PDE - Iteration Not Working? (Lesson)

Iteration Not Working?

Okay, you coded your iteration (loop), but nothing seems to be working.  The program goes on, but the loop does not happen.  What is causing this?  What should you look for?

Let's look at some examples:

  1.  Examine the count loop below.  All looks great with this basic loop.

                   count up to cntr

      turn 5 times

      move up 1

      move down 1

So what can break this loop and make it not work?

Initializing the cntr variable to 0 and never setting the value above 0 before the loop starts.  To count up to 0 is to do nothing.

  2.   Examine the while loop below.  All looks great with this basic loop.

while cntr > 2

      turn 5 times

      move up 1

      move down 1

      cntr = cntr - 1

So what can break this loop and make it not work? Initializing the cntr variable to 2 or below prior to the loop starting.  The condition of truth is 2 > 3  which is false and the loop will not be entered.

 3.     Here is another common issue, mistakenly comparing the same number in an inequality.

Here the comparison of nbrCk < nbrCk comes  up as false as a number cannot be either less than as this is or greater than itself.

 4.  Another situation is the comparison to the array.length incorrectly in an inequality using an iteration.    

When nbrCk is initialized to 0 as you would normally have to run a full array, nbrCk is not greater than the nbrArray.length as the array minimum would be 1, but we are requiring a minimum of 2, preferably 3 depending on the the program.  This loop is passed by.

 

Now let's look at a different situation.  You want to read your array, the program starts running and then appears to hang or crash.

     Examine the while loop below.  All appears to be the basic loop setup. 

index = 0

while index <= array.length

      array[index].turn 5 times

      array[index].move up 1

      array[index].move down 1

      cntr = cntr + 1

However, the inequality is less than and equal to the array.length rather than less than the array.length.  Remember the array starts with index 0, but the array.length is the exact number of positions in the array that are used. 

So an array of 5 would have indices 0, 1, 2, 3, 4, whereas the array length is 5.  There is not 5 index stored for this array.  When the loop iterates, the cntr variable gets closer to 5 and then finally 5.  cntr = 5 is the same value as the array.length and goes in the loop again, but what happens when 5 is used for an index to find a new item that does not exist?  

The program appears to freeze or crash, nothing else works.  In some IDEs (Interactive Development Environment), of which Alice is one, an out of bounds message is provided.  This is what has happened and the program can no longer work as access to something that does not exist has been executed.  The bounds for this array were the indices 0, 1, 2, 3, and 4 for array items, but the array was attempted to accessed with a 5 for the 6th item of the array, but the 6th item does not exist.