PHP: One-Armed Conditional Statements I

Derek Bridge

Department of Computer Science,
University College Cork

PHP: One-Armed Conditional Statements I

Aims:

• to learn how to write some expressions that evaluate to Booleans
• to learn the one-armed conditional statement

Comparison operators

• Revision of operators: 2 * 3 evaluates to 6
• A comparison operator compares two values, and evaluates to either true or false
• E.g.
  • 2 < 3 evaluates to true
  • 2 > 3 evaluates to false
• These expressions are called Boolean expressions

PHP's comparison operators

Operator	Paraphrase	Example use
==	Equals	$a == $b
===	Identical	$a === $b
!=	Not equal	$a != $b
!==	Not identical	$a !== $b
<>	Not equal	$a <> $b
<	Less than	$a < $b
>	Greater than	$a > $b
<=	Less than or equal to	$a <= $b
>=	Greater than or equal to	$a >= $b

Using Boolean expressions I

• The result of evaluating these expressions can be
  • stored in a variable
  • echoed into the output, but strangely...
    • true is output as '1'
    • false is output as ''

Class exercise: What's the output of each of the following?

1. echo 2 < 3;
2. echo 2 == 3;
3. echo '2 == 3';

$my_age = 20;
$your_age = $my_age;

4. echo $my_age == $your_age;
5. echo $my_age < $your_age;
6. echo $my_age <= $your_age;

$your_age = $your_age + 1;
        
$is_equal = $my_age == $your_age;

7. echo $is_equal;

$is_less_than = $my_age < $your_age;

8. echo $is_less_than;

$is_less_than_or_equal_to = $my_age <= $your_age;

9. echo $is_less_than_or_equal_to;

Using Boolean expressions II: the conditional

• The (one-armed) conditional statement, also called the if-statement:

  if ( Boolean expression )
{
    zero, one or more statements
}

• If the Boolean expression evaluates to true,
  • the statements in the curly braces are executed
  • then execution moves on to statements after the closing curly brace
• If the Boolean expression evaluates to false,
  • the statements in the curly braces are not executed
  • execution skips to statements after the closing curly brace

Class exercise: What is the output of the following?

$my_age = 20;
$your_age = 23;
if ( $my_age < $your_age )
{
    echo '<p>';
    echo 'I\'m younger than you!';
    echo 'The difference is ' . ($your_age - $my_age) . ' year(s).';
    echo '</p>';
}
echo "<p>I am {$my_age} and you are {$your_age}.</p>";

Also, what is the output if 23 is changed to 17? What is the output if 23 is changed to 20?

Flowchart

Some observations

• The Boolean expression must be enclosed in parentheses
• Then there comes a block of statements
  • i.e. zero, one or more statements enclosed in curly braces
  • the curly braces are optional if there is only one statement
  • Advice: it is better practice to always use them
• If you use the curly braces, the conditional statement is not terminated by a semi-colon
• Good spacing and indentation are essential for human-readability
  • Advice: use my layout, not others you will see elsewhere

Class exercise

• In the country of Nerdland (where the currency is the eurine), up to the first 20,000 eurines of salary is tax-free; anything above 20,000 is taxed at 22%
• Suppose salary_form.html contains this form:

  <form action="tax.php" method="get">
      <input type="text" name="salary" value="0" />
      <input type="submit" />
  </form>

• Complete tax.php (on the next slide), which outputs the amount of tax the user must pay on his/her salary

Incomplete version of tax.php

$salary = (int) $_GET['salary'];

$tax = 0.0;
if 




    
echo "<p>Your tax is {$tax} eurines</p>";

More example of conditionals

• A sequence of conditional statements is quite common
• Class exercise: Explain the different possible outputs from the following:

  if ( $firstname == '' )
  {
      echo "You must supply a firstname.";
  }
  if ( $surname == '' )
  {
      echo "You must supply a surname.";
  }

Class exercise

• When citizens of Nerdland return from abroad they pay duty on the value of any items they import. Duty is paid at 10%. But people who have been away for less than three weeks pay an additional 5% in duty. Similarly, duty goes up by 5% also if the value of the imports exceeds 2000 eurines. In all cases, if the duty owing is less than 5 eurines, then it is waived.
• Suppose declaration_form.html contains this form:

  <form action="duty.php" method="get">
      <input type="text" name="import_value" value="0" />
      <input type="text" name="days_away" value="0" />
      <input type="submit" />
  </form>

• Complete duty.php (on the next slide), which outputs the amount of duty the user must pay

Incomplete version of duty.php

$import_value = (int) $_GET['import_value'];
$days_away = (int) $_GET['days_away'];

$tax_rate = 0.1;














echo "<p>You must pay {$duty} eurines</p>";