Posts tagged Web Programming

PHP #6: Even More Control Structures

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So far we have learned about the following control structures:

  1. if, elseif, else
  2. while
  3. for

But there are still more. We'll first look at the switch construct.

Switch

The syntax for a switch statement looks something like this:

switch (expression) {
    case value1:
        //some code
    case value2:
        //some code
    case value3:
        //some code
    default:
        //some code
}

When this code is executed, each case is looked at in order. If(value1 == expression), then all code under that case and the code under all subsequent cases will be executed. If not, it will check if(value2 == expressions) and so on. If none of the cases match, the default case is used. You can have as many or as few cases as you want, and having the default case is optional. An important thing that is often forgotten is that all code underneath a true case is executed, even the code within the following cases.

$n = 3;
switch($n){
    case 0:
        print "$n is equal to 0\n";
    case 1:
        print "$n is less than or equal to 1\n";
    case 2:
        print "$n is less than or equal to 2\n";
    case 3:
        print "$n is less than or equal to 3.\n";
    case 4:
        print "$n is less than or equal to 4.\n";
    case 5:
        print "$n is less than or equal to 5.\n";
}

When the above code is executed, the following will be printed:

3 is less than or equal to 3.
3 is less than or equal to 4.
3 is less than or equal to 5.

Notice that once an appropriate case is found, all code in the following cases is executed as well. There is a way to prevent this though. Using the break; command you can jump to the closing brace of the switch statement preventing any more code from being executed.

Also, a default case will be used when no other cases match.

switch ($n) {
    case 0:
        print "$n is zero.";
        break;
    case 1:
    case 4:
    case 9:
        print "$n is a perfect square.";
        break;  
    case 2:
        print "$n is an even number.";
    case 3:
    case 5:
    case 7:
        print "$n is a prime number.";
        break;
    case 6:
    case 8:
        print "$n is an even number.";
        break;
    default:
        print "Only single-digit numbers are allowed.";
        break;
}

Test for Understanding

  1. What will print if $n is set to 7?
  2. What will print if $n is set to 1?
  3. What will print if $n is set to 20?

Answers:

  1. 7 is a prime number.
  2. 1 is a perfect square.
  3. Only single-digit numbers are allowed.

HTML 4/XHTML 1.0 Tag List

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DTD referrs to the doc type and indicated which tags are allowed F = Frameset, S = Strict T = Transitional.

Tag Description DTD XHTML 1.1
<!--...--> Comment in HTML FST YES
<!DOCTYPE> Sets document type FST YES
<a> Sets an anchor FST YES
<abbr> Sets an abbreviation FST YES
<acronym> Sets an acronym FST YES
<addres> Sets contact information for the author/owner of a document FST YES
<applet> Deprecated: Sets an embedded applet FT NO
<area /> Sets an area inside an image-map FST NO
<b> Sets bold text FST YES
<base /> Sets a default address or default target for all links on a page FST YES
<basefont /> Deprecated: Sets a default font, color, or size for the text in a page FT NO
<bdo> Sets text direction FST NO
<big> Sets big text FST YES
<blockquote> Sets a long quotation FST YES
<body> Sets a document's body FST YES
<br /> Sets a single-line break FST YES
<button> Sets a push button FST YES
<caption> Sets a table caption FST YES
<center> Deprecated: Sets center text FT NO
<cite> Sets a citation FST YES
<code> Sets computer code text FST YES
<col /> Sets attribute values for one or more columns in a table  FST NO
<colgroup> Sets a group of columns in a table for formatting FST NO
<dd> Sets a description of a term in a definition list FST YES
<del> Sets deleted text FST NO
<dfn> Sets a definition term FST YES
<dir> Deprecated: Sets a directory list FT NO
<div> Sets a block-level section of a document FST YES
<dl> Sets a definition list FST YES
<dt> Sets a definition term (or item) FST YES
<em> Sets emphasized (or italic) text FST YES
<fieldset> Sets a border around elements in a form FST YES
<font> Deprecated: Sets font, size, and color for text FT NO
<form> Sets a form for user input FST YES
<frame /> Sets a window in a frameset F NO
<frameset> Sets a set of frames F NO
<h1> to <h6> Sets different headings FST YES
<head> Sets the head section of an HTML document FST YES
<hr /> Sets a horizontal line FST YES
<html> Sets an HTML document FST YES
<i> Sets italic (or emphasized) text FST YES
<iframe> Sets an inline frame FT NO
<img /> Sets an image FST YES
<input /> Sets an input field in a form FST YES
<ins> Sets inserted text FST NO
<isindex> Deprecated: Sets a searchable index related to a document FT NO
<kbd> Sets keyboard text FST YES
<label> Sets a label for an input element FST YES
<legend> Sets a legend for a fieldset FST YES
<li> Sets a list item FST YES
<link /> Sets the relationship between a document and an external resource FST YES
<map> Sets an image-map FST NO
<menu> Deprecated: Sets a menu list FT NO
<meta /> Sets metadata about the document FST YES
<noframes> Sets alternative content for users who do not support frames FT NO
<noscript> Sets alternative content for users who do not support client-side scripts e.g. JavaScript FST YES
<object> Sets an embedded object FST YES
<ol> Sets an ordered list FST YES
<optgroup> Sets a group of related options in a form select list FST YES
<option> Sets an option in a form select list FST YES
<p> Sets a paragraph FST YES
<param /> Sets a parameter for an object FST YES
<pre> Sets preformatted text FST YES
<q> Sets a short quotation FST YES
<s> Deprecated: Sets strikethrough text FT NO
<samp> Sets sample computer code FST YES
<script> Sets a script type FST YES
<select> Sets a drop down list FST YES
<small> Sets small text FST YES
<span> Sets a inline-section of a document FST YES
<strike> Deprecated: Sets strikethrough text FT NO
<strong> Sets strong (or bold) text FST YES
<style> Sets an internal style sheet FST YES
<sub> Sets subscript text FST YES
<sup> Sets superscript text FST YES
<table> Sets a table FST YES
<tbody> Groups a table body FST NO
<td> Sets a table cell FST YES
<textarea> Sets a textarea for an input field FST YES
<tfoot> Groups a table footer area FST NO
<th> Sets a table header cell FST YES
<thead> Groups a table header area FST NO
<title> Sets the title of the document FST YES
<tr> Sets a table row FST YES
<tt> Sets teletype text FST YES
<u> Deprecated: Sets underlined text FT NO
<ul> Sets an unorderd list FST YES
<var> Sets a variable part of a text FST YES
<xmp> Deprecated: Sets preformatted text None NO

PHP #5: More Loops

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There are a few other types of loops in addition to while loops that we learned about in Part 4.

Do-While Loop

A do-while loop is very similar to a while loop. The only difference is that the condition is evaluated after each loop iteration, rather than before. The syntax is like this:

do {
  // some code here
} while (condition)

Since the condition isn't evaluated until after the loop body, the loop body is always executed at least once, whether or not the condition is true. Once it does get to the condition, if it is true, it goes back up to the top of the loop body. If it is false, the loop terminates.

$i = 11;
do {
   print "The number is $i<br />";
   ++$i;
} while ($i <= 10)

The above code prints:

The number is 11&lt;br /&gt;

even though the condition is not true. The loop body always executes once.

For Loop

The for loop is a bit more complicated. The syntax looks like this:

for (initial-expression; condition; loop-end-expression) {
  //some code goes here
}

The initial-expression is evaluated only once, at the very beginning of the loop. The condition is then evaluated. If the condition is true, the body of the loop executes. Then the loop-end-expression is run. The process then starts over again with evaluating the condition. A for loop is generally used to do simple bounding loops, like this:

for ($i = 1; $i <= 10; ++$i) {
   print "The number is $i<br />"
}

As you'll notice, the same exact thing can be done with a while loop, but many prefer the for loop as it is more compact:

$i = 1;
while ($i <= 10) {
   print "The number is $i<br />";
   ++$i;
}

More Looping Examples

Let's try using a for loops to create a multiplication table.

$start = 1;
$end = 10;

print '<table border="1" cellspacing="0" cellpadding="5">';
for ($y = $start - 1; $y <= $end; ++$y) {
	print "<tr>";
	for ($x = $start - 1; $x <= $end; ++$x) {
		if ($y == $start - 1) {
			print "<td><b>$x</b></td>";
			
		} else if ($x == $start - 1) {
			print "<td><b>$y</b></td>";
		} else {
			print "<td>"
            print $x*$y;
            print "</td>";
		}
	}
	print "</tr>";
}
print "</table>";

Notice that there is a for loop inside of another for loop. These are called nested for loops. The outer loop loops through each row, and while the inner loop loops through each column. This means on every iteration of the outer loop, the entire inner loop runs. Trace the above code carefully to see how it works. This code will produce the following table:

0 1 2 3 4 5 6 7 8 9 10
1

1 2 3 4 5 6 7 8 9 10
2

2 4 6 8 10 12 14 16 18 20
3

3 6 9 12 15 18 21 24 27 30
4

4 8 12 16 20 24 28 32 36 40
5

5 10 15 20 25 30 35 40 45 50
6

6 12 18 24 30 36 42 48 54 60
7

7 14 21 28 35 42 49 56 63 70
8

8 16 24 32 40 48 56 64 72 80
9

9 18 27 36 45 54 63 72 81 90
10

10 20 30 40 50 60 70 80 90 100

Test for Understanding

  1. In the code above how many times does line 6 run?
  2. In the code above how many times does line 8 run?

Answers:

  1. Line 6 runs while $y <= 10. $y starts at 0 ($start - 1) and goes up by 1 each time. So it runs while $start = 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10. That's 11 times!
  2. Line 8 runs on every iteration of the inner loop. The inner loop runs while $x <= 10. $x starts at 0 and goes up by 1 each time. So the inner loop runs 11 iterations. But the inner loop happens again on every iteration of the outer loop, which also iterates 11 times. 11 x 11 = 121 times.

PHP Video Tutorial 8: Comparison and Logical Operators

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PHP Video Tutorial 7: If-Else Statements

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