to explore information with code.

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