PHP: Nested Conditionals

Derek Bridge

Department of Computer Science,
University College Cork

PHP: Nested Conditionals

Aims:

• to learn about nested if-else statements
• to practice reasoning about supposedly equivalent programs
• to learn how to avoid the dangling-else problem

Example

• Following a coup d'etat, the Nerdland tax laws have changed again. The following tables show the rate at which people must pay tax on income above a threshold

  Thresholds			Tax rates
  	Single	Married		Single	Married
  Female	20000	32000	Female	30%	25%
  Male	18000	32000	Male	40%	25%

• Suppose tax.html contains this form:

  <form action="tax.php" method="get">
      <input type="radio" name="status" value="single" />
      <input type="radio" name="status" value="married" />
      <input type="radio" name="sex" value="F" />
      <input type="radio" name="sex" value="M" />
      <input type="salary" name="salary" value="0" />
      <input type="submit" />
  </form>

• The next slide shows one possible version of tax.php

A program with nested-ifs

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

$threshold = 0;
$tax_rate = 0.0;
$tax = 0.0;

if ( $status == 'single' )
{
    if ( $sex == 'F' )
    {
        $threshold = 20000;
        $tax_rate = 0.3;
    }
    else // i.e. $sex == 'M'
    { 
        $threshold = 18000;
        $tax_rate = 0.4;
    }
}
else // i.e. $status == 'married'
{
    $threshold = 32000;
    $tax_rate = 0.25;
}

if ( $salary > $threshold )
{
    $tax = $tax_rate * ($salary - $threshold);
}
echo "<p>Send us {$tax} eurines or your first-born child</p>";

Class exercise

• Rewrite the if-else to eliminate the nested-if
• Compare the two versions

Class exercise

• Write the output of the following:

  $x = 1;
  $y = 1;
  if ( $x == 2 )
      if ( $y == 2 )
          echo '<p>$x and $y are both 2</p>';
  else 
      echo '<p>$x was not 2</p>';

The dangling-else problem

• Confusion can arise when a statement mixes a one-armed conditional with a two-armed conditional
• There will be two ifs and one else
• So which if does the else belong to?
• PHP is uninfluenced by your indentation
• PHP has a simple rule:

  The else belongs to the most recent if, unless braces dictate otherwise

• Class exercise: Rewrite the program using new indentation to show how the else actually related to the ifs

Avoiding the dangling-else problem

• Always include braces...and think carefully about where the closing brace goes
• E.g. this is what the programmer intended

  $x = 1;
  $y = 1;
  if ( $x == 2 )
  {
      if ( $y == 2 )
      {
          echo '<p>$x and $y are both 2</p>';
      }
  }
  else 
  {
      echo '<p>$x was not 2</p>';           
  }

Avoiding the dangling-else problem

Class exercise

• In the final-year of the BSc in Alchemy at the National University of Nerdland, students are judged on their exams, continuous assessment and a project, as follows

  Exams ≥ 40%	Y	Y	Y	Y	N	N	N	N
  Cont. ass. ≥ 40%	Y	Y	N	N	Y	Y	N	N
  Project ≥ 40%	Y	N	Y	N	Y	N	Y	N
  Pass	×
  Resit exams					×	×	×
  Resubmit cont. ass.			×	×			×
  Re-do project		×		×		×
  Fail								×

• Write outcome.php, which takes data from this form, and tells the student the outcome:

  <form action="outcome.php" method="get">
      <input type="text" name="exam" />
      <input type="text" name="cont" /> 
      <input type="text" name="proj" />      
      <input type="submit" />
  </form>

• NB Think hard to make your script as concise and clear as possible