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PHP Tutorial
Part 4 - Loops and
Arrays
Introduction
In
the last parts of this tutorial I have showed you how to
deal with text and variables in PHP and how you can use
IF statements to compare them and to make decisions. In
this part I am going to show you how to use another
important part of PHP, loops.
The WHILE
Loop
The WHILE loop is one of
the most useful commands in PHP. It is also quite easy
to set up and use. A WHILE loop will, as the name
suggests, execute a piece of code until a certain
condition is met.
Repeating A Set Number Of
Times
If you have a piece of
code which you want to repeat several times without
retyping it, you can use a while loop. For instance if
you wanted to print out the words "Hello World" 5 times
you could use the following code:
$times = 5;
$x = 0;
while ($x
< $times) {
echo "Hello World";
++$x;
}
I will now explain this code. The first
two lines are just setting the variables. The $times
variable holds the number of times you want to repeat
the code. The $x variable is the one which will count
the number of times the code has been executed. After
these is the WHILE line. This tells the computer to
repeat the code while $i is less than $times (or to
repeat it until $i is equal to $times). This is followed
by the code to be executed which is enclosed in {
}.
After the echo line which prints out the text,
there is another very important line:
++$x;
What
this does is exactly the same as writing:
$x = $x +
1;
It adds one to the value of $x. This
code is then repeated (as $x now equals 1). It continues
being repeated until $x equals 5 (the value of times)
when the computer will then move on to the next part of
the code.
Using
$x
The variable counting the
number of repeats ($x in the above example) can be used
for much more than just counting. For example if you
wanted to create a web page with all the numbers from 1
to 1000 on it, you could either type out every single
one or you could use the following code:
$number = 1000;
$current
= 0;
while ($current < $number)
{
++$current;
echo
"$current<br>";
}
There are a few
things to notice about this code. Firstly, you will
notice that I have placed the ++$current; before the
echo statement. This is because, if I didn't do this it
would start printing numbers from 0, which is not what
we want. The ++$current; line can be placed anywhere in
your WHILE loop, it does not matter. It can, of course,
add, subtract, multiply, divide or do anthing else to
the number as well.
The other reason for this is
that, if the ++$current; line was after the echo line,
the loop would also stop when the number showed 999
because it would check $current which would equal 1000
(set in the last loop) and would stop, even though 1000
had not yet been printed.
Arrays
Arrays
are common to many programing languages. They are
special variables which can hold more than one value,
each stored in its own numbered 'space' in the array.
Arrays are extremely useful, especially when using WHILE
loops.
Setting Up An
Array
Setting up an array is
slightly different to setting up a normal variable. In
this example I will set up an array with 5 names in
it:
$names[0] =
'John';
$names[1] = 'Paul';
$names[2] =
'Steven';
$names[3] = 'George';
$names[4] =
'David';
As you can see, the parts of an
array are all numbered, starting from 0. To add a value
to an array you must specify the location in the array
by putting a number in [ ].
Reading From An
Array
Reading from an array is
just the same as putting information in. All you have to
do is to refer to the array and the number of the piece
of data in the array. So if I wanted to print out the
third name I could use the code:
n
echo "The third name is
$names[2]";
Which would output:
The
third name is Steven
Using Arrays And
Loops
One of the best uses of a
loop is to output the information in an array. For
instance if I wanted to print out the following list of
names:
Name 1 is John
Name 2 is Paul
Name 3
is Steven
Name 4 is George
Name 5 is
David
I could use the following code:
$number = 5;
$x =
0;
while ($x < $number) {
$namenumber = $x +
1;
echo "Name $namenumber is
$names[$x]<br>";
++$x;
}
As
you can see, I can use the variable $x from my loop to
print out the names in the array. You may have noticed I
am also using the variable $namenumber which is always 1
greater than $x. This is because the array numbering
starts from 0, so to number the names correctly in the
output I must add one to the actual value.
Part
5
In the next part I will show
you how you can send e-mail using PHP.
© 1999 - 2001
David Gowans
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