PHP: Boolean Operators I

Derek Bridge

Department of Computer Science,
University College Cork

PHP: Boolean Operators I

Aims:

• to learn how to write programs that use nested conditional statements
• to learn PHP's Boolean operators

Ifs within ifs

• In a conditional statement

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

  the block can contain any kind of statement…including conditional statements

Class exercise: What is the output of the following?

$age = 23;
$nationality = 'Nerd';

if ( $age > 18 )
{
    echo '<p>You are able to pay tax in Nerdland</p>';
    if ( $nationality == 'Nerd' )
    {
        echo '<p>You are able to stand for election in Nerdland</p>';
    }
}

Also, what if the person is a 17 year old Nerd? Or a 17 year old Geek? Or a 23 year old Geek?

Common error

• Compare this, which is OK:

  if ( $num_of_faults != 0 )
  {
      if ( $total_downtime / $num_of_faults >= 3 )
      {
          echo 'Average downtime is too high!';
      }
  }

• ...with this, which isn't:

  if ( $total_downtime / $num_of_faults >= 3 )
  {
      if ( $num_of_faults != 0 )
      {
          echo 'Average downtime is too high!';
      }
  }

Class exercise

• In the National University of Nerdland, a student passes AL1101 Intro to Alchemy if s/he obtains at least 40% in continuous assessment and at least 40% in the written exam
• Complete al_outcome.php (next slide), which takes data from this form, and determines whether the student passes or not:

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

Incomplete version of al_outcome.php

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

$message = '<p>You have failed</p>';
if



        
    

echo $message;

Boolean operators: &&

• In effect, a student passes AL1101 if $cont >= 40 and $exam >= 40
• We can instead write this using a Boolean operator
• The && operator (pronounced "and") combines several tests into a single test that is true if and only if all the tests are true
• E.g.

  if ( $age > 18 && $nationality == 'Nerd' )
  {
      …
  }

• Class exercise: Rewrite al_outcome.php (next slide) using &&

Another incomplete version of al_outcome.php

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

$message = '<p>You have failed</p>';
if 

        

echo $message;

Boolean operators: &&

See

• Conjunction in CS1105
• AND gates in CS1101

Class exercise

• In the National University of Nerdland, a student passes WC1101 Intro to Witchcraft if s/he obtains at least 40% in continuous assessment or at least 40% in the written exam
• A solution using conjunction (&&) is shown (next slide)
• Complete wc_outcome.php (next slide but one) to determine whether a student passes or not without using conjunction (Hint: more than one if)

A version of wc_outcome.php that uses conjunction

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

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

Incomplete version of wc_outcome.php

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

$message = '<p>You have failed</p>';
if 

    



    

echo $message;

Boolean operators: ||

• In effect, a student passes WC1101 if $cont >= 40 or $exam >= 40
• We can instead write this using a Boolean operator
• The || operator (pronounced "or") combines several tests into a single test that is true if at least one of the tests is true
• Class exercise: Rewrite wc_outcome.php (next slide) using ||

Another incomplete version of wc_outcome.php

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

$message = '<p>You have failed</p>';
if 

        

echo $message;

Boolean operators: ||

See

• Disjunction in CS1105
• OR gates in CS1101

Comments

• Comments can make computer software more intelligible to humans

<!-- This is how comments are done in HTML -->

/* This is how comments are done in CSS */

/* It is also one way that comments are done in PHP */

#  But this is another way: from the hash to the end of the line

// And this is a third way: from the slashes to the end of the line is a comment

-- This is a common way in SQL (but using /* */ and # are also allowed)