Programming

What exactly is programming? What can be programmed? What are programming languages? What does one need to know to get started programming?

Definitions

Programming, or scripting, is the act of creating instructions that someone or something will carry out. You can program robots, computers, media players, consumer devices, factory machinery, etc.

A programming language is a language for writing programs in. An interpreter for a programming language runs programs written in that language. Some languages don't have interpreters, so you have to use a compiler to translate programs written in that language into another language that does have an interpreter.

An algorithm is the (abstract) set of instructions for solving a problem. Technically each instruction has to take (in principle) a finite time to carry out and the entire instruction set must always terminate. An algorithm doesn't get carried out on a machine, but a program does.

The "Hello, World" Tradition

There's a tradition that the first program you write when you are learning a new language is one that displays the message 

    hello, world

on some output device. Obviously you can't necessarily to this on a language for programming VCRs or the brakes or suspension in your car, but you can do this for languages that let you write programs on a desktop or laptop computer.

Here is the hello, world program in some popular computer languages:

Python

print "hello, world"

Ruby

puts "hello, world"

Perl

print "hello, world\n"

JavaScript

In a pop-up window:

alert("hello, world");

In the browser page:

document.write("hello, world");

Standard ML

print("hello, world\n");

C

#include <stdio.h>
int main() {
    printf("hello, world");
    return 0;
}

C++

#include <iostream>
int main() {
    std::cout << "hello, world";
    return 0;
}

C#

using System;
class Greeter
{
   public static void Main()
   {
      Console.WriteLine("hello, world");
   }
}

Java

public class Greeter {
    public static void main(String[] args) {
        System.out.println("hello, world");
    }
}

ActionScript

To the Flash console:

trace("hello, world");

As a Flash Movie:

class Test {
    static function main(mc) {
        mc.createTextField("message", 1, 0, 0, 100, 100);
        mc.message.text = "Hello World";
    }
}

HTML

hello, world

XHTML

<?xml version="1.0"?>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN"
    "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
    <head>
        <title>A Greeting</title>
    </head>
    <body>
        <p>hello, world</p>
    </body>
</html>

Ada

with Ada.Text_IO;
use Ada.Text_IO;

procedure Hello is
begin
  Put_Line ("hello, world");
end Hello;

Lisp

(print "hello, world")

Scheme

(display "hello, world")

Smalltalk

Transcript show: 'hello, world'.

PHP

echo("hello, world\n")

HQ9+

H

You might also be interested in Wikipedia's Hello World Program article as well as some on-line collections of Hello World programs, including:

Here are some collections that include Hello World programs

How do you write, and run, programs? That depends on a lot of things...

Programmer's Mental Landscape

David Eck's first chapter in his book on programming describes the "mental landscape" for learning to program as consisting of:

Skim (or read) the chapter (but don't try to memorize all facts), and do the exercises.

Kinds of Computer Programs

Here are four kinds of computer programs (there are others — it all depends on what you mean by a kind of program):

Type Description Examples
Local Application A standalone program designed for user interaction. Runs directly on the user's computer. iTunes, Firefox, TextEdit, notepad, Adium, Terminal, cmd, iChat, Pidgin, OpenOffice, Word, Google Earth, javac, java, Photoshop, etc.
Daemon A program providing, usually, a system service, that is always running in the background. Device drivers, web servers, database servers, printer spoolers, updaters, etc.
Web Application
(Webapp)		 A full-fledged program designed that interacts with a user through a variety of web pages, within a browser. GMail, Yahoo Finance, Your bank's online access, PROWL.
Applet A small program running inside of another (usually a web browser). There is a Java applet on this page (as well as many of the pages of the course textbook), and a Flash movie on this page, and all over youtube, too.
Exercise: Do some reading on the trend of applications moving "from the desktop" to "the web". How are vendors that sell local applications trying to cope? Which applications do you think will remain local applications?

Computer File System Basics

On virtually every computer system in the planet, data, and programs, are stored in files. A computer's file system is its mechanism for storing and accessing these files. You can't be an effective programmer without understanding the basics of a file system.

So.... Here's where we spend one or two whole classes taking a tour of the file system. There are no online notes for this tour; we will wing it in class. We will cover at least these topics, plus whatever else comes up: