CS 215 ­ Fundamentals of Programming II Homework 1 solution

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The purpose of this homework is to practicing using the development environment expected in this course.   Therefore, please type in your program from scratch in the UNIX environment and used the clang++ compiler  for this homework.  Please ask for assistance sooner rather than later if you need it. A cipher is a form of cryptography in which individual letters of a message are replaced by other letters or  symbols.  The original form of the message is called the plaintext.  It is enciphered using the cipher into the  ciphertext.  The act of recovering the plaintext is called deciphering. A very simple cipher was used by the Romans and is now called the Caesar shift cipher.  In the Caesar shift  cipher, an alphabetic character (i.e., a letter) is replaced by the letter located n places to the right in the alphabet  circularly.  Another way to say this is that the plaintext letter is shifted circularly to the right n places to obtain  the ciphertext letter.  For example, if the shift is 8, then ‘A’ becomes ‘I’, ‘B’ becomes ‘J’, etc. to ‘Z’ becomes ‘H’   This idea also can be represented as a substitution table, as shown below for a shift of 8. Plaintext A B C D E F G H I J K L M N O P Q R S T U V W X Y Z Ciphertext I J K L M N O P Q R S T U V W X Y Z A B C D E F G H Using the corrected file copying program from Lecture 3 as a base, modify it to perform a Caesar shift  encipherment of only the alphabetic characters in the input file before writing the (enciphered) character to the  output file.  The source code for this program must be in a file named cipher.cpp. The encipherment must be case sensitive.  That is, an uppercase letter must map to another uppercase letter (e.g., ‘A’ to ‘I’), and a lowercase letter must map to another lowercase letter (e.g., ‘a’ to ‘i’).  The non­alphabetic  characters (i.e., digits, punctuation, and whitespace) must be passed through as plaintext. The program must accept exactly three command­line arguments that are the name of an input file, the name of  an output file, and an integer.  The input file is interpreted a plaintext message.  The output file will contain the  corresponding ciphertext.  The integer is the shift distance.  The program must do proper error checking of the  number of command line arguments and the file opens.  Here are some things to note: • Since argv is an array of C­strings, the third command­line argument will need to be converted to an  integer using the function atoi that is defined in .  It is used as follows:     int shift = atoi(argv[3]); • To compute the right circular shift of a lowercase alphabetic character ch, we can use the following  formula:    ch = (((ch ­ ‘a’) + shift) % 26) + ‘a’; 01/12/2017 Page 1 of 2 D. Hwang Computing the shift of an uppercase alphabetic character is similar using ‘A’ instead of ‘a’. • Deciphering a Caesar shift is the opposite of enciphering, but subtracting the shift value could result in a  negative number.  The equivalent positive shift value is 26 minus the shift value.  This means that we  can decipher any Caesar shift message in ciphertext with the same program by giving a shift value that is 26 minus the shift value used to encipher.  E.g., if we encipher using shift value of 8, then to decipher  the resulting ciphertext, we run the program using the ciphertext as the input file and a shift value of 18. A reminder of how to compile this program using clang++:    $ clang++ cipher.cpp ­o cipher ­Wall  The ­Wall option is used to turn on all of the warning messages.  As usual, there shouldn’t be any warning  message unless you understand why it is being ignored.  An example run of this program would look like:    $ ./cipher plaintext.txt ciphertext.txt 8 If file plaintext.txt contains the following “message”:    abcdefghijklmnopqrstuvwxyz     ABCDEFGHIJKLMNOPQRSTUVWXYZ  Then file ciphertext.txt should contain the following result:    ijklmnopqrstuvwxyzabcdefgh     IJKLMNOPQRSTUVWXYZABCDEFGH  Note: you create the plaintext.txt test file yourself using your text editor. To decipher the above ciphertext.txt, you would run:    $ ./cipher ciphertext.txt newplaintext.txt 18 and newplaintext.txt would be exactly the same as the original plaintext.txt.  You can check this  by using the UNIX diff command:    $ diff plaintext.txt newplaintext.txt When two files are exactly the same, the diff command produces no output. At the beginning of class on Wednesday, January 18, the procedure for making electronic submissions will be demonstrated and you should try to submit your program at that time.  If your program does not pass the  submission tests at that time, you will have until 11:59pm to correct and resubmit for full credit.  Please note that the automated submission system requires that all files be named exactly as specified in an assignment, and also  requires that the output of the program be exactly as expected including whitespace.