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This chapter is from the book

This chapter is from the book

PHP for the Web: Visual QuickStart Guide, 4th EditionPHP for the Web: Visual QuickStart Guide, 4th Edition

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This chapter is from the book

This chapter is from the book

PHP for the Web: Visual QuickStart Guide, 4th EditionPHP for the Web: Visual QuickStart Guide, 4th Edition

Learn More Buy

Incrementing and Decrementing a Number

PHP, like Perl and most other programming languages, includes some shortcuts that let you avoid ugly constructs such as

$tax = $tax + 1;

When you need to increase the value of a variable by 1 (called an incremental adjustment) or decrease the value of a variable by 1 (a decremental adjustment), you can use ++ and −−, respectively: 

$var = 20; // 20
$var++; // 21
$var++; // 22
$var−−; // 21

Solely for the sake of testing this concept, you’ll rewrite the handle_calc.php script one last time.

To increment the value of a variable:

  1. Open handle_calc.php in your text editor or IDE, if it is not already open (Script 4.4).
  2. Change the tax rate calculation from Script 4.3 to read as follows (Script 4.5): 
    $taxrate = $tax/100;
$taxrate++;

    Script 4.5. Incrementing or decrementing a number is a common operation using ++ or −−, respectively.

    1    <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"
2       "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
3    <html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en" lang="en">
4    <head>
5       <meta http-equiv="Content-Type" content="text/html; charset=utf-8"/>
6       <title>Product Cost Calculator </title>
7       <style type="text/css" media="screen">
8          .number { font-weight: bold;}
9       </style>
10   </head>
11   <body>
12   <?php // Script 4.3 - handle_calc.php #4
13   /* This script takes values from calculator.html and performs
14   total cost and monthly payment calculations. */
15
16   // Address error handling, if you want.
17
18   // Get the values from the $_POST array:
19   $price = $_POST['price'];
20   $quantity = $_POST['quantity'];
21   $discount = $_POST['discount'];
22   $tax = $_POST['tax'];
23   $shipping = $_POST['shipping'];
24   $payments = $_POST['payments'];
25
26   // Calculate the total:
27   $total = (($price * $quantity) + $shipping) − $discount;
28
29   // Determine the tax rate:
30   $taxrate = $tax/100;
31   $taxrate++;
32
33   // Factor in the tax rate:
34   $total = $total * $taxrate;
35
36   // Calculate the monthly payments:
37   $monthly = $total / $payments;
38
39   // Apply the proper formatting:
40   $total = number_format ($total, 2);
41   $monthly = number_format ($monthly, 2);
42
43   // Print out the results:
44   print "<p>You have selected to purchase:<br />
45   <span class=\"number\">$quantity</span> widget(s) at <br />
46   $<span class=\"number\">$price</span> price each plus a <br />
47   $<span class=\"number\">$shipping</span> shipping cost and a <br />
48   <span class=\"number\">$tax</span> percent tax rate.<br />
49   After your $<span class=\"number\">$discount</span> discount, the total cost is
50   $<span class=\"number\">$total</span>.<br />
51   Divided over <span class=\"number\">$payments</span> monthly payments, that would be $<span class=\"number\">$monthly</span> each.</p>";
52
53   ?>
54   </body>
55   </html>

    The first line calculates the tax rate as the $tax value divided by 100. The second line increments this value by 1 so that it can be multiplied by the total to determine the total with tax.

  3. Save the script, place it in the same directory as calculator.html, and test it in your browser circle-a.jpg and circle-b.jpg.
    04_figure_10.jpg

    circle-a.jpg The last execution of the form.

    04_figure_11.jpg

    circle-b.jpg It won’t affect your calculations if you use the long or short version of incrementing a variable (compare Scripts 4.4 and 4.5).

TIP

Although functionally it doesn’t matter whether you code $taxrate = $taxrate + 1; or the abbreviated $taxrate++, the latter method (using the increment operator) is more professional and common.

TIP

In Chapter 6, “Control Structures,” you’ll see how the increment operator is commonly used in conjunction with loops.

Arithmetic Assignment Operators

PHP also supports a combination of mathematical and assignment operators. These are +=, −=, *=, and /=. Each will assign a value to a variable by performing a calculation on it. For example, these next two lines both add 5 to a variable: 

$num = $num + 5;
$num += 5;

This means the handle_calc.php script could determine the tax rate using this: 

$tax = $_POST['tax']; // Say, 5
$tax /= 100; // Now $tax is .05
$tax += 1; // 1.05

You’ll frequently see these shorthand ways of performing arithmetic.

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