PHP: Cascading Conditionals

Derek Bridge

Department of Computer Science,
University College Cork

PHP: Cascading Conditionals

Aims:

• to learn how to write cascading conditionals
• to learn to reason about whether programs are equivalent
• to learn to reason about their relative efficiency, clarity, and maintainability

Cascading conditionals

• By using elseif, we can have a conditional with more than two arms:

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

• You can have as many elseifs as you need

Example

if ( $grade < 40 )
{
    echo '<p>Fail</p>';
}
elseif ( 40 <= $grade && $grade < 45 )
{
    echo '<p>Pass degree</p>';
}
else
{
    echo '<p>Honours degree</p>';
}

Class exercise

• Describe how would you alter the previous program to handle the following criteria from the National University of Nerdland

Another example

• Following the election of a new Nerdlandish government, tax is now computed as follows

  If your salary is over	But not over	Your whole salary is taxed at
  0	20000	15%
  20000	50000	28%
  50000		45%

• Suppose tax.html contains this form:

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

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

Version 1 of tax.php

$salary = (int) $_GET['salary'];
if ( $salary <= 20000 )
{
    $tax = 0.15 * $salary;
}
elseif ( $salary <= 50000 )
{
    $tax = 0.28 * $salary;
}
else
{
    $tax = 0.45 * $salary;
}
echo "<p>Send us {$tax} eurines or your first-born child</p>";

Class exercise

• For each of versions 2-5,
  • Is it equivalent to version 1, i.e. does it give the same output for all inputs as version 1?
  • If you think they are not equivalent, give a piece of test data on which they would differ
  • If you think they are equivalent, is one of them more efficient? clearer? more maintainable?

Version 2

if ( $salary <= 50000 )
{
    $tax = 0.28 * $salary;
}    
elseif ( $salary <= 20000 )
{
    $tax = 0.15 * $salary;
}
else
{
    $tax = 0.45 * $salary;
}

Version 3

if ( 0 <= $salary && $salary <= 20000 )
{
    $tax = 0.15 * $salary;
}
elseif ( 20000 < $salary && $salary <= 50000 )
{
    $tax = 0.28 * $salary;
}
elseif ( 50000 < $salary )
{
    $tax = 0.45 * $salary;
}

Version 4

if ( 20000 < $salary && $salary <= 50000 )
{
    $tax = 0.28 * $salary;
}
elseif ( 0 <= $salary && $salary <= 20000 )
{
    $tax = 0.15 * $salary;
}
elseif ( 50000 < $salary )
{
    $tax = 0.45 * $salary;
}

Version 5

if ( 0 <= $salary && $salary <= 20000 )
{
    $tax = 0.15 * $salary;
}

if ( 20000 < $salary && $salary <= 50000 )
{
    $tax = 0.28 * $salary;
}

if ( 50000 < $salary )
{
    $tax = 0.45 * $salary;
}