Intro to programming using Python

Equipo Academy’s College Simulation Day

Hugo O. Rivera

2019.11.27

Intros

Intros

Useful Websites

(you can use a smartphone if you don’t have a laptop)

What to Expect

What is programming?

Programming is the art and science of translating a set of ideas into a program - a list of [exact] instructions a computer can follow. The person writing a program is known as a programmer (also a coder).

What is programming, really?

youtu.be/Ct-lOOUqmyY

Computer science

Computer science, the study of computers and computing, including their theoretical and algorithmic foundations, hardware and software, and their uses for processing information.

(Follow the rabbit hole)

What’s computer science used for?

What’s Python used for?

Program 1

# 1.0.py
name = "Alice"
message = "Hello," + name
print(message)

from the computer’s point of view, what’s happening???

Program 1

# 1.0.py
name = "Alice"
message = "Hello," + name
print(message)

Output

$ python 1.0.py
Hello,Alice

Program 1.1

# 1.1.py
name = "Alice"
message = "Hello, " + name
print(message)
print("Hello, " + name + "!")

Output

$ python 1.1.py
Hello, Alice
Hello, Alice!

Program 2: lists.py

favorite_foods = ["TAMALES", "nigiri sushi", "nutella"]

number_of_favorite_foods = len(favorite_foods)
index_of_favorite_food = 0

print("of my ",
      number_of_favorite_foods,
      " favorites, ",
      favorite_foods[index_of_favorite_food],
      " are my most favorite food")

Output

of my 3 favorites, TAMALES are my most favorite food

Questions about lists:

Strings are lists

>>> len("Hello!")
>>> [1, 2] + [3, 4]

The three arrows >>> mean try it in the REPL.

Program 3: Slices vs. indices

Python supports slices.

definitions:

most_states = ["Alabama", "Alaska", "Arizona", "Arkansas", "California", "Colorado"]
top_states = most_states[0:3]
print(top_states)

Program 4: more slices

print("Hello, world!"[0:5])

Program 4 output

print("Hello, world!"[0:5])
Hello

Program 5: negative indices

print("Hello, world!"[-1])
print("Hello, world!"[-4:-1])
print("Hello, world!"[-4:-1])

Exercises

Let’s use CodingBat!

Program 6: For-loops! Append! Pop!

Look at each element using for x in elements.

numbers = [2, 4, 8]
total = 0
for number in numbers:
    total = total + number
# MUST BE 4 SPACES!!!!

highest_known_power_of_two = numbers.pop()

numbers.append(6)
numbers.append(8)
numbers.append(10)

print(total)
print(numbers)

Output:

14
[2, 4, 6, 8, 10]

Program 7: dictionaries_for_polyglots.py

greetings = {
  "english": "Hello, ",
  "español": "hola, "
}

greetings["sinhala"] = "හෙලෝ "
greetings["es"] = greetings["español"]
greetings["en"] = greetings["english"]

print(greetings["en"] + name)

Dictionaries assign keys to values.

Program 7: dictionaries_for_polyglots.py output

Traceback (most recent call last):
  File, line 8, in <module>
NameError: name 'name' is not defined

too bad :(

Thought-experiment

lists can be thought of as “weaker” dictionaries: think of a list as a dictionary with its keys set to numbers 0, 1, 2, …, len(list)-1

list = ["a", "b", "c"]
list[0]
silly_dictionary = {-1: "negative", 0: "zero", 1: "one", "1": 1, "one": 1}
print(dictionary[0])
print(dictionary[-1])
print(dictionary[1])
print(dictionary["1"])

Questions about dictionaries:

Program 8: functions.py

def greet(person)
  message = "Hello " + person
  print(message)

Program 8: functions.py output

you defined the function “greet” but didn’t do anything with it! (that’s ok)

Questions about functions:

Program 9: python.py

def run_code(instructions):
    names = dict()
    stack = list()

    for each_step in instructions:
        instruction, associated_data = each_step

        if instruction == "PRINT":
            value = stack.pop()
            print(value)
        elif instruction == "STORE_TO_VAR":
            value = stack.pop()
            names[associated_data] = value
        elif instruction == "LOAD_VAR":
            stack.append(names[associated_data])
        elif instruction == "LOAD_LITERAL":
            stack.append(associated_data)
        elif instruction == "ADD_LAST_TWO":
            result = stack.pop() + stack.pop()
            stack.append(result)
        else:
            print("ERROR: unknown instruction", instruction)

run_code([ ["LOAD_LITERAL", "Alice"],
           ["STORE_TO_VAR", "name"],
           ["LOAD_VAR", "name"],
           ["LOAD_LITERAL", "Hello,"],
           ["ADD_LAST_TWO", []],
           ["PRINT", []]               ])
# CHALLENGE: MAKE THIS INTERPRETER INTERPRET ITSELF!!!!

hints:

Program 9 Output

Hello,Alice

More about python.py

See full source code at roguh.com/intro_to_cs.equipo.1127/python.py.

See aosabook.org/en/500L/a-python-interpreter-written-in-python.html for my inspiration.

It supports relatively advanced code for 117 lines of Python. The fun is when you go and make it more powerful.

run_code(parse_string("a = 1\na = a + a\nb='birds'\nprint(a, ' ' + b)"))

Learning resources

All you need is a computer and some time.

Reach out to me with any questions you have!

hugo@roguh.com

Language doesn’t matter

Python is friendly and has many real-world uses.

Try JavaScript! Or C if you want more control over the machine, or Haskell if you want to peek into the future of programming.

Learning resources are bountiful. Ask me if you want advice.

I can help you make a GitHub repo so we can work on stuff together

Route 0: install Arch Linux and use the vim and git command line tools

Route 1: use visual studio code and nice collaboration plugins (GitHub)

Route 2: use websites like CodingBat and other online coding platforms

JavaScript

magically somehow friendlier than Python, but I don’t like the language

Learn from Mike Bostock, https://observablehq.com/