Introduction to Programming

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Where are we?

• Web Technology Stack

• 10 Minute Digression on Servers– Client/server model

– Server Architecture

– Server Software Background

• The physical vs. logical– Where do my files live?

– Where are they processed?

• Introduction to Programming

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Web Technology Stack

Structure – What does this logically mean?

Presentation – What does it look like?

Behavior – What does it do?

Richness of the

Experience

Behavior – What does it do?

Data – What does it know?

Basic Server Architecture

Web Server

PHP

Script

Database Server

HTTP request

HTTP response`

Web Browser

A server is a computer optimized to share resources, such as files, printers, web sites, databases, and email, over a network.

This is the model!

What is our physical architecture?

About Apache

• The Apache http server project is an

effort to develop and maintain an open

source http server for modern operating

systems (Linux, MS, OSX, …). The goal is

to provide a secure, efficient, and

extensible server that meets all standards

• http://httpd.apache.org

• Maintained by the Apache foundation

Stats of Web Server types

http://www.greatstatistics.com/serverstats.php

What the Busiest 1M Websites use

Totals for Active Servers Across All Domains - May 2010

Apache

59%Microsoft

19%

Google

13%

nginx

9%

lighttpd

0%

About PHP

• PHP stood for Personal Home Page (like a shell script)

• PHP now stands for PHP: Hypertext Preprocessor (yes, it’s a recursive definition)

• PHP is HTML-centric and lives inside/along side HTML

• PHP is used to generate dynamic web pages

• PHP runs on the web server– When the user requests a dynamic web page (typically a

.php file), the web server calls the PHP interpreter to read the requested file

– The PHP interpreter parses the PHP commands (code) and typically generates an .html file, which is returned to the user

– PHP can use data passed to it from the web page and access data in a database server

About MySQL

• MySQL is an open source database server that is available for all/most operating systems– SQL: Structured Query Language

• Introduced in 1995 with version 3.23

• Now, version 5.X

• How open source is MySQL?– MySQL is owned by MySQL AB, a for profit firm

– In 2008, Sun Microsystems acquires MySQL AB

– In 2009, Oracle Corporation acquires Sun Microsystems

Apache, PHP & MySQL

• These open source products are easily installed by developers and used commercially

• Your OS, Apache, MySQL, PHP – [X A M P]!– LAMP, WAMP, and MAMP

• LAMP stacks are widely used to serve many content

applications

– Webmail, Blogs, Wikis, CMSs, etc.

• Great for Virtual Hosting!– It is the server stack you get for $5/month from

HostGator, GoDaddy, etc.

Introduction to Programming

• Slides are from a course teaching “young people” to program

– Apologize if too elementary

– Apologize if too advanced

• There will be some opportunities to program

– All examples are online and expectation is that you will run the examples and review them here and for homework

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What are the Attributes of Good Programmers?

• Humility

• Love of Learning

• Detail-orientedness

• Adaptability

• Passion

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What is programming?

• Very simply, programming means telling a computer to do something– Computers are dumb machines

• A computer program is made up of a number of instructions– An instruction is a basic command given to a

computer, usually to do a single, specific thing

• Software is a program or a collection of programs that run on your computer– Sometimes the software runs on another computer

that yours is connected to, like a web server.

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Programming Languages

• Inside, all computers use a binary (0/1) language– Humans don’t speak binary

very well

• A programming language lets humans write in ways we “understand”– It is then translated into

binary for the computer to use

• There are lots of programming languages– HTML, CSS, Javascript, PHP,

Perl, Python, Basic, C, C++, Java, etc.

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Vocabulary

• Instructions– echo “Hello there!”;

• Keywords– A keyword is a special word that is part of a programming language (also

called a reserved word).

• Coding/Editing– Writing a number of instructions that will perform a specific set of functions

• Compiling– Converting the “human readable” instructions into the binary language that

the computer understands

• Executing – A fancy way of saying “running your program”

• Interpreting– Sometimes, converting to binary and executiing takes place instruction by

instruction, this is interpreting

• Debugging– The process of finding errors (bugs) and correcting them

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What could go wrong?• Syntax bugs – Errors prior to execution (perhaps in your editor)

– Syntax is the spelling and grammar rules for a programming language, a syntax error means that something is typed that just won’t work

• Runtime bugs - Errors during execution– Runtime errors usually crash or stop the execution of your program with

some clues, called a Traceback - like what file, line number, or even error

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*Class caveats & kludges*

• All programming examples are on my server:– http://jamesmarcus.net/bwa/introprog

• We will use our browsers as an output device to print simple results – our test jig!– We are not printing (real) web pages (yet)

– We do not have the ability to get input values (yet)

• The example PHP code is embedded in a comment tag and can be viewed using “View Source”– Examples must be changed to run on your web server

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The programming cycle

• Edit

– Write code

• Execute

– Run a program

– Get a webpage

• Test & Debug

– “Inspect what you expect”

– If you find a bug, back to editing!

• Edit and Save <1hello1.php>

• Execute <1hello1.php>

– Browser: http://localhost/bwa/introprog/1hello1.php

• Test & Debug the results

– Review results for a variety of inputs

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Our first PHP program• XAMPP Control – start web server!• Create in /htdocs/bwa/introprog the file

<1hello1.php>

<?php

print “Hello”;

print “ and Hello to you.”;

?>

• Start and end of a PHP code block• A PHP code block• PHP instructions end with a ‘;’

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Analyzing our first PHP program

• File naming convention– Files that contain PHP code have a .php extension

• PHP code blocks start and end with:– <?php and ?>

– PHP instructions end with a ‘;’

• PHP has two simple output statements:– print

– echo

• Using the browser as a text output device– Yet, it is still a browser

• We can add simple text formatting. How?

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Playing with our first PHP program

• Add simple text/HTML formatting to <1hello1.php> to create <1hello2.php>

• Introduce errors to <1hello2.php>

– Syntax errors: Where/when do syntax errors occur?

– Runtime errors: Where/when do runtime errors occur?

• Multiple code blocks: <1hello3.php>

– What is going on in between the code blocks?: <1hello4.php>

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Memory & Variables - Remember Me!

• Most all ‘useful’ programs:

– Get input

– Process the input

– Produce output

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Feed Me, Seymour

• The computer needs input

– But in order to do something with the input, the computer has to remember it or keep it somewhere

– The computer keeps things, including input (and the program itself), in its memory

• Computer memory - a bunch of on/off switches

– You can write to the memory (set the switches)

– You can read from the memory (look at how the switches are set, without changing them)

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Naming Memory

>> Teacher = “Mr. Morton”

>> print Teacher

• What do you expect from this “pseudo-code” code snippet?

Mr. Morton

<2teacher1.php>

• What happened?– We created a thing that

is made up of (a string of) characters and gave it a name, Teacher

• The equal sign ‘=‘ says to assign or “make equal to” – You assigned the name

Teacher to the string of letters “Mr. Morton”

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But wait…

• It’s just like if someone said, “Write down your address.”

– What would you write?

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Variables

• Programmers do not have to think about how and where memory stores things (like the string of letters)

– We assign a value to a name and then retrieve the value later

• The name assigned to the value, like Teacher, is called a variable

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Types of Variables

• Variables exist for– Strings (series of characters)

– Numbers (integers, reals)

– And other interesting stuff• Boolean: true/false

• Groups of groups of letters or numbers – arrays!

• Explicit variable declaration

• Implicit variable declaration

• And in between

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Using Variables

print 5 + 3;

$First = 5;

$Second = 3;

print $First + $Second;

$Third = $First + $Second;

print $Third;

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<3variables1.php>

What’s her name?

$MyTeacher = “Mrs. Goodyear”;

$YourTeacher = $MyTeacher;

print $MyTeacher;

Mrs. Goodyear

print $YourTeacher;

Mrs. Goodyear

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What if…

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What’s in a name?

• In most modern programming languages, you can call a variable anything you want (well, almost):

• It can be as long as you want• It can have letters and numbers in it, as well as special

characters, like the underscore character (_) • It usually is case-sensitive (uppercase and lowercase

matter) – Are teacher and TEACHER two different names?

• It cannot have spaces• It may require a special starting character• In PHP, variables start with $, i.e., $Teacher• In other languages, read the documentation

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String Variables

• A character, or series of characters (letters, numbers, or punctuation), is called a string

– The way you state that you are making a string is to put quotes around the characters

– PHP and other languages are not too fussy about whether you use single or double quotes

$teacher = “Mr. Morton”;

$teacher = „Mr. Morton‟;

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Numbers as StringsStrings as Numbers

$first = 5;

$second = 3;

print $first + $second;

$first = „5‟;

$second = „3‟;

print $first + $second;

• Huh? Adding two strings? – Is that what we mean to do?

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<4concat1.php>

Fancy word time: concatenate

• It’s not really correct to say we want to “add” strings. (Though PHP lets us do so.)

• When you put characters or strings together to make a longer string, we call it concatenation.

• In PHP, two strings are concatenated by the dot ‘.’ operator:

$newstring = $string1 . $string2;

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<4concat2.php>

How variable is a variable?

• Variables are called “variables” for a reason

– It’s because they are . . . well . . . variable!

– The value assigned to them can vary or change

– Remember the MyTeacher example

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How variable is variable?

$teacher = “Mr. Morton”;

$teacher = “Mr. Smith”;

$print teacher;

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The new me!

• You can make a variable equal to itself:

$Score = 7;

$Score = $Score;

• Big woop! It is just the same old me.

$Score = $Score + 1;

print $Score;

• Woop, woop! It is a whole new me!

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<5increment1.php>

Incrementing a variable

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Variable Wrap-up!

• A variable can be reassigned (the tag can be stuck on a new thing) at any time by a program

– One of the most common “bugs” in programming is changing the wrong variable or changing the right variable at the wrong time

– One way to prevent this is to use variable names that are easy to remember and have contextual meaning

• $t = 'Mr. Morton' or $x1796vc47blahblah = 'Mr. Morton' work but have no meaning

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Quiz

• Once you have created a variable, can you change the value assigned to it?

• With variable names, is $TEACHER the same as $TEACHEr?

• Is ‘Blah’ the same as “Blah”?

• Is ‘4’ the same as 4?

• Is “10” a number or a string?

• How do you tell PHP that a variable is string or a number?

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Basic Math

• The four basic operations:– Addition: +– Subtraction: -– Multiplication: *– Division: /

• Exponentiation [PHP: pow($base, $exponent)]– Raising a number to a power: 25= 2*2*2*2*2

• Modulus [PHP: %]– Clock arithmetic

• Increment and decrement$Score = $Score + 1;++$Score;

$Score += 1;

• Really big numbers, really small numbers– Overflow, underflow, e-notation

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Order of Operation

• PEMDAS– Parentheses

– Exponents

– Multiply

– Divide

– Add

– Subtract

• But be kind and use parentheses!– (2 + 3) * 4 = 20

– 2 + (3 * 4) = 14

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Data Types

• So far, we have talked about three data types – Strings– Integers– Reals/decimals (floats)

• Sometimes we have have to be careful about what types we use

• Fancy words alert: Data type Conversion– A programming language will have explicit ways to convert

from one data type to another, also called type casting!– Integer to float: $newFloat = (float) $oldInteger;– Float to integer: $newInteger = (int) $oldFloat;– String to float/integer: $number = $stringasnumber;

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Gotchas will getcha

• Be careful casting an unknown fraction (float) to an integerprint (int)( (0.1+0.7) * 10 );

• Result is …

• 7 Why?

• There are many helper functions for casting:– floor()

– ceil()

– round()

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Quiz

• What symbol does PHP use for multiplication?

• How would you get the remainder for 8 / 3?

• How would you get the integer part for 8 / 3?

• What’s another way of calculating 6 * 6 * 6 * 6 in PHP?

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• End here.

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• Exercises

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Exercise 1

• Write a short PHP program, as we just did, that will print three lines:

– Your name

– Your birth date

– Your favorite color

• My name is James Marcus.

• I was born January 26, 1961.

• My favorite color is blue.

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Exercise 2

• Calculate the number of minutes in a week using variables

– $DaysPerWeek

– $HoursPerDay

– $MinutesPerHour

• How many minutes are there in a week in the Bilky Way, a near parallel galaxy, if there are 26 hours on a day there?

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