PHP: For Loops I

Derek Bridge

Department of Computer Science,
University College Cork

PHP: For Loops I

Aims:

• to practice writing simple for-loops for bounded repetition

Doing something repeatedly

• PHP, like most programming languages, has commands that allow us to write loops, which execute a block of statements repeatedly
• We've already seen one such command: foreach
  • In PHP, this allows us to visit each element in an array and execute the same block of statements for each element
• Here we look at a related command: for
  • This is most commonly used for bounded repetition, where the number of times the block of statements is executed is known in advance

for-loops

• Example: this will execute the echo statement exactly 5 times:

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

Observations: terminology

In the example,

• variable $i is referred to as the loop variable (or 'loop counter')
• the expression $i = 0 initialises the loop variable
• the expression $i < 5 is called the loop test (or 'loop condition')
• the set of statement(s) inside the curly braces (in this case, just an echo statement) is called the loop body
• each time the body is executed is referred to as one iteration

Observations: how it works

• Initialisation:
  • $i is initialised to zero: $i = 0
• Iteration:
  • At the beginning of each iteration, the loop test is evaluated, in this case: $i < 5
  • If the loop test evaluates to true, the loop body is executed once
  • At the end of this iteration (i.e. after executing the body), the loop variable is incremented ($i++)
  • Then the next iteration begins
• Termination:
  • If the loop test evaluates to false, execution of the loop ends: any statements after the curly braces are now executed

Flowchart

Class exercises

The example is a for-loop where $i counts from 0 to 4 inclusive but stops when $i is 5 (hence the body is executed 5 times)

1. Write a loop in which $i counts from 0 to 99 inclusive. How many times is the loop body executed?
2. Write a loop in which $j counts from 0 to 100 inclusive. How many times is the loop body executed?
3. Suppose variable $num_exams contains the number of exams a student must sit. Write a loop that executes its loop body exactly $num_exams times

Class exercise

• Suppose exercise.html contains this form:

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

  Write exercise.php, which outputs (as an unordered list) a multiplication table ('times table') from 1 up to 10 for the number that the user enters
• For example, if the user enters 5, the output is
  • 1 times 5 is 5
  • 2 times 5 is 10
  • ...
  • 10 times 5 is 50

Class exercise

• Using the same form as in the previous exercise (exercise.html), write exercise.php, which outputs the sum of all the integers from 1 up to the number the user entered (inclusive)
• For example, if the user enters 5, the output is 15 (i.e. 1 + 2 + 3 + 4 + 5)
• By the way, this is a terrible way of summing all these numbers: see discussion in lecture

Class exercise

• Define a function with the following prototype:
  int factorial( int $n )
  which returns $n! ($n factorial)
• Recall from school maths that $n factorial is the product of all the integers from 1 up to $n inclusive
• For example, if the actual parameter is 5, the function returns 120 (i.e. 1 × 2 × 3 × 4 × 5)
• Mathematicans define 0! to be 1
  Does your function correctly compute 0!?
• Write some PHP that invokes your function to compute the factorial of all the integers from 0 to 10 inclusive