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Lab 3: Sorting

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COE428 Lab 3: Sorting

1. IMPORTANT: Two week lab

Note that you have two weeks to complete this lab. This lab must be submitted at

least 48 hours before the beginning of your next lab period.

2. Prelab preparation

Before coming to the lab you should:

• Read the lab. Try to prepare any questions you may have about the lab.

• Refer to Lab Guide.

• Create your lab directory for lab3. (i.e. use mkdir lab3 within your coe428 directory.)

• Change to your coe428/lab3 and unzip the lab3.zip file with the command:

unzip /home/courses/coe428/lab3/lab3.zip

• Ensure that you have downloaded properly by entering the command: make

No errors should be reported..

3. Requirements

The requirements to complete the lab are summarized below.

1. Implement InsertionSort using the mySort() function in the file:

insertionSort.c.

2. Implement MergeSort using the mySort() function in the file

mergeSort.c.

3. Edit your README file to include:

1. A brief summary of what you accomplished and what (if any) parts you did not

complete or bugs that you are aware of but have not fixed.

2. Analysis (including equations for number of moves, swaps and compares as a

function of n) for the best-, average- and worst-case behaviors of InsertionSort and

MergeSort,

4. Introduction

This lab revisits sorting algorithms. You will:

1. Implement and analyze two sort algorithms:

a. InsertionSort

b. MergeSort

2. Using programming conventions (or patterns) that allow the

performance of your implementations of the algorithms to be

measured.

3. Use the make development tool to keep your project up to date.

5. Theory

5.1. The Insertion Sort Algorithm

You are required to analyze and implement another sorting strategy: InsertionSort. The

InsertionSort algorithm applied to a deck of cards can be described as:

Step 1:

If there are no cards to sort, then Stop.

Step 2:

Otherwise, remove the top card from the unsorted pile, figure out where it should go in the

sorted pile and insert it there.

Step 3:

Go back to step 1.

 

The important characteristics of the algorithm are:

• As it runs, it divides the deck into a sorted portion (initially empty) and an unsorted

portion (initially the whole deck).

• Each time Step 2 is performed, the sorted portion has one more card and the unsorted

portion has one less card.

• Step 2 is performed n times (where n is the number of cards in the deck).

• Step 2 describes how this is done: in this case, get the first unsorted card and insert

it into the proper position in the sorted part.

 

It is useful to compare this algorithm with the similar SelectionSort algorithm.

Step 1:

If there are no cards to sort, then Stop.

Step 2:

Otherwise, find the smallest card, remove it and place it on top of the sorted card pile.

Step 3:

Go back to step 1.

 

It should be evident that both algorithms share some basic characteristics. Both increase the

sorted portion by one each time Step 2 is performed. They differ only in the way this is

done:

• SelectionSort has to examine all the cards in the unsorted portion, remove the smallest

one and then place it at the end of the sorted portion.

• InsertionSort takes the first card from the unsorted portion (i.e. it does not have to scan

through the unsorted cards) and inserts it into the sorted portion (and it has to scan the

sorted portion to figure out where it should go).

5.2. Implementing sort algorithms with metrics

The algorithms described previously use the deck-of-cards metaphor. This may be useful for

human understanding of the algorithm, but is incomprehensible to computers.

 

In this lab, we require that all sort algorithms be implemented in C using a function mySort

that sorts an array (or a subarray) of int’s. The signature for mySort() is:

void mySort(int data[], unsigned int first, unsigned int last);

Note

The signature for mySort is in the file mySort.h.

An implementation of the mySort function must modify the data array such that all elements in the

range data[first], data[first+1]…data[last] are in sorted order.

5.2.1. Example: SelectionSort implementation

The SelectionSort algorithm is described in Chapter 2 of Introduction to Algorithms, exercises

2.2-2 on page 27. It is strongly recommended that you review the idea of SelectionSort.

5.2.1.1. The initial implementation

For reference, the initial implementation of SelectionSort is shown below:

void mySort(int array[], unsigned int first, unsigned int last)

{

int i;

/* Step 1: Is there nothing to sort? */

while (first < last)

/* Step 2: Swap… */

for(i = first+1; i <= last; i++) {

/* Find smallest one in rest of array */

if(array[first] > array[i])) {

/Step 2..continued…swap them */

int tmp;

tmp = array[first]

array[first] = array[i];

array[i] = tmp;

}

first++;

}

return;

5.2.2. Using metrics

All sort algorithms require that elements in the array to be sorted can be compared with

something else of the same data type. Indeed, the number of times an algorithm needs to

compare a data item with something else (of the same type) is fundamental metric (i.e.

measure-of-performance or benchmark) that is often used to evaluate different sort algorithms.

 

You are provided with a framework that counts the total number of times that data elements

are compared. Although you do not need to write (or even understand) this metrics framework,

you must respect its contract.

 

In order to respect the contact so that the total number of comparisons is tracked, it is

forbidden to compare a data element with something else using C comparison operators (such

as ==, <, >, etc.) Instead, you must use the myCompare() function.

Note: Replacing “(data[j]<foo)” with myCompare()

Consider the following code that directly compares a data element with something else:

if (data[j] < foo) {

The myCompare(x, y) function returns a negative number if and only if x < y. Hence, the previous

code can

transformed to:

if (myCompare(data[i], foo) < 0) {

 

In addition to comparing elements in the data array, all sort algorithms also have to change the

positions of individual data elements. In order to be able to keep count of these operations, we

require that elements be moved using the myCopy() or mySwap() functions.

 

Note: Replacing “data[i] = tmp” with myCopy()

For example, instead of writing something like:

data[i] = tmp;

you would use the myCopy():

myCopy(&tmp, &data[i]);

 

Note: Using mySwap() to interchange elements

Step 2 of SelectionSort was initially implemented in C (as shown previously) with:

int tmp;

tmp = array[first]

array[first] = array[i];

array[i] = tmp;

The effect of this code was to interchange (or swap) the elements array[first] and array[i]. The

same result can be achieved with the mySwap() function:

mySwap(&array[first], &array[i]);

5.3. Using make

Unlike previously labs where you had to type in the compile and link commands, you can

perform these steps efficiently with the single command make.

6. Suggested approach

There are several requirements in the lab. A suggested approach is outlined below.

6.1. Use make

The files furnished to you include stub implementations of mySort() and a real main()

function.

 

By default, the command make ensures that the commands insertionSort and

mergeSort exist and correspond to the most recent modifications to source code files. The

command make works “right out of the box” (which you should have verified in your

prelab).

 

Although the “out-of-the-box” commands are compiled without error, they do not really work!

(Actually, they do work if they are asked to sort nothing. Try it! (i.e. the command

insertionSort < /dev/null will “sort nothing”, write “nothing” to stdout and write

statistics to stderr indicating that no comparisons, move, or swaps were needed to “sort

nothing”.)

6.2. Implement InsertionSort without metrics

As described previously, the InsertionSort sort algorithm is quite similar to SelectionSort.

6.3. Modify InsertionSort to include metrics

Once you are reasonably confident that the command insertionSort works, modify the

implementation so that the metric functions are used.

The most important measurement is the number of comparisons; start by using

myCompare() instead of direct comparisons. When this works, the output should still be

sorted and the statistics (displayed to stderr) will indicate the number of comparisons required.

 

Finally, replace data element movements with myCopy() and/or mySwap(). The statistics

should now reflect the number of copies/swaps performed by the algorithm.

6.4. Complete InsertionSort theoretical analysis

You should now attempt to complete a theoretical analysis of the number of

compares/copies/swaps that your implementation performs for worst-, best- and average-case

inputs of n. Ideally, you should develop an equation (a function of n) that gives these metrics.

 

6.5. Implement and analyze MergeSort

You should aim to complete the implementation and analysis of InsertionSort in the first week

of the lab.

MergeSort is a bit trickier to implement and analyze. Note, in particular, that the algorithm

itself is actually very easy to implement in C (although, of course, it does use recursion). The

tricky part is the merge sub-algorithm.

 

Although the merge is not itself recursive, it does require at least one temporary array and it

does require a fair amount of copying of elements to an from the temporary. Note, however,

that it is not necessary to use dynamic memory allocation functions (such as malloc()); you

can simply declare a local variable for your temporary array as (for example):

int temp[MAX_SIZE_N_TO_SORT};

 

Submit your lab

1. Go to your coe428 directory

2. Zip your lab3 directory by using the following command:

zip -r lab3.zip lab3/

3. Submit the lab3.zip file using the following command:

submit coe428 lab3 lab3.zip

by Ken Clowes, revised by Ivan Lee, revised by Olivia Das

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