First Let's Talk About Natural Languages

A Programming Language…

This Makes Reading and Writing Programs Much Different From Natural Languages

Reading a Programming Language

• reading a program requires analysis of a program's structure
• information density can make comprehension a slow process
• again, meaning is usually unambiguous
  • behavior / meaning usually backed by formal specifications
  • though language is unambiguous, programmer intent may not be!

Writing a Program

• easy to introduce minor syntactic errors (you must be careful with syntax and grammar)
• may have multiple implementation possibilities
• may offer constructs for abstraction and preventing repetition
  • potentially further improves on human readability
  • important for refactoring, improving your program, and future maintenance
• don't necessarily have to know entire language to write a program!

Sooo Many Programming Languages…

These Are 20 of the Most Used Languages Today

That's a Lot!

Here Are Some Ways That We Can Describe Programming Languages

Some Characteristics of Programming Languages Include:

1. high-level and low-level
2. interpreted and compiled
3. by paradigm
4. esoteric and mainstream

High-Level
and Low-Level
Languages

Low-Level Language

A programming language designed for execution by a computer. (Not so friendly.)

• loosely speaking, a computer can only directly execute programs written in low level languages
• generally makes no attempt to hide the machinery that it instructs
• usually fast and resource efficient
• some examples include:
  • machine code - as close as you can get to the machine
  • assembly language - just a step above machine code

Low-Level Languages Look Like:

Assembly

MOV AL, 1h        ; Load AL with immediate value 1
MOV CL, 2h        ; Load CL with immediate value 2
MOV DL, 3h        ; Load DL with immediate value 3

Machine Code

10110000 01100001

High-Level Language

A programming language that is designed to be easy for humans to read and write.

• must be compiled or interpreted to a lower-level language so that a computer can execute it
• provides abstractions that free the programmer from dealing with the details of the underlying machine
• meant for the programmer rather than the computer
• usually prioritizes usability over efficiency
• more likely to be correct
• portable across different operating systems/architectures

Examples of Some High-Level Languages:

Ruby

100.times do |i|
	puts i if i % 2 == 0
end

PHP

<?php
$count = 0;
while($count < 100) {
	print($count);
	$count = $count + 2;
} ?>

High-Level vs Low-Level

• high level
  • more human readable, but also usually more verbose
  • some are closer to natural languages
  • usually abstracts away intricacies of dealing with underlying machine / computer
  • loosely speaking, meant for programmers
• low level
  • usually terse / less verbose
  • usually does not look like a natural language
  • need some knowledge of underlying machine / computer
  • loosely speaking, meant for machines

Compiled and Interpreted

Two Ways to Execute a Program

Before a program can be executed, the source code of a high-level language must be translated into something that the computer can understand. You can:

• compile the source code into another (usually low-level) language using a compiler… then execute
• execute the source code immediately using an interpeter

Compiling a Program

• when we talk about source code (or just code), we're referring to the program prior to compilation
• programs called compilers perform this translation
• a compiler parses your source code in order to do this translation
  • parse - to examine a program and analyze the syntactic structure
  • we do this for natural languages too!
• the result of this translation is usually referred to as object code
• the object code is what is run by the computer

Compiling a Program… Continued

General Workflow

1. Write code
2. Compile
3. Execute program
4. Go back to step #1

Examples of Compiled Languages

• Basic
• C/C++
• Java (there are some subtleties here that we'll talk about later)

Interpreting a Program

• a program can be executed using an interpeter
• an intpreter is a program that executes source code without the need for compilation
  • the way that a program is executed behind the scenes varies by language and implementation
  • perhaps compilation is done behind the scenes
  • …or the interpreter acts as a virtual machine where the codes just runs
• Sometimes an interpeter may also allow a program to be run interactively
  • also called an interactive shell or an interactive console

Interpreting a Program… Continued

General Workflow

1. Write code
2. Execute program
3. Go back to step #1

Examples of Interpreted Languages

• Ruby
• PHP
• Lisp

Compiled vs Interpreted

• compiled languages are usually considered faster and more efficient than interpreted languages
• interpreted languages are generally thought of as easier to use (less steps, immediate feedback, etc.)

And What Does This Mean for Us?

Despite the pros and cons of compiled and interpreted languages, for our purposes, the main difference between a compiled and interpreted language is:

• a compiled language requires an explicit compilation step before the program can be run.
• an interpreted language can be run immediately

Some Important Caveats

• distinction is wholly dependent on implementation, not language design
• a programming language may be considered compiled, interpreted or even both!
• hybrids exist (to further complicate things!)
  • a language may be compiled to an intermediary form
    • this form may be run by an interpeter!
    • still considered compiled because of explicit compilation step
  • a language may be compiled to an intermediary form that's run immediately by the interpreter

Programming Paradigms

Some Programming Paradigms

• Procedural
• Imperative
• Object Oriented
• Functional
• Aspect Oriented
• Declarative
• and more…

For This Class…

We'll mostly be concerned with imperative and procedural programming. (So don't worry about the others).

• this approach most closely resembles our definition of what a program is
• solves a problem through a sequence of commands
• these commands (or statements) can change the program's state (data)

The tools that we use may include bits of object oriented programming.

And, Lastly… Is it Practical?

Esoteric vs Mainstream

• mainstream
  • widely adopted
  • may adopt features found in esoteric languages
• esoteric
  • experimental - not necessarily meant for practical use
  • may serve as a proof of concept
  • recreational - a thought exercise, or even a joke!
  • examples - Piet, LOLCODE, and Befunge

Let's Use Python