PHP: While Loops I

Derek Bridge

Department of Computer Science,
University College Cork

PHP: While Loops I

Aims:

• to practice writing simple while-loops for unbounded repetition

Doing something repeatedly

• Commands that we have seen for writing loops:
  • foreach-loops are used to visit each element in an array and execute the same block of statements for each element
  • for-loops are most commonly used for bounded repetition, where the number of times the block of statements is executed is known in advance
• while-loops are most commonly used for unbounded repetition, where the number of times the block of statements is executed is not known in advance

The while-loop

• The while-loop:

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

• Iteration:
  • At the beginning of each iteration, the Boolean expression is evaluated
  • If it evaluates to true, the loop body is executed once
  • Then the next iteration begins
• Termination:
  • If the Boolean expressions evaluates to false, execution of the loop ends: any statements after the curly braces are now executed

Flowchart

while-loop example

• One way to calculate the (ceiling of the) square root of a non-negative integer is to subtract successive odd numbers until you reach zero. The number of subtractions you need is the square root
• E.g. 25 - 1 = 24; 24 - 3 = 21; 21 - 5 = 16; 16 - 7 = 9; 9 - 9 = 0
  It took five subtractions to reach zero, so the square root is 5
• Suppose root.html contains this form:

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

  The next slide shows root.php, which computes the square root of the user's number using the above method

root.php

$number = (int) $_GET['number'];
$root = 0;
$odd = 1;
while ( $number > 0 )
{
    $number -= $odd;
    $root++;
    $odd += 2;
}
echo "<p>The square root is {$root}</p>";

Common errors

• The following logic errors are common:
  • a loop whose body never gets executed
  • infinite loop: a loop that never terminates
  • off-by-one errors: a loop whose body is executed one too many, or one too few, times
• Class exercise: There follow a number of variations of the program we wrote earlier. For each say what the error is

Exercise I

$number = (int) $_GET['number'];
$root = 0;
$odd = 1;
while ( $number >= 0 )
{
    $number -= $odd;
    $root++;
    $odd += 2;
}
echo "<p>The square root is {$root}</p>";

Exercise II

$number = (int) $_GET['number'];
$root = 0;
$odd = 1;
while ( $number != 0 )
{
    $number -= $odd;
    $root++;
    $odd += 2;
}
echo "<p>The square root is {$root}</p>";

Exercise III

$number = (int) $_GET['number'];
$residue = $number;
$root = 0;
$odd = 1;
while ( $number > 0 )
{
    $residue -= $odd;
    $root++;
    $odd += 2;
}
echo "<p>The square root of {$number} is {$root}</p>";

Class exercise

• Suppose shift.html contains this form:

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

  This allows the user to enter the duration (secs) of his/her shift at the local factory
• And suppose we have an array which states, for each job the user must carry out, the time it will take (secs) e.g.

  $job_lengths = array(127, 195, 3012,…);

• Using a while-loop, write shift.php which says how many of the jobs will get finished during the user's shift, assuming they are tackled in the order given