Hey guys,
Time for tutorial #2 in my series of Programming and
Problem Solving posts! Today the first thing we're going to do is go
over some things that weren't touched on in detail in the last tutorial
that you should be familiar with before we proceed.
The first
thing to notice is that in the last tutorial the code had a lot of
semi-colons ";" this is a symbol used by the interpreter to determine
where the end of each command is. Generally speaking(although there are
exceptions which we will touch on) every line should end with a
semi-colon, you can also stack two commands onto a single line like
this:
<?php
echo "Hello"; echo " "; echo "Dickwad\n";
?>
If
anyone was expecting that last echo to say world, im sorry to disapoint
but thats just a little toooooo cliche. the previous segment of code,
to the interpreter, is identical to:
<?php
echo "Hello";
echo " ";
echo "Dickwad\n";
?>
and
<?php
echo "Hello Dickwad\n";
?>
note
that although you CAN stack commands onto one line, unless you have a
specific reason to you should avoid doing such, even though it will
execute fine on the computer end, if you or anyone else has to actually
maintain the code it can make things a lot more painful.
Another
convention that Im going to do everything I can to drill into you guys
is proper indentation. I dont give 2 fucks if you think you can
obfuscate your code by leaving it unindented and sloppy, trust me a half
retarted monkey could reverse just about any of the obfuscation ive
seen in skiddy php shells and scripts, all it serves to do is make you
look like you dont know what the fuck you're doing and like you don't
belong anywhere near a modern computer. When you have a script of
hundreds, thousands, or even millions of lines of code that you have to
maintain and manage, INDENTATION IS KEY. say it again because its that
important. INDENTATION IS KEY. Until we touch on conditional statements
later in this tutorial I won't be able to give much of a concrete
example so this is all ill say on indentation for now.
You may
have noticed in the code examples that they always begin with <?php
and end with ?>, these tags tell the php interpreter what to
process(since you can splice php code among html you can have a script
entirely in html with a single <?php include 'menu.php'; ?> inside
the html, so these tags exist to tell the interpreter exactly what to
process and what to ignore.) There is short tags available as well which
let you use simply <? and ?>, this is a setting that needs to be
changed in the php.ini configuration file, but regardless, <?php
?> should work no matter what.
One more thing to catch up on
before we dig any deeper. PHP is a recursive acronym, it stands for PHP
Hypertext Preprocessor. The key word we're looking for here is
pre-processor. If any of you have written in C or C++ you may be
familiar with the concept of a preprocessor, its what takes all the
#include files(like #include <iostream> and #include "stdlib.h")
and embeds those libraries into your current file. PHP also has this
feature available so you can include libraries and scripts into new
applications you write(this may seem like a somewhat complicated concept
at the moment but trust me its pretty simple and makes scripts way more
reuseable) with this in mind we have 4 ways of including external
scripts into our main script
For more information on Preprocessors:
http://en.wikipedia....ki/Preprocessor
include()
include_once()
require()
require_once()
as you can see they're all quite similar ill start first with the differences between include and require
basically
if you require a file and its unavailable, the program will stop
execution and throw you an error saying that you're a real jerkface
telling it to look for files that don't exist. If you use include it
will continue execution and proceed but without the code that you
intended to include so it very well may break down if any of that code
is directly referenced or you may end up with a website that only half
renders, etc... but the key difference between require and include is
that with require if its not there its not loading.
now to touch
on require_once and include_once, they both have the same differences
that require and include have. the _once indicates that if in your
script you have several references to a file and require or include it
in more than one spot, to reduce overhead it will check and only
actually load that file into memory once and use that location for all
future references, instead of loading the same library into memory
multiple times.
That being said, I almost exclusively use
require_once, it ensures that overhead is limited and that my script
won't errenously proceed without its requirements. However you should
assess each situation and determine which is the most appropriate for
your specific needs.
OK so we're caught up and I think we're
ready to move onto something new. For once we'll get into some real
actual coding... This next part is going to be broken into two parts, as
PHP is primarily used in two ways: as a web application scripting
language and as a command line scripting language. So we'll run through a
few examples of how to obtain user input through both the command line
as well as through using html forms on a web page.
We'll start
off by doing it using the command line which may seem a little bit more
complicated but by and large is a lot less work than creating a web page
to read the info and send it off to be processed.
<?php
$input;
print "Enter your name: ";
$input = trim(fgets(STDIN));
print "Your name is: " . $input . "\n";
?>
so
what we've done here is created a variable named $input, we then prompt
the user to enter their name(notice the print "Enter your name: "
doesn't have the \n character in it. This means that when the user
enters text it will appear directly to the right of the prompt instead
of the line below it) Then we come to the line: $input =
trim(fgets(STDIN)); in this line we are assigning the result of
trim(fgets(STDIN)) to the variable $input. trim() is a function used to
remove whitespace and endline characters as well as a few others. The
reason for this is if they enter "Harold" and hit enter, the value of
input will be "Harold\n" so if you print $input it will jump a line,
trim removes the \n and leaves us with just the data we want.
fgets(STDIN) extracts the data from the standard input buffer(in this
case whatever was typed on the keyboard) Thats it, this is really all
you need to know to get started with command line scripts and user
input.
Obtaining input from a web form takes a few more lines of
code so i'll write out an example first then well go through it piece by
piece.
[code]
<?php
if(isset($_GET['sentinel']))
{
print "Your name is: " . $_GET['name'] . "\n";
}
else
{
?>
<form method="GET" action="<?php echo $_SERVER['PHP_SELF']; ?>">
<input type="text" id="name" name="name" placeholder="Enter your name..." />
<input type="hidden" id="sentinel" name="sentinel" value="1" />
<input type="submit" value="Submit" />
</form>
<?php
}
?>
NOTE**:
the line of an if statement does NOT require a semi-colon, just
if(condition){} each line within the {} requires a semi-colon but the if
statement itself does not.
Theres a few things in this code that
we haven't taken a look at yet so I'll give a brief overview here but
we'll go more into detail on them later.
We see here says: if(isset($_GET['sentinel']))
What
we're really telling the computer is that if the variable
$_GET['sentinel'] exists then we want to execute the following indented
block of code, in this case a single print statement. If that variable
does not exist we want to skip that line and instead execute the other
block of indented code which presents an html form to the user asking
for their name. This is what we refer to as a control structure, the
concept of if/else clauses allows us to create several alternative
courses of action that will be executed depending on which conditions
are met, in this case the condition is the existence of the
$_GET['sentinel'] variable. We call these kinds of operations
"Conditionals" or "Conditional Statements". Why do we care if
$_GET['sentinel'] exists? lets take a closer look at the second block of
code
<form method="GET" action="<?php echo $_SERVER['PHP_SELF']; ?>">
<input type="text" id="name" name="name" placeholder="Enter your name..." />
<input type="hidden" id="sentinel" name="sentinel" value="1" />
<input type="submit" value="Submit" />
</form>
if
you're familiar with html most of this should be pretty straight
forward, by using the GET method we're basically saying "Send these
variables in the clear via the URL" and you'll notice when you submit
the form the URL changes to something like:
http://192.168.1.146...rian&sentinel=1
Trailing
from the URL are the values: name=Brian&sentinel=1 notice in the
Form we have two input fields name and sentinel that correspond with
these, so when the form is submitted two variables are being created:
$_GET['sentinel'] and $_GET['name'] if these don't exist we can safely
assume the user has not submitted the form, therefore we want to display
the form to get them to fill it out. However if these variables do
exist we can reasonable assume they have filled out the form and instead
want to display the results back to them. Technically there are 3 input
types, the submit is one as well but fuck submit thats irrelevant.
If
you want to submit form data but have it not appear in the URL(say
someones logging into a website, sending the username and password
clearly in the URL is obviously a bad idea) then you can use the POST
method instead of GET, all you have to do is substitute method="GET" for
method="POST" and prefix your variables with $_GET instead of $_POST.
This is also an example of a self-contained script. We wrote the input
and processing stages all to one file. But what if we need to send the
data from a form to a different file? the action="" determines where to
send the request, we have sent it to $_SERVER['PHP_SELF'] which is a
variable that contains the name of the file we're using so basically it
says "send it back to me, ill reload and re-evaluate" but you can
instead put in any file name you like.
So now we've got an
introduction into some basic user input, as well as conditional
statements. But conditionals, as we've seen them thusfar, seem kind of
limited don't they? right now we can only choose between two options, if
this do this otherwise do that. For some problems this will be
sufficient but its not hard to see this being insufficient for more
complex problems. So lets look at the If/Else claus a little more
thoroughly and look into a concept known as nesting.
Nesting is
the concept of inserting one block of code into another block of code,
for example the form in the previous example was nested under the else
statement and the print was nested under the if statement. Lets write a
simple script to determine if its morning or afternoon, as well as if
its a weekday or if its the weekend:
<?php
$day = date("l");//returns the full name of the current day e.g.: Monday, Friday
$timeday = date("a");//returns either am or pm in lower case
if($day == "Saturday" || $day == "Sunday")
{
print "It's the weekend!\n";
if($timeday == "am")
print "\t It's the morning!\n";
else
print "\t It's the Afternoon!\n";
}
else
{
print "It's a weekday :(\n";
if($timeday == "am")
print "\t It's the morning!\n";
else
print "\t It's the Afternoon!\n";
}
?>
Alright
so a few things we can ignore here are basically the date() function,
it's beyond the scope of this tutorial but I've included in the comments
exactly what data is being sent to those variables to give you an idea
of what its doing.
In this code we have an if else block embedded
inside of the if else block, this way we can create even more potential
flows of execution. A few other things that we havn't seen before are
|| and == we'll discuss || and some other corresponding operators later
on but for now lets focus on ==. Remember in the previous tutorial when I
said to make a note of one = sign meaning assignment? e.g. $var1 = 5;
assigns the value of 5 to $var1, when we use two equal signs its to
compare the values of two items, so:
if($day == "Saturday" || $day == "Sunday")
is
checking to see if $day is equal to the value Saturday or if $day is
equal to the value Sunday(|| is the logical or operator, if $day is
equal to either of those values, the condition returns true and the
block of code is executed, only if each condition is wrong will it
continue on to the else) so basically the first thing we're checking is
if its a weekend or a weekday, then outputting those results, from there
we check if its morning or afternoon then output that as well. You
might notice that the outer blocks use {} brackets to contain their
blocks whereas the internal if/else statements don't. This is a matter
of preference but to break things down:
if there is ONE and only
ONE line of code, you may omit the brackets(still be sure to indent so
that its readily apparant, if you dont indent it will still run
perfectly fine but could be a bit boggling to read) if there is more
than one line of code you MUST i repeat MUST use {} brackets to contain
it. Heres why:
say you have 3 or 4 conditions you want to check
but only have an if/else and nothing to say this or this or this or
this(note i dont mean this || this || this i mean if this else if this
else if this else this)
we can write it such as:
if(condition 1)
process 1
else
if(condition 2)
process 2
else
if(condition 3)
process 3
else
process 4
this
might take a second to set in, and understandably so but basically this
lets us check through a number of options, it is however written in a
overly complex way, so people have constructed the concept of the else
if statement written this way for clarity:
if(condition 1)
process 1
else if(condition 2)
process 2
else if(condition 3)
process 3
else
process 4
THIS
IS the correct way to write it. using if/else if/else statements we can
effectively check through any combination of conditions that we
need(well not exactly but with the help of || and some other operators
we can achieve that goal)
Apart from if/else statements theres
another option for running through conditionals, its used less often but
excels in certain situations so for the sake of a full background we'll
go over the switch statement next:
the format of a switch
statement is to take a variable then check its value against a number of
options, if any are true then a block of code is executed, lets look a
little closer:
switch($variable)
{
case "Sunday":
print "Sunday\n";
break;
case "Monday":
print "Monday\n";
break;
case "Tuesday":
print "Tuesday\n";
break;
case "Wednesday":
print "Wednesday\n";
break;
case "Thursday":
print "Thursday\n";
break;
case "Friday":
print "Friday\n";
break;
case "Saturday":
print "Saturday\n";
break;
}
in
this example we're reviewing the value of $variable, if it corresponds
to any of the days of the week that day will be printed, so when we see
case "" what lies between the "" is what we are comparing it to, you can
also use case 0: case 1: case 2: etc... if you're using a numerical
value(it can be useful when iterating through an array, more on that
later!)
notice the break statement at the end of each block too,
break tells the program to break outside of the current block(basically
it looks for the next } character and executes the line after it) if we
don't include break after every case it will continue executing the code
from each following case, so if we remove break from sunday and
$variable == "Sunday" then we will have sunday and monday print out.
Knowing this we can actually create teh equivalent of || statements by
omitting break statements:
switch($variable)
{
case "Saturday":
case "Sunday":
print "Weekend!\n";
break;
case "Monday":
case "Tuesday":
case "Wednesday":
case "Thursday":
case "Friday":
print "Weekday :(\n";
break;
}
So now if the day is a sat or sun it will print weekend otherwise it will print weekday, pretty cool huh?
Alright
so you guys have been patient and we've been taking in a lot of fairly
boring info without getting down and dirty and writing any real code so
nows your reward! the following are a few case studies, I'll help by
presenting you with an algorithm and leave it to you to write up these
programs and test them on your own. If anyone has any difficulty or
questions don't hesitate to shoot me a pm or post a reply and ask and
ill be glad to help as quickly as I possibly can! All of these except
for the last can be done in command line or in a web application, its up
to you which you'd prefer to work in or if you're a hardcore you can do
it in both
the last is specific to web pages(alternating background colours for table rows!)
Case Study 1:
Modify
the earlier script that prompts the user for a name, have it prompt
them for a name and an age and print out both results back to them
Algorithm:
prompt user for username
prompt user for age
print results back to user
Case Study 2:
Change Sorter
A
script to prompt the user for the amount of change they have(in cents)
the script then performs some basic calculations on the data to
determine the number of dollars, quarters, dimes, nickels, and pennies
in the change
Algorithm:
prompt user for amount of change
divide change by 1.00 to get the number of dollars
modulus divide change by 1.00 to get remaining change
divide remaining change by .25 to get the number of quarters
modulus divide the remaining change by .25 to get the remaining change
divide the remaining change by .10 to get thenumber of dimes
modulus divide the remaining change by .10 to get the remaining change
divide the remaining change by .05 to get the number of quarters
modulus divide the remaining change by .05 to get the number of pennies
print the results to the user
Case Study 3:
Basic Calculator
Create
a calculator that allows the user to enter two numerical values as well
as an operator to perform a mathematical calculation and output the
results
Algorithm:
prompt user for first number
prompt user for second number
prompt user for operator(+-/*%)
perform calculation and output results
Case Study 4:
Alternating Row Colours
often
when generating html tables we want to stylize them to have alternating
background colours, you can do this manually but its tedious and
laborious, using modulus division we can create an infinite on-off
sequence, imagine we have each row keyed to a value 0-9 well say if the
row key % 2 is equal to 0 then make it colour a otherwise make it colour
B
key:
0 %2 == 0
1 %2 == 1
2 %2 == 0
3 %2 == 1
4 %2 == 0
5 %2 == 1
6 %2 == 0
7 %2 == 1
8 %2 == 0
9 %2 == 1
). A variable is a container for data. Think back to math when you were a kid and you had to solve this problem: