PHP: For Loops II

Derek Bridge

Department of Computer Science,
University College Cork

PHP: For Loops II

Aims:

• to learn about off-by-one errors
• to learn more of the flexibility of for-loops

The for-loop

• The for-loop in general:

  for (expr1; expr2; expr3)
{
    zero, one or more statements
}

• Initially expr1 is executed
• Iteration:
  • At the beginning of each iteration, expr2 is evaluated
  • If it evaluates to true, the loop body is executed once
  • At the end of this iteration (i.e. after executing the body), expr3 is executed
  • Then the next iteration begins
• Termination:
  • If expr2 evaluates to false, execution of the loop ends: any statements after the curly braces are now executed

Flowchart

Some observations

• The three expressions must be enclosed in parentheses, separated by semi-colons
• Then there comes a block of statements
  • i.e. zero, one or more statements enclosed in curly braces
  • the curly braces are optional if there is only one statement
  • Advice: it is better practice to always use them
• If you use the curly braces, the loop is not terminated by a semi-colon
• Good spacing and indentation are essential for human-readability
  • Advice: use my layout, not others you will see elsewhere

Common error

• Class exercise: What's the error here? Will there be an error/warning message?

  $current_year = (int) date('Y');
  for ( $year = 1948; $year <= $current_year; $year++ );
  {
      if ( $year % 4 == 0 )
      {
          echo "<p>{$year} was an Olympic year</p>";
      }
  }

Off-by-one errors

• The most common logic errors with for-loops are off-by-one errors, where the programmer writes a loop that executes one too many, or one too few, times
• Class exercise: A programmer wants a loop that executes exactly 5 times. Which of these are correct?
  1. for ( $k = 0; $k < 5; $k++ )
  2. for ( $k = 0; $k <= 5; $k++ )
  3. for ( $k = 1; $k < 5; $k++ )
  4. for ( $k = 1; $k <= 5; $k++ )

Class exercise

Assuming $m and $n contain non-negative integers and that $n >= $m, how many iterations will there be of the following loops?

1. for ( $i = $m; $i < $n; $i++ )
{
    echo "<p>{$i}</p>";
}
2. for ( $i = $m; $i <= $n; $i++ )
{
    echo "<p>{$i}</p>";
}

More general for-loops

• In a for-loop,
  • expr1 conventionally initialises the loop variable, but can do anything you want
  • expr2 conventionally tests the loop variable, but can do anything you want (but will always be treated as a Boolean to determine whether to continue/stop looping)
  • expr3 conventionally increments the loop variable, but can do anything you want
• Each can even be missing (although the semi-colons are needed)
• Each can even have several statements, in which case separate the statements by commas!

Example

$current_year = (int) date('Y');
for ( $year = 1948; $year <= $current_year; $year += 4 )
{
    echo "<p>{$year} was an Olympic year</p>";
}

Class exercise

The version below is also correct but the previous version is better than this version. Why?

for ( $year = 1948; $year <= (int) date('Y'); $year += 4 )
{
    echo "<p>{$year} was an Olympic year</p>";
}

Class exercises

1. Write a for-loop that echoes the numbers from 36 to 1 (inclusive) in that order
2. Write a for-loop that never terminates (an infinite loop), forever printing consecutive integers starting from 0
3. Rewrite the following so that as much code as possible is moved into the for-loop header

   $number = (int) $_GET['number'];
   
   $total = 0;
   for ( $i = 1; $i <= $number; $i++ )
   {
       $total += $i;
   }
   echo "<p>The sum of the numbers from 1 to {$number} is {$total}</p>";

Advice

• Just because a programming language offers extra flexibility doesn't mean you should exploit it
• Advice:
  • use a for-loop when you have a counter that moves from start to end with constant increments made at the end of the loop body
  • in general, use expr1, expr2 and expr3 only for initialising, testing and incrementing the counter respectively