PHP: While Loops III

Derek Bridge

Department of Computer Science,
University College Cork

PHP: While Loops III

Aims:

• to compare for-loops with while-loops

for-loops versus while-loops

• 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
• But a while-loop can always be used in place of a for-loop, and vice versa

Class exercise: Are these equivalent?

• for-loop:

  for ( $i = 0; $i < 5; $i++ )
  {
      echo "<p>{$i}</p>";
  }

• while-loop:

  $i = 0;
  while ( $i < 5 )
  {
      echo "<p>{$i}</p>";
      $i++;
  }

Class exercise

• Rewrite the following to use a for-loop:

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

Compound interest

You deposit 1000 eurines in your Allied Nerdish Bank account. At the end of each year, 10 per cent cumulative interest is added

Class exercise I

• Suppose interest1.html contains this form:

  <form action="interest1.php" method="get">
      <input type="text" name="deposit" />
      <input type="text" name="interest_rate" />
      <input type="text" name="num_years" />    
      <input type="submit" />
  </form>

• Write interest1.php which outputs the amount in the user's account if s/he deposits deposit eurines for num_years years at an interest rate of interest_rate%

Class exercise II

• Suppose interest2.html contains this form:

  <form action="interest2.php" method="get">
      <input type="text" name="deposit" />
      <input type="text" name="interest_rate" />
      <input type="text" name="desired_deposit" />    
      <input type="submit" />
  </form>

• Write interest2.php which outputs the the number of years it will take for the amount in the user's account to grow to at least desired_deposit eurines if s/he deposits deposit eurines at an interest rate of interest_rate%

Pointers for advanced students

• Look up the keyword break
  • allows you to prematurely exit a loop
  • generally avoid it: it makes the flow of control in your script more complex
• Look up the keyword continue
  • allows you to skip the rest of the loop body and jump ahead to the next test of the loop condition
  • generally avoid: it makes the flow of control in your script more complex
• Look up the do…while loop
  • the loop test comes after the body is executed (see flowchart)
  • use it if you know the body must be executed at least once

Flowchart