PHP: Boolean Operators II

Derek Bridge

Department of Computer Science,
University College Cork

PHP: Boolean Operators II

Aims:

• to learn about short-circuit evaluation of Boolean operators
• to learn the negation operator
• to learn how to handle precedence and associativity

Class exercise

• What is the output of the following when $x contains 2, $y contains 4 and $z contains 6?

   if ( $x > 1 && $y == 4 )
   {
      $z = $z / $x;
   }
   if ( $x < 2 || $z == 3 )
   {
      $z = $z + 1;
   }
   echo $z;

• And what is its output when $x contains 3, $y contains 5 and $z contains 3?

Short-circuit evaluation of && and ||

• The && and || operators in PHP are evaluated using short-circuit evaluation:
  • evaluation of each test is from left-to-right
  • evaluation stops early if the remaining tests cannot change the result
• In the case of &&
  • if a test is false, the rest are skipped: the whole combined condition must be false
  • e.g. consider if ( $sex == 'F' && $age >= 60 ) in the case where $sex contains 'M'
• Class exercise: State what happens in the case of ||
• Class exercise: Look again at the previous slide when $x contains 0, $y contains 1 and $z contains 2

Short-circuit evaluation of &&

• Programmers often rely on short-circuit evaluation, e.g.:

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

• The following is incorrect:

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

• A difference between CS1105 and CS1109
  • in CS1105, AND and OR are commutative
  • in CS1109, due to short-circuit evaluation, && and || are not

Boolean operators: !

• The ! operator (pronounced "not") is a unary operator which 'inverts' or 'negates' a test
• E.g.

  if ( ! $nationality == 'Nerd' )
  {
      …
  }

Boolean operators: !

See:

• Negation in CS1105
• NOT gates in CS1101

Boolean operators: !

• In most cases, you can avoid the ! operator by expressing the test differently
• E.g. these are the same:
  • if ( ! $nationality == 'Nerd' )
  • if ( $nationality != 'Nerd' )
• Class exercise: Similarly, get rid of the ! operator in the following:
  1. if ( ! $age > 18 )
  2. if ( ! $age >= 12 )
  3. if ( ! $age < 100 )
  4. if ( ! $age <= 65 )
  5. if ( ! $nationality != 'Nerd' )

Boolean expressions: precedence and associativity

• Class exercise: Evaluate each of these in different orders:
  1. 3 < 2 + 4
  2. false && false || true
• But precedence, associativity and parentheses come to the rescue

Precedence and associativity of operators we have covered

Precedence and associativity of operators we have covered

• Class exercise: Hence, evaluate each of these as PHP would:
  1. 3 < 2 + 4
  2. false && false || true
• Advice: make your intention clear with parentheses (hence no need to learn the table)

Precedence and associativity of operators we have covered

• Don't write this: if ( $sex == 'M' && $age > 65 || $age < 18 )
  • Either write this: if ( ( $sex == 'M' && $age > 65 ) || $age < 18 )
  • Or write this: if ( $sex == 'M' && ( $age > 65 || $age < 18 ) )
  • Class exercise: Paraphrase them into English
• Don't write this: if ( ! $nationality == 'Nerd' && $age > 18 )
  • Class exercise: Write down the two alternatives and paraphrase them into English

Class exercise

• In the Gregorian calendar, a year is a leap year if it is divisible by 4. However, years that are divisible by 100 are not leap years unless they are divisible by 400
• Suppose year.html contains this form:

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

• Complete year.php (next slide), which outputs whether the year entered by the user is a leap year or not (use just one if statement)

Incomplete version of year.php

$year = (int) $_GET['year'];
    
$message = "<p>{$year} is not a leap year</p>";
if 
{
    $message = "<p>{$year} is a leap year</p>";
}
echo $message;