Lesson 9: Programming

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Paradigms

Programming is a big topic.

Bill Nye the Science Guy, 'Huge!'

We will cover the conceptual basics of programming before doing any programming. Don’t worry, you’ll be coding soon.

Note: Pseudo-code

We will be using pseudo-code (fake code) to express and demonstrate concepts. This isn’t necessarily code you can run on a computer; it is close to human language to enable teaching concepts without getting bogged down by exact syntax necessary in a formal programming language.

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

Variables are a name used to refer to a piece of data. In languages that use a syntax derived from C, variable assignment will usually look something along the lines of <variable name> = <variable value>. Some languages have mutable variables, meaning a variable can change its value, and others have immutable variables, meaning once a variable is assigned its value does not change. A large majority of languages support both kinds.

Constants are variables that are static (known before the program is run).

>>> 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.
Static Vs Dynamic Typing

In statically typed languages, types are either known or deduced at compile time. When you run a program, the computer knows which variables are integers, which variables are strings, etc. Examples of statically typed languages include C, C++, and Java.

In dynamically typed languages, you don’t know what the type of a variable is until it has been assigned. Examples of dynamically typed languages include Python, Ruby, and PHP.

Strong Vs Weak Typing

Strongly typed languages enforce type safety. What this means is that you can’t, for example, use an int in the place of a float. However, this can be useful for catching bugs that would otherwise hide themselves in large codebases and cause problems.

Weakly typed languages are permissive in allowing types to act like different types. In reality, though, no language is either strongly or weakly typed. However, some languages are more strongly typed than others.

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’]

You can even combine types to create data structures. Data structures are another important computer science concept and a topic for another day, but they are immensely useful tools.

Flow Control

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

Think of your program’s flow-control as water running in a stream. Sometimes the stream hits a rock and flows around, or it encounters a fork, causing it to diverge permanently. Occasionally the stream will go into a pool and whirl around for a bit and then return to the main path. Eventually, the water will exit into a sea or ocean.

This isn’t a perfect metaphor, but visualizing this as your program starting at some single point and having the possibility to branch and loop may help you understand what it means for a program to flow and for you to control that flow.

Consider a problem: You are writing a program and you want it to make a decision. Given a number you want it to either multiply it by 2 if it’s too small, or divide it by 2 if it’s too big. This is the perfect use-case of an ‘if’ statement, or more formally a Conditional statement.

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

In this conditional block, the first conditional statement is evaluated to either True or False. If it’s True, the program continues on inside the block with do something. If the conditional evaluates to False, then the conditional inside else if is evaluated and handled accordingly. You can have any number of else if statements. At the end, the code inside the else statement executes if none of the other conditions were met. The else if and else aren’t required, but they can be useful when you have more sophisticated control flow.

Now consider a different problem: You want to look through a list of names and print out every one that starts with the letter ‘Q’. This is where the Loop comes in to play. A loop is exactly what it sounds like– it is any construct that allows you to carry out some operation multiple times, without having to copy and paste code for each time the commands are executed.

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

Programs aren’t very useful unless they can get data from the outside world and return the results they’ve found. I/O is the concept of

  1. Getting input from a user, a file, or the outside world.
  2. Doing something with that data.
  3. Outputting a result.
>>> 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

IO is useful for making a user-interface and for debugging a program. You can print out variables that you think might be getting set wrong, loops which might not be working the way you want, etc.

Functions

Functions are how we put a set of functionality in a box. Instead of typing the same code over and over we write a function which runs those lines of code for us. When you make a change in the function it changes the behavior of the program every time that function is called.

Functions take arguments (input) and return values (output). Some functions return nothing, i.e. a value called “Null”.

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

In this pseudo-code we’ve specified the steps for reading the contents of a file to a string (text). The function returns the contents of the file if the file exists and returns Null if the file does not exist. This type of return value is useful because when we use it we can check “did this function return a string or Null” to determine if we passed a ‘good’ filename.

We also called (used) functions in this function, which is totally allowed! The function we used were file_exists, read_file_to_string and print. The file_exists function we can assume returns a bool (true/false) and the read_file_to_string returns a string type.

Structs

Structs are collections of logically grouped variables. They can be treated as variables with sub-variables.

Here is a declaration of a struct dog:

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

To use a struct you access it’s members like so:

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)

In our declaration we specified each member and its type for the struct. Each language’s implementation (or lack thereof) of structs differs slightly but all act about the same.

Objects

Objects are collections of logically grouped functions and variables. They are declared in a class and an instance of a class is an object. The class can be thought of as the blueprint for an object.

Objects usually have an initialization function that takes arguments so you can create a unique object.

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

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

To create an instance of a class (object), you use some syntax like the following:

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

Note that we treated structs like a data type (declaring var = struct struct_type while we treat objects more like functions. The variable my_chair is assigned to the return value of the chair‘s init function. Once you have an object you can call the object’s functions, formally referred to as methods.

Libraries

Libraries are collections of functions and constants external to the file you’re working on. They may be written by somebody else or written by you in a separate file.

The usual syntax for using a library looks like this:

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.
Answers:
for i starting at 1 and ending at 20:
    print(i)

input = prompt_for_input("Please enter a number: ")
print("Your input was ", input)

i = 2
while i < 100
    for j starting at i going down to 1:
        if j is 1:
            print( i, "is a prime number")

        if j / i is a round number:
            Skip to the next iteration of this inner loop.

Python

$ sudo <apt or yum> install python
python programming language logo

Python is a programming language designed for learning. This means it is relatively simple to pick-up and run with, and it looks a lot like pseudo-code you might write. It is very popular, modestly fast, and commonly used in the programming industry.

Python is a scripting language. While other languages need to be compiled and then have a binary run, you run the script directly with $ python my_script.py.

One advantage to scripts is that you can write functions in a file and Python will just run them– you don’t need to write a main() function or anything!

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.
_images/duckly.gif
Type Example
boolean True
integer 7
long 18,446,744,073,709,551,615
float 12.4
string "Hello World!"
list ['first', 'second']
dict (map) {'key1': 'value', 'key2', 'value2'}
tuple ('value','paired value')
object anObjects.variable == <value>
None

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

Below is an example of flow-control in Python.

Note You test equality (is x equivalent to y) with == not =.

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

Python Functions

Below is the exact syntax for declaring a function in Python.

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

print myfunction(1, 5)
_images/function-machine.png

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

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:

Python (Virtual) Environments

When developing a Python project you will want to use a virtual environment. This isolates the dependencies of your project from the Python software installed on your computer.

$ 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
_images/environments.jpg

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.