Programming Assignment 2 Solution

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In this assignment you will implement the Binary Search and Merge Sort algorithms discussed in class.

You may begin by studying the examples posted on the webpage posted at:

Your task will be to adapt these methods to operate on String arrays rather than int arrays. The key operation to alter is the comparison of array elements. Given Strings s1 and s2, the expression s1.compareTo(s2)<0 returns true if and only if s1<s2 in the lexical ordering induced by the Unicode Character Set, s1.compareTo(s2)==0 if and only if the Strings are identical, and s1.compareTo(s2)>0 if and only if s1>s2 in the same ordering. Go through the Binary Search and Merge Sort examples and replace int comparison with String comparison where appropriate. You will write a program called that takes command line arguments giving a file to be searched and target word(s) to search for. The executable jar file will be named Search so the command line will look like:

% Search file target1 [target2 target3 ..]

As always % represents the Unix prompt. The items in brackets [] represent optional arguments.

Input File Format and Program Output

Each line of the input file for this project will contain a single word, i.e. a string containing no spaces or tabs. Your program will determine whether or not the target word is amongst the words in the input file, print a message to stdout stating whether or not the target was found, and (optionally) state the line on which the target was found, if it is found. You may assume that your program will be tested only on properly formatted input files. For example suppose file1 contains the following lines:











Running the program on file1 with several targets results in:

% Search

Usage: Search file target1 [target2 ..]

  • Search file1 key happy dictionary key found on line 5

happy not found

dictionary found on line 9

Observe that line numbering starts at 1. If you choose the option to not report where the target is found, the corresponding output would be:


  • Search file1 key happy dictionary key found

happy not found dictionary found

Actually this is not an option if you aspire to get an A or better on this assignment. In other words if your program does not state the line on which target is found, your maximum possible score will be 90 out of 100 (which is A-). The functionality you choose to implement must be clearly stated in your README file. It is recommended that you begin by completing the project without reporting the line number where the target is found, submit your project, then start working on the additional functionality.

Program Operation

Your program should begin by determining the number of lines in the input file. See the examples and on the webpage at

to see some ways to do this. If the number of lines (and therefore the number of words) is n, allocate a String array of length n and scan the file again, storing each word in the array. Your program will use Binary Search to find the target word(s). Recall however that Binary Search requires the array to be in increasing order. As the above example indicates though, you cannot expect the input file to be sorted. Therefore you must first call Merge Sort on the array before you search it. Binary Search returns -1 if the target is not found, and a non-negative integer giving it’s index in the array if it is found. You can simply test this return value to determine if the target was found. Unfortunately the index returned by Binary Search is not the original index of the target in the unsorted array, which is what you need to determine the position of the target in the input file.

This is only a problem if you are choosing the more ambitious option of reporting the line on which the target is found. A tempting approach to overcome the problem is to simply do a linear search of the word array for the target. This would actually be the simplest way to write a program that transforms the input into the required output. However this is not the task before you and not the point of the exercise. You must sort the word array using Merge Sort, then search it using Binary Search, which you’ll recall is more efficient than a linear search.

The most efficient way to determine the line number on which the target is found is by altering the mergeSort() method so as to pass in an int array that keeps track of the line number for each of the words in the array being sorted. Likewise the merge() method must also pass such an array. The signatures of mergeSort() and merge() would then be:

static void mergeSort(String[] word, int[] lineNumber, int p, int r)

static void merge(String[] word, int[] lineNumber, int p, int q, int r)

Recall from our discussion in class that merge()is where the real work of mergeSort() is done. When a call to mergeSort() returns, lineNumber[k] should be the line number where word[k] is located in the input file. For instance:


String[] word = {“ccc”, “bbb”, “ddd”, “aaa”}; int[] lineNumber = {1, 2, 3, 4};

mergeSort(word, lineNumber, 0, 3);

  • now:

  • word is {“aaa”, “bbb”, “ccc”, “ddd”}

  • lineNumber is {4, 2, 1, 3}

Think of the sorting action as performing a permutation of the input array which places it in a certain order. The trick is to write mergeSort() and merge() so as to perform the same permutation on the subarray lineNumber[p….r] as is performed on subarray word[p….r]. Actually mergeSort() will just pass this problem along to merge(), which as usual, is where the real work is done.

See the project description for lab2 to see how to deal with command line arguments and file input. It is recommended that you begin your project by manually initializing a String array in function main(), write functions mergeSort() and merge() to sort that array, adapt function main() to read a file given on the command line, then work on getting the output to display line numbers of found targets.

What to turn in

Submit the files README, Makefile, and Your Makefile must create an executable jar file called Search, and must include a clean utility that removes all .class files as well as Search itself. See Lab Assignment 1 to learn how to do this. Submit all files to the assignment name pa2.