Lesson 9: Programming

Overview

• Programming Paradigms.
• Psuedo-Code exercise.
• Python Overview.
• Python practice exercise.

Paradigms

Programming is a big topic.

Note: Pseudo-code

function f(x):
    # This line is a comment, not run by the computer.
    # Comments are only for human eyes.
    if x is less than than 5
        print "x is less than 5"
    else if x is less than than 10
        print "x is greater than five and less than 10"
    else
        print "x is greater than 10"

Variables & Constants

>>> x = "value"
>>> print(x)
value
>>> x = "different value"
>>> print(x)
different value

Data Types

Data types dictate how a piece of data should be handled within a program.

Data Types

Common types:

Int	1, 5, 10000
Float	1.5, 3.14159
Char	'a', 'f', 'g'
String	"foo", "BAR"
Array	[1, 2, 3], ['a', 'b', 'c']

Flow Control

Flow Control allows you to execute code only if certain conditions are met.

Conditionals: If / Else If / Else

Conditionals are used to tell the program when to execute commands.

In pseudocode, they usually look something like

if some conditional statement is true
    do something
else if some other conditional
    do something else
else
    do a final thing

Flow Control

Loops: For / While / Do While

Loops are used to do multiple things, usually an indefinite number of things.

For instance:

for every element, let's call it "foo", in a list "my_list"
    if foo is greater than five
        print(foo)
    else
        print(foo + " is too small")

While loops execute indefinitely (while something continues to be true).

For loops iterate over a list (array) of elements or to a specific number.

Input & Output

>>> user_input = get_input("Where would you like to go today? ")
>>> -> Where would you like to go today? Nebraska
>>> print(user_input)
>>> -> nebraska
>>> print(reverse(user_input))
>>> -> aksarben

Functions

function read_file(x):
    # Also check that it exists! How convenient!
    if file_exists(x)
        v = read_file_to_string(x)
        return v
    else
        print("file does not exist")
        return Null

Structs

struct dog {
    breed: String
    height: Float
    color: String
    age: Integer
}

spot = struct dog      # Create a new variable of type `struct dog`
spot.breed = "corgie"  # Assign each member a variable.
spot.height = 1.5
spot.color = "Blond"
spot.age = 1
print(spot.breed, spot.height, spot.color, spot.age)

Objects

class chair():
    function init(material):
        self.material = material

    function rock():
        print("The ", self.material, " chair rocks slowly.")

>>> my_chair = chair.init("plastic")
>>> my_chair.rock()
>>> -> The plastic chair rocks slowly.

Libraries

import math_lib

print(math_lib.pi, math_lib.pow(2, 5), math_lib.tan(79.3))
# prints out "3.14 32 .951"

TODO: Write Pseudo-Code

Write pseudo-code to do the following tasks:

• Count to 20 (hint: for loop).
• Get user input and print it.
• Generate prime numbers.

Hints:

• Break the problem down to the simplest steps.
• Don't worry about the details.
• This is pseudo-code! Get creative.

Python

$ sudo <apt or yum> install python

Python Datatypes

• You don't need to declare the type of your variables, Python will assume the type of your variable and type it for you.
• Python is a duckly-typed language. If it walks like a duck and quacks like a duck, then Python treats it like a duck. As long as an object implements the proper interfaces, it can act like any type it wants.

Python Datatypes

Python Variables

# This is a comment
boolean = True # boolean
name = "Lucy" # string
age = 20 # integer
pi = 3.14159 # float
alphabet = ['a', 'b', 'c']
dictionary = {"pi":3.14159, "sqrt 1":1}
winter = ('December', 'January', 'February', 'March')

print(name + " is " + str(age+1) + " this " winter[3])

REPL: Try it out

Open a REPL (Read Evaluate Print Loop):

$ python
>>> print("I'm in a REPL!")
>>> name =      # <Your name>
>>> age =       # <Your age>
>>> print(name + " is " + str(age))
>>> # We need to convert age from int to string so it can print!

Python Control Flow

if name == "Lucy":
    for month in winter:
        print name + " doesn't like " + month
else:
    print "My name isn't Lucy!"

Python Functions

def myfunction(arg1, arg2):
    return arg1 + arg2

print myfunction(1, 5)

Python Libraries

There are a few ways to use other code in your code:

from math import pi
x = pi

from math import *
x = pi

Python Libraries

There are hundreds of Python libraries. If you're trying to do something and think "This has probably been solved...", Google it!

Some libraries to know:

• sys
• os
• dateutil
• future
• And more

Python (Virtual) Environments

$ sudo apt-get install python-virtualenv
$ sudo yum install

# In each project you work on, you'll want to run
$ virtualenv venv
$ source venv/bin/activate
(venv)$ pip install <package>
(venv)$ deactivate

TODO: Practicing Python

Formalize the last TODO by writing them in Python.

Prove the program works by running the code!

Further Reading

Python on Learnpython.org

The Python programming language's website offers some good (free) tutorials and reference documentation.

Python on Codecademy

Codecademy is a great resource for learning many programming languages and offers a good (free) beginner's guide to Python.

CS 160, 161, 162

These OSU courses focus on programming fundamentals covered in this lesson in greater detail. Python is used in CS 160 and C/C++ is used in CS 161 and CS 162.