1 Attachment(s)
PHP Tutorial for Beginners - Part 3
while Loops - An Important Conditional Statement
A while loop is a little less sophisticated than a for loop, but it approximately carries out the same function. It is composed of a body containing some instructions and an exit condition. At the beginning of the cycle and each successive iteration, all the instructions in the body are executed, the exit condition consisting of a boolean expression, is checked up. The loop ends when the exit condition returns a false value.
The syntax of a while loop is as follows
PHP Code:
while (condition)
{
BodyOfLoop;
}
The condition is the control condition of the loop. The first check of the condition's validity takes place at the beginning of the cycle, before the first iteration. Also in this case, it can happen that the instructions included in the loop's body are never executed. This happens when the exit condition immediately returns a false value. As with the for loops, even with the while loops there is the danger to create a never ending loop. This happens when the exit condition never returns a false value.
The example below, prints numbers from 1 to 10 (Just like the example of for loops). I have shown only a part of the program. A more complete and complex example is shown later.
PHP Code:
$k = 1;
while($k <=10){
echo("$k <BR>");
$k++;
}
Observe that in this case it is necessary to supply to the increment of the control variable $k, by adding an increase instruction ($k++) in the loop's body. The output of the program is shown below
1
2
3
4
5
6
7
8
9
10
I have also taken up the example that displays the cell along with the line-cell numbers. This time I have generated the same output using the while loop instead of the for loop.
PHP Code:
<HTML>
<BODY>
<?
$j=1;
echo ("<TABLE ALIGN=CENTER BORDER=1 CELLSPACING=1 CELLPADDING=5 >");
while ($j<=5) {
echo ("<TR>");
$k=1;
while ($k<=3) {
echo ("<TD> Line $j, Cell $k </TD>");
$k++;
}
echo("</TR>");
$j++;
}
echo ("</TABLE>");
?>
</BODY>
</HTML>
The output of this program is similar to the one using the for loop. The table below is the output.
Line 1, Cell 1 Line 1, Cell 2 Line 1, Cell 3
Line 2, Cell 1 Line 2, Cell 2 Line 2, Cell 3
Line 3, Cell 1 Line 3, Cell 2 Line 3, Cell 3
Line 4, Cell 1 Line 4, Cell 2 Line 4, Cell 3
Line 5, Cell 1 Line 5, Cell 2 Line 5, Cell 3
PHP also provides an alternative syntax for while loops, too. It is possible to avoid the use of the brackets "{" and "}" by enclosing the instructions that constitute the body of the loop with the endwhile keyword. The syntax of a while loop using the endwhile keyword follows
PHP Code:
while (condition)
BodyOfLoop;
endwhile;
Just as in the if-then-else statement, we can set up a while loop intermingled with HTML code. The syntax is as follows
PHP Code:
<?
while (condition) : ?>
HTML instructions
<? endwhile ?>
The table below serves as a quick reference for the syntax of for loops and while loops. Both these return the same result. You could use any of them.
Comparisons of for and while loops
for Loop
while loop
PHP Code:
for ($i=0; $i<=10; $i++) {
echo("Iteration no. $i<BR>");
}
$i=0;
while ($i<=10) {
echo ("Iteration no. $i<BR>");
$i++;
}
Functions
Functions are blocks of instructions used by the main program in order to achieve its goal. A function generally executes a basic and complete operation. Many such functions together would carry out the main objective of the program.
Every function has a name that identifies it. A function can get as input an arbitrary number of parameters (variables), which can be used within that function. Also if necessary, an output value can be returned to the main program from the function. This value can be saved and used when necessary. Functions are a basic instrument in every programming language. This allows software developers to spilt the application's main objective into many smaller blocks each concerned with a specific job. This simplifies the application's building, testing, debugging and editing process. This technique leads to what is called modular programming since the entire program is divided into separate modules (functions).
A function is constituted by a head and a body. In PHP, functions are defined as follows
PHP Code:
function FunctionName($param1, $param2, ..., $paramN){
FunctionBody;
}
FunctionName is the name of the function. $param1 , $param2 ..., $paramN are the N parameters that the function receives. The parameters sent to a function do not necessarily have to be primitive data types (integers, char, floats, etc.), they can be any type of objects defined by the software developer.
Between the brackets ' { ' and ' } ' is the function's body, containing the instructions to be executed when the function is called. Once these are executed, a value called function's output can be returned to the program that called this function.
Now we want to define a function that displays some text in a HTML document using a paragraph TAG. This text is passed as parameter. We can write a function like the following one
PHP Code:
function writeString ($str)
{
echo("<FONT FACE=\"Comic Sans MS\" SIZE=3><P ALIGN=\"JUSTIFY\">$str</P></FONT>");
}
To use the writeString() function, we will write the following line inside the PHP code
writeString("Text to be formatted and displayed by the function");
The PHP code of the page in which the writeString() function is used is shown below
PHP Code:
<HTML>
<BODY>
<?
function writeString ($str)
{
echo("<FONT SIZE=+1><P ALIGN=\"JUSTIFY\">$str</P></FONT>");
}
writeString ("Text to be formatted and displayed by the function");
?>
</BODY>
</HTML>
The code will generate the following output
Text to be formatted and displayed by the function
The next example is quite a complex example. The PHP script consists of a function that creates a beautiful checkered board using only HTML tags. The number of checkered squares on the board can be passed to the function as input parameters.
PHP Code:
<HTML>
<BODY>
<?
function createBoard ($lines, $cols)
{
$j=1;
echo ("<TABLE ALIGN=CENTER BORDER=1 CELLSPACING=0>");
while ($j<=$lines) {
echo ("<TR>");
$k=1;
while ($k<=$cols) {
if (($j+$k)%2>0)
echo ("<TD WIDTH=30 HEIGHT=30 BGCOLOR=#000000> </TD>");
else
echo ("<TD WIDTH=30 HEIGHT=30 BGCOLOR=#FFFFFF> </TD>");
$k++;
}
echo("</TR>");
$j++;
}
echo ("</TABLE><BR><BR>");
}
createBoard(8,8);
createBoard(4,4);
?>
</BODY>
</HTML>
The output of the program shown above would be the following
Attachment 17
The createBoard(8,8) function call creates the 8x8 checkered board. And the next line createBoard(4,4) creates the smaller 4x4 checkered board. The most important part of the program is the nested for loops (one for loop inside another for loop). The inner for loop would run complete number of times for each iteration of the outer for loop. So if the outer for loop has 10 iterations and the inner for loop has 5 iterations. And suppose there is an echo() statement within the inner for loop, this echo() statement would be called a total of 10x5 times - 50 times.
I hope you liked this article. In my next article I shall explain the different datatypes in PHP. Also I shall deal with strings and explain their usage.
