Week 1: Intro to Python

DSUA111: Data Science for Everyone, NYU, Fall 2020

TA Jeff, jpj251@nyu.edu

https://github.com/jpowerj/dsua111-sections

We'll be coding in Python - Why?

  • Python rocks
  • Very easy to write and to read
  • Huge library of outside code you can use
  • Now the default language for machine learning
  • But also applied to many other usecases, such as the web (for example Instagram's backend)

What is Jupyter?

  • We'll be writing python code - but there are many different tools to write that code in
  • For example, there are many different ways to write word documents, like Microsoft Word, Google Docs, text edit, etc.
  • The words are the same, but different platforms have different tools
  • The same is true for coding, there are many different platforms or IDEs (Integrated Development Enviroment) such as Pycharm, sublimetext, Spyder, or even just text edit
  • We'll be using Jupyter Notebooks, which is probably the most popular IDE for datascience

So what is a programming language? What is Python?

  • From wikipedia:
    • "A programming language is a formal language, which comprises a set of instructions that produce various kinds of output. Programming languages are used in computer programming to implement algorithms."

Programming Languages

  • In theory, they tell the computer what to do.
  • But in practice, computers dumb - they only understand a small number of VERY specific instructions
  • So learning to program is basically two things:
    1. Learning those instructions
    2. Figuring out how to put them together (into a program) that does what you want it to do
  • Let's start

First, let's create a variable

  • We'll create a variable (AKA an object), called 'a', and assign it the value 5
  • We'll type it in a cell (the square thing where we write text), then press SHIFT + ENTER (or press run above)
In [1]:
a = 5
In [2]:
a == 6
Out[2]:
False
In [3]:
a == 5
Out[3]:
True
In [4]:
print(a)

5

What is a?

  • we assign (or map) 'a' to the number 5 with just one '='
  • Ok, so we created 'a', what does this mean?
  • Well, we created a mapping to 'a'. When I tell the computer, "hey, use 'a' to..." the computer will look at 'a', and see what it maps to
  • So let's ask the computer what it maps to, we can usually do this with a print statement, which is just..
In [5]:
print(a)

5
In [6]:
a
Out[6]:
5

Jupyter's default output behavior

  • In Jupyter I can also just run a line of Python code by itself in a cell to output the result
In [7]:
c = 47
In [8]:
d = 5
In [9]:
c + d
Out[9]:
52
In [10]:
2 + 5
Out[10]:
7
In [11]:
answer = c + d
In [12]:
answer
Out[12]:
52

So how is this useful?

  • First things first, computers can do arithmetic (it's the basis for all the other fun things it can do!)
  • Addition and subtraction:
In [13]:
12 + 25
Out[13]:
37
In [14]:
120030498 + 12375
Out[14]:
120042873
In [15]:
123 - 5
Out[15]:
118

Multiplication and Division

In [16]:
123/65
Out[16]:
1.8923076923076922
In [17]:
1/123123321321321874981239847298
Out[17]:
8.121938145172706e-30
In [18]:
3/4
Out[18]:
0.75
In [19]:
12*4
Out[19]:
48
In [20]:
c/d
Out[20]:
9.4

Exponents!

  • For example we can compute $3^4$ as:
In [21]:
3**4
Out[21]:
81
In [22]:
new_answer = c**2
In [23]:
new_answer
Out[23]:
2209

And remember, we can do roots as exponents by making them fractions:

  • So $\sqrt{16} = 16^\frac{1}{2} = $
In [24]:
16**(1/2)
Out[24]:
4.0
In [25]:
16**(1/4)
Out[25]:
2.0

Math with variables

  • First off, notice how we use parenthesis, just like in normal math (Python knows PEMDAS)
  • But we can get fancier than just typing in specific numbers like 16, 1/4, etc.
  • Python can do arithmetic on objects that map to numbers too
  • so let's write a simple program
In [26]:
a = 15

b = 30

answer = a + b

print(answer)

45
In [27]:
answer
Out[27]:
45
In [28]:
a
Out[28]:
15

From Numbers to Variable and back to Numbers

  • Notice, I reassigned 'a' to a new value, and created b
  • I then created a new object, called 'answer', that maps to their new value
  • and finally I printed that out
  • Ok, so how could this be useful?
  • Now lets do it for the Pythagorean (pythogorean?) theorem
In [29]:
a = 6

b = 8

c = (a**2 + b**2)**(1/2)

print(c)

10.0
In [30]:
c
Out[30]:
10.0

When you make the computer angry

  • Finally, computers are very, very sensitive
  • They can't handle it when they don't understand the code
  • This is called an error!
  • It is NO BIG DEAL (srsly, it's not... more on that later)

Let's make it angry, for fun

In [31]:
c_hello

---------------------------------------------------------------------------
NameError                                 Traceback (most recent call last)
<ipython-input-31-f0a6956ee073> in <module>
----> 1 c_hello

NameError: name 'c_hello' is not defined

What's happening?

  • All of my variables still exist, nothing is broken
  • Before python runs the code, it reads it
  • If there are instructions in there that are not Python (for example typos)
  • Python 'throws' an error - it doesn't actually run it.
  • All the variables you created before are still in memory - the computer hasn't forgotten them
  • Python even tells you what is wrong
  • So make mistakes, experiment, etc. Python doesn't care while you learn

Please try: https://www.learnpython.org/