PHP: Indexed Arrays II

Derek Bridge

Department of Computer Science,
University College Cork

PHP: Indexed Arrays II

Aims:

• to learn PHP's compound assignment operator and its increment and decrement operators
• to learn some of PHP's built-in functions for processing indexed arrays
• to practice writing PHP for processing indexed arrays

Average rainfall

• Consider the PHP we wrote for computing average rainfall:

  $rainfalls = array(3, 2, 2, 1, 3);
  $total = 0;
  $size = 0;
  echo "<table>";
  foreach ($rainfalls as $rainfall)
  {
      echo "<tr><td>{$rainfall}</td></tr>";
      $total = $total + $rainfall;
      $size = $size + 1;
  }
  echo "</table>";
  $avg = $total / $size;
  echo "<p>{$avg}</p>";

Compound assignment operators

• In programs, assignment statements such as the following are very common

  $total = $total + $rainfall;

• There is a shorthand using a compound assignment operator:

  $total += $rainfall;

• There are other compound assignment operators including -=, *=, /=, %= and .=
• Class exercise: Say what $x contains after each statement in this sequence:

  $x = 5;
  $y = 6;
  $x += 5;
  $x /= 2;
  $x += $y;
  $x %= 10;

Increment and decrement operators

• In programs, incrementing a variable using an assignment statement such as the following is very common

  $size = $size + 1;

• Class exercise: Rewrite this using a compound assignment operator
• But two further shorthand versions can be used
  • This one we will tend to use in CS1109:

    $size++;

  • And this one which we will tend not to use:

    ++$size;

• Similarly, to decrement a variable, we can use any of these:

  $x = $x - 1;
  $x -= 1;
  $x--;
  --$x;

Post-increment and pre-increment

• Class exercise (revision): Explain what happens here:

  echo $x = 22;

• The following table summarises similarities and differences between $x++ and ++$x:

  Operator	Name	Effect on $x	Value returned
  $x++	Post-increment	Incremented	Original
  ++$x	Pre-increment	Incremented	New

• Similarly for the post-decrement ($x--) and pre-decrement (--$x) operators
• Class exercise: Say what $x contains after each statement in this sequence, and give the output:

  $x = 5;
  $x++;
  ++$x;
  echo $x++;
  echo ++$x;
  echo $x;

Average rainfall again

• A shorter version of the PHP to compute average rainfall:

  $rainfalls = array(3, 2, 2, 1, 3);
  $total = 0;
  $size = 0;
  echo "<table>";
  foreach ($rainfalls as $rainfall)
  {
      echo "<tr><td>{$rainfall}</td></tr>";
      $total += $rainfall;
      $size++;
  }
  echo "</table>";
  $avg = $total / $size;
  echo "<p>{$avg}</p>";

Built-in functions

• PHP has 70+ built-in functions for array processing
• count( $a ) returns the number of elements in array $a (sizeof is an alias for count)
• array_sum( $a ) returns the sum of every element in the array
• Class exercise: Rewrite the average rainfall program to use these two functions so that it is even shorter

Class exercises

• Which of the two versions of the average rainfall program do you think is the more efficient?
• Class exercise: Rewrite the average rainfall program to use only the count function and not the array_sum function

More built-in functions

• explode( $delim, $str ) splits a string into an array, e.g.:

  $string_of_planets = 'mercury venus earth';
  $array_of_planets = explode( ' ', $string_of_planets );

  It's handy for getting input from the user and creating an array from it
• trim( $str ) removes spaces from the beginning and end of a string, e.g.:

  $string_of_planets = '   mars jupiter saturn     ';
  $trimmed_string_of_planets = trim( $string_of_planets );

• Class exercise: What will $trimmed contain:

  $trimmed = trim( ' ' );

Example

• The user types a sentence into this form:

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

• Here is word_count.php, which tells the user how many words s/he entered:

  $sentence = trim( $_GET['sentence'] );
  if ( $sentence != '' )
  {
      $array_of_words = explode( ' ', $sentence );
      echo '<p>You typed ' . count( $array_of_words ) . ' words.</p>';
  }

Class exercise

• strlen( $str ) returns the length of (number of characters in) string $str, e.g.:

  $how_many = strlen( 'uranus' ) + strlen( '' ) + strlen( ' ' );
  echo $how_many;

• The user types a sentence into this form:

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

• Complete non_space_count.php (next slide), which tells the user how many characters s/he entered, excluding the spaces between words
• E.g. if s/he enters fish and chips, the program echoes 12

Your answer

$sentence = trim( $_GET['sentence'] );