PDE - Swapping Values (Lesson)

Swapping Values

Swapping Values Array

Let's learn the basics of swapping the order of 2 numbers.

Suppose you have an 

            a[0] = paper

            a[1] = pencil

and you want to have pencil in a[0]  and paper in a[1]. 

Try this at your home.  Put the paper and pencil side by side on the table, pencil on the left and paper on the right.  With one hand, can you swap the two around only picking up one at a time to put the paper directly into the place of the pencil and the pen directly into the place of the pen?

Hmm... hard with one hand.  What about two hands?  Can you do it now?

Yes, but what happened?  You had to pick up both, thus holding on to the pencil with your right hand and moving the paper with your left hand to the spot the paper had and then moving the paper to the pencil location.  Or maybe you moved the paper first and held onto the pencil and then placed the pencil down.  Either way you had to have a place to hold one prior to moving the item to the final location. Otherwise one would have been on top of the other.  

So... in the above example, a holding place is required.  We will call this tempHold

a[0] = paper

a[1] = pencil

to swap

variable hold is defined to be the same datatype as the array datatype

hold = a[{0}] 

a[0] = a[1]

a[1] = hold

Creating an Array

In the code below an array is created to be used to demonstrate the swap of an array using the orca whale.  This is essentially the same code used for the orca in the previous page.

Swapping the Order of the Array

The order of this array in the code below is swapped, meaning the last letter is put in the position 0 and the first letter is put in position 3.  Remember, array is zero relative so the positions start with 0 and range from 0 to 3 for an array of 4 items.  The length of the array is literal length, there array length is 4 items.

Note the use of "zz", a value that is clearly not in my set of information in the array to initialize to.  Initialization could have been null, by using "" without space.   (end of the above code)

Here are two different ways to use loops to swap and array.  Note that most of the interior loop code is the same.   There are multiple ways to solve problems and still have exactly the same answer.

After both examples, we will examine why the division by 2 is required and how it works.  

Note: 

• the use of the variable "temp" initialized in the  code .
• the initialization of an index called index.

Swap Example 1

Swap Example 2

Below is the same swap as above, but using the while loop to read the information.  In both we are going through all of the positions in the array.  The key is we cannot go off the end of the array.  In the for each above, the for each procedure knows where the end of the array is.  The "while" loop must be told when to stop when it reaches the end of the array even though we don't "intend" to go that far this time.  

Video of Above Code Run

Note the "for each" and the "while" give the exact same result.  Also the result is the same output as the original where the same array was read left to right and then right to left.  Why the difference?  Different techniques for different problems.   A swap is an important concept in programming.  Swapping the order may save overall time depending on what is needed in the program.

Analyzing the Differences in the use of the  "for each" or "while"

In both we are going through all of the positions in the array.  The key is we cannot go off the end of the array.  In the for each above, the for each procedure knows where the end of the array is.  The "while" loop must be told when to stop when it reaches the end of the array.

Use the following example for all of the explanations below.

Example:    Given letters[0] = a 

letters[1] = b

letters[2] = c

letters[3] = d    

array.length - 1  -->  The position of the index only go up to 3 and not 4.  The length of the array is 4 but the position numbers start at zero,.  Four positions, zero relative is 0, 1, 2, 3

 letters[3] = d  the value of 3 is equal to letters.length - 1 since letters.length = 4 as there are 4 positions used.

Why the variable temp?

 letters[3] = d  the value of position 3 is equal to letters.length - 1 ( 4 - 1) since letters.length = 4 as there are 4 positions used.  The minus 1 brings the length down to 3 which is the true value of the last position of the array.

temp is used to hold one of the swapping items as otherwise the value would be overwritten and lost.

  letters[0] = letters[3]    //   letters[0] is overwritten by the value or 3 which is d.

  letters[3] = letters[0]    //   letters[3] is overwritten by the value or 0 which is now d.

  "a" is lost without the temp to hold the value "a" as in the following:

 temp = letters[0]

  letters[0] = letters[3]    //   letters[0] is overwritten by the value or 3 which is d.

  letters[3] = temp            //   letters[3] is overwritten by the value temp which is a.   

                "a" is not lost with the use of a temporary holding storage location!!! 

Why Divide by 2?  Dividing the array in half avoids folding the array back on itself and putting the changes back to the original.   

 In this swap, the outer two items are swapped first.

 temp = letters[0]

  letters[0] = letters[3]    //   letters[0] is overwritten by the value of position 3 which is d.

  letters[3] = temp            //   letters[3] is overwritten by the value temp which is a.   

The second swap is the items just inside the outer two

 temp = letters[1]

  letters[1] = letters[2]    //   letters[1] is overwritten by the value of position 2 which is c.

  letters[2] = temp            //   letters[3] is overwritten by the value temp which is b.   

The third swap would be the one's inside the outer 2, or the middle two again as the index continues to go up by 1 and the length decrease by 1.

 temp = letters[2]               //  currently letters[2] is c. 

  letters[2] = letters[1]    //   letters[2] is overwritten by the value of position 3 which is b.

  letters[1] = temp            //   letters[3] is overwritten by the value temp which is c.   

The fourth swap would be the one's on the outside again.

 temp = letters[3]               //  currently letters[3] is d. 

  letters[0] = letters[3]    //   letters[1] is overwritten by the value of position 3 which is a.

  letters[3] = temp            //   letters[3] is overwritten by the value temp which is d.   

Oops!!!! We are back to where we started without stopping in the middle. 

If the terms to be swapped were and odd number, the middle term remains the middle term as it does not need to move.

Why index and not i?  The index indicating the position of the data storage in the array variable is not required to be "i" or any other letter.  By convention, when typed it is typically "i".  This could be anything.  Note that the word index was used in the second problem instead of "i" found in the first problem that read the array in left to right order and then right to left order without changing anything about the array.  

Tracing the code:  Reading the code, line by line, putting in the variable values and following the changes will help you understand the swap method and be able to recognize it.  

What else is swapping used for?  Swapping using various algorithms like this one or slightly different allow arrays to be put in order for quick access during searches.  Ordered arrays are typically quicker to find an item in than in an unordered array.

IMAGES AND VIDEO CREATED BY GAVS.