Lab Assignment 2 Solution

The goal of this assignment is to practice using command line arguments, file input-output, and manipulation of Strings in java. File input-output and command line arguments will be necessary to complete pa2, and future assignments.




Command Line Arguments




A java main function always reads the operating system command line from which it was called, and stores the tokens on that line in the args array. Use the following Java program to create an executable jar file called CommandLineArguments (see lab1 to learn how to do this)




CommandLineArguments.java class CommandLineArguments{



public static void main(String[] args){ int n = args.length; System.out.println("args.length = " + n);

for(int i=0; i<n; i++) System.out.println(args[i]);

}

}




then do %CommandLineArguments zero one two three four and observe the output. Run it with several other sets of tokens on the command line. These tokens are called command line arguments, and can be used within a program to specify and modify the program's behavior. Typically command line arguments will be either strings specifying optional behavior, text to be processed by the program directly, or names of files to be processed in some way.




File Input-Output




The java.util package contains the Scanner class, and the java.io package contains classes PrintWriter and FileWriter. These classes perform simple input and output operations on text files. Their usage is illustrated in the program FileCopy.java below, which merely copies one file to another, i.e. it provides essentially the same functionality as the Unix command cp (with respect to text files only.)




FileCopy.java
Illustrates file IO



import java.io.*;

import java.util.Scanner;




class FileCopy{




public static void main(String[] args) throws IOException{




Scanner in = null;

PrintWriter out = null;

String line = null;

int n;




check number of command line arguments is at least 2 if(args.length < 2){



System.out.println("Usage: FileCopy <input file <output file");




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System.exit(1);

}




// open files

in = new Scanner(new File(args[0]));

out = new PrintWriter(new FileWriter(args[1]));




read lines from in, write lines to out while( in.hasNextLine() ){



line = in.nextLine(); out.println( line );




}




close files
in.close();

out.close();

}




}




As you can see, the Scanner constructor takes a File object for initialization, which is itself initialized by a String giving the name of an input file. The Scanner class contains (among others) methods called hasNextLine() and nextLine(). Read the documentation for Scanner at




http://docs.oracle.com/javase/7/docs/api/




to learn about the proper usage of these methods. The PrintWriter constructor takes a FileWriter object for initialization, which is in turn initialized by a String giving the name of an output file. PrintWriter contains methods print() and println() that behave identically to the corresponding methods in System.out, except that output goes to a file instead of stdout. Note that the FileWriter initialization can fail if no file named args[1] exists in the current directory. If it fails, it will throw an IOException. This is a checked exception, which cannot be ignored, and therefore function main() must either catch the exception, or throw it up the chain of function calls. (In the case of function main(), the "calling function" is the operating system). In this example, we deal with this by declaring main to throw an IOException, causing the program to quit if the exception is encountered. Similar comments apply to the initialization of the Scanner object. See the java documentation for more details.




Compile and run FileCopy.java, and observe that a Usage statement is printed if the user does not provide at least two command line arguments. This Usage statement assumes that the program is being run from an executable jar file called FileCopy. All of your programs which take command line arguments should include such a usage statement.




String Tokenization




A common task in text processing is to parse a string by deleting the surrounding whitespace characters, keeping just the discrete words or “tokens” which remain. A token is a maximal substring containing no whitespace characters. For instance, consider the preceding sentence to be a string. The 10 tokens in this string are: "A", "token", "is", "a", "maximal", "substring", "containing", "no", "whitespace", "characters.". Whitespace here is defined to mean spaces, newlines, and tab characters. This is one of the first tasks that a compiler for any language such as Java or C must perform. The source file is broken up into tokens, each of which is then classified as: keyword, identifier, punctuation, etc. Java's String class contains a method called split() which decomposes a string into tokens, then returns a String array containing the tokens as its elements. Compile and run the following program FileTokens.java illustrating these operations.




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FileTokens.java
Illustrates file IO and tokenization of strings.



import java.io.*;

import java.util.Scanner;




class FileTokens{

public static void main(String[] args) throws IOException{




Scanner in = null;

PrintWriter out = null;

String line = null;

String[] token = null;

int i, n, lineNumber = 0;




check number of command line arguments is at least 2 if(args.length < 2){



System.out.println("Usage: FileCopy <input file <output file"); System.exit(1);

}




open files
in = new Scanner(new File(args[0]));

out = new PrintWriter(new FileWriter(args[1]));




read lines from in, extract and print tokens from each line while( in.hasNextLine() ){
lineNumber++;




trim leading and trailing spaces, then add one trailing space so
split works on blank lines
line = in.nextLine().trim() + " ";




split line around white space token = line.split("\\s+");



print out tokens
n = token.length;




out.println("Line " + lineNumber + " contains " + n + " tokens:"); for(i=0; i<n; i++){

out.println(" "+token[i]);

}

}




close files in.close(); out.close();
}

}




What to turn in




Write a java program called FileReverse.java that takes two command line arguments giving the names of the input and output files respectively (following the preceding examples). Your program will read each










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line of input, parse the tokens, then print each token backwards to the output file on a line by itself. For example given a file called in containing the lines:




abc defg

hi

jkl mnop q




rstu v

wxyz




the command %FileReverse in out will create a file called out containing the lines:




cba

gfed

ih




lkj

ponm

q

utsr

v

zyxw




Your program will contain a recursive method called stringReverse() with the following signature:




public static String stringReverse(String s, int n)




This function will return a String that is the reversal of the first n characters of s. Note that reversing a String recursively is very similar to reversing an array. Study the methods in the String class documented at http://docs.oracle.com/javase/7/docs/api/ to determine how this might be done. See especially the instance methods charAt() and substring(), as well as the static method valueOf(). Base your reversal strategy on one of the recursive methods reverseArray1() or reverseArray2() discussed in class, or on the methods of the same name in pa1. Use stringReverse() to perform the reversal of tokens from the input file.




Submit the files README, Makefile, and FileReverse.java to the assignment name lab2. This is not a difficult assignment, since most of the items mentioned here should be review, but please don't wait until the last minute to start.





























































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