Iteration

• Iteration is another term for looping
• A function or program utilizing looping will run a specific block of code until certain crtierion is met
• Examples in Visual Basic HERE
• Three main types of iteration exist; while, do-while and for
  
  
  • WHILE Loops (Test-first)
    
    • A while loop can be seen as a repeating IF statement
    • At the start of the statement, a boolean condition is given
    • While this statement is true, the code within the WHILE loop will execute
    • Once this statement is false, the compiler or interpreter will move on to the next block of code
    • This is known as test-first because the expression is always considered before running the loop, meaning that this block of code has the option to never run if the condition isn't met
      
      
    • Example:
      $i = someFunction($value);
      

      while ($i >= 2){
          // Do something
      }

      # Our variable $i will have an arbitrary value from some undefined function.
      # While that number is greater than or equal to 2, we'll "do something".
      

     

  • FOR Loops (Fixed)
    
    • FOR loops run until a particular condition is satisfied
    • A boolean expression is given at the start of the control structure
    • There is an incremental counter in the for loop expression that tells the function when to stop
    • This is known as a fixed control structure as there's a finite number of times the loop will run (as per determined by the expression)
      
      
    • Example:
      for ($i = 1; $i <= 200; $i++){
      

          // Do something
      }

      # In a FOR loop, we state what value our counter ($i) will begin with (in this case 1)
      # We then tell the control to repeat the code while $i is LESS THAN or EQUAL TO 200
      # Finally, we increment our counter ($i) by 1 to continue the loop.
      # This is a fixed loop because we know that the boundaries are 1 <= $i <= 200 (thus looping 200 times)
      

     

  • DO-WHILE Loops (Test-last)
    
    • A DO-WHILE loop will run a condition regardless of the previous outcomes
    • This is because the boolean expression is evaluated at the end of the control structure
    • This means that a DO-WHILE loop is garuanteed to run at least once
      
      
    • Example:
      do {
          // Do something
          $i++;
      } while ($i < 4);
      
      # In this example, we're telling the code to run some functions and increment $i by 1.
      # Since this is a test-last, we will always have this code executed at least once, since $i isn't evaluated until the end of the block.
      # $i will increment by 1 until it is equal to 4.