PHP: Multidimensional Arrays I

Derek Bridge

Department of Computer Science,
University College Cork

PHP: Multidimensional Arrays I

Aims:

• to see examples of multidimensional arrays (arrays of arrays)
• to practice writing programs that use nested for-loops to handle multidimensional arrays

Multidimensional arrays

• In an indexed array,
  • elements are indexed by integers
  • elements may be integers, floats, strings, Booleans,... and even arrays
• In an associative array,
  • the keys are strings
  • the values may be integers, floats, strings, Booleans,... and even arrays
• An array that contains other arrays is called a multidimensional array
• Specifically, an array that contains arrays of simple values is called a 2-dimensional array
• Higher dimensional arrays (e.g. 3-dimensional arrays, i.e. arrays of arrays or arrays of values) are possible

2-dimensional arrays: example I

• E.g. Hugh Jeegoh is studying Alchemy at the National University of Nerdland and uses a multidimensional array to record his coursework grades in modules AL1101, AL1102 and AL1103 respectively:

  $hughs_grades = array
      (
          array(72, 75, 91, 60),
          array(85,  0,  0,  0),
          array(57, 57, 57, 57)
      );

2-dimensional arrays: example II

• 2-dimensional tables, matrices and grids are often represented using 2-dimensional arrays
• Most commonly, the array contains an array for each row of the table/matrix/grid
• E.g. the matrix on the left can be represented using the array on the right

  				$a = array         (             array(12, 15, 0, 22),             array(38, 14, 38, 19),             array(0, 6, 90, 19)         );
  12	15	0	22
  38	14	38	19
  0	6	90	19

• Use $a[$i][$j] to access the element in the $ith row, $jth column

  $a[0][0]	$a[0][1]	$a[0][2]	$a[0][3]
  $a[1][0]	$a[1][1]	$a[1][2]	$a[1][3]
  $a[2][0]	$a[2][1]	$a[2][2]	$a[2][3]

• Class exercise: What is the value of $a[2][1]? How would you access the value in column 3 of row 0?

Class exercise: accessing the elements

What do you think $x, $y and $z will contain after the following assignment statements:

1. $x = $hughs_grades[1];
2. $y = $x[0];
3. $z = $hughs_grades[1][0];

Class exercise: accessing the elements

Write statements to perform the following:

1. assign Hugh's AL1101 grades into $AL1101_grades
2. assign his grade for the second piece of AL1102 coursework into $AL1102_lab2_grade
3. echo his grade for the last piece of AL1103 coursework

Nested for-loops, again

• The standard way to visit each element in a 2-dimensional array is with nested for/foreach-loops
• The outer loop visits each 'row'
• The inner loop visits each element in the 'row' (i.e. each 'column' in the 'row')
• E.g. this outputs array $a as a table:

  echo "<table>";
  foreach ($a as $row)
  {
      echo "<tr>";
      foreach ($row as $element)
      {
          echo "<td>{$element}</td>";
      }
      echo "</tr>";
  }
  echo "</table>";

Class exercise

1. Write PHP to output Hugh's grades as a table
2. Modify your PHP so that it also computes and echoes the average grade for each module

2-dimensional arrays: example III

• E.g. Ann O Domini stores her grades, like Hugh does, but she realises she gets a more self-explanatory representation if she uses an associative array of indexed arrays:

  $anns_grades = array
      (
          'AL1101' => array(100, 95, 90, 85),
          'AL1102' => array(50,  51,  51,  52),
          'AL1103' => array(60, 55, 60, 65)
      );

Class exercise: accessing the elements

Write statements to perform the following:

1. assign Ann's AL1101 grades into $AL1101_grades
2. assign her grade for the second piece of AL1102 coursework into $AL1102_lab2_grade
3. echo her grade for the last piece of AL1103 coursework

Class exercise

Write PHP to output the grades as a table but include the module codes as row headers