PHP: Two-Armed Conditionals

Derek Bridge

Department of Computer Science,
University College Cork

PHP: Two-Armed Conditionals

Aims:

• to learn the two-armed conditional statement (if-else)

The two-armed conditional statement

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

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

Class exercise: What is the output of the following?

$grade = 57;
if ( $grade < 40 )
{
    echo '<p>You have failed your degree.</p>';
}
else
{
    echo '<p>You have passed your degree.</p>';
}
echo '<p>Goodbye!</p>';

Also, what is the output if 57 is changed to 12?

Flowchart

Class exercise

• Are there errors here? What type (compile-time, run-time, logic)? Will there be an error/warning message?

  if ( $sex == 'F' )
  {
      echo '<p>Hello sir</p>';
  };
  else
  {
      echo '<p>Hello madam</p>';
  }

Writing a sequence of ifs instead of an if-else

• Compare this with the version given earlier

  $grade = 57;
  if ( $grade < 40 )
  {
      echo '<p>You have failed your degree.</p>';
  }
  if ( $grade >= 40 ) 
  {
      echo '<p>You have passed your degree.</p>';
  }
  echo '<p>Goodbye!</p>';

Multiple ways of writing the same program

• The point is: there are often numerous ways of writing the same program
• They may give the same output but they may differ in
  • efficiency
  • clarity
  • maintainability (how easy they are to modify)
• Class exercise: Compare the two versions for efficiency, clarity and maintainability

Class exercise

• Rewrite the following on the next slide so that it uses a two-armed conditional:

  $cont = (int) $_GET['cont'];
  $exam = (int) $_GET['exam'];
  
  $message = '<p>You have failed</p>';
  if ( $cont >= 40 && $exam >= 40 )
  {
      $message = '<p>You have passed</p>';
  }
  echo $message;

Your answer

$cont = (int) $_GET['cont'];
$exam = (int) $_GET['exam'];

$message = '';
if 

   

else

    
    

echo $message;

Class exercise

• Nerdland homes are heated by Nerd Gas, a company that supplies natural methane. Customers on tariff A pay a fixed annual charge of 500 eurines and a variable charge of 2 eurines per unit used. Customers on tariff B pay no fixed charge but pay 5 eurines per unit. All customers pay a 10% tax if they use more than 100 units
• Suppose gas.html contains this form:

  <form action="gas.php" method="get">
      <input type="radio" name="tariff" value="A" />
      <input type="radio" name="tariff" value="B" />
      <input type="text" name="units" value="0" />
      <input type="submit" />
  </form>

• Complete gas.php (next slide), which outputs the customer's bill

Incomplete version of gas.php

$tariff = $_GET['tariff'];
$units = (int) $_GET['units'];

$fixed_charge = 0.0;
$variable_rate = 0.0;
$total_charge = 0.0;

// Set the fixed charge and variable rate, depending on the tariff
if 

    
    



    
    



// If high consumption, add energy tax
if 

    


echo "<p>Pay us {$total_charge}, or we'll send the boys round</p>";