CS107 - Lab 6

Due Wednesday 10/27 at the beginning of class.


This assignment is on C++ programming with emphasis on functions and characters and their ASCII codes. The problems you will see today are selection sort (again!, this tme with functions) and cryptography using Caesar's cypher.

As you write your program please remember to pay attention to issues of style, including:


  1. Re-write your program for selection sort using functions. Write the following functions:
    //input: n is given 
    //function: reads n values  from the user into array a[] 
    void getArray(int a[], int n) { 
      //fill in 
    //input: array a[] of n numbers, and n are given
    //function: prints the n values stored in array a[]
    void printArray(int a[], int n) {
      //fill in
    //input: array a[] of n numbers and n are given; also two indices begin and end 
    //between 0 and n-1
    //function: finds the smallest number in a[begin]...a[end]
    //return value:  the index of the position of the smallest element 
    int findSmallest(int a[], int n, int begin, int end) {
      //fill in
    //input: array a[], two indices i and j between 0 and n-1
    //function: swaps a[i] with a[j]
    void swap(int a[], int i, int j) {
      //fill in
    //input:  array a[] of n numbers and n are given
    //function: sorts a[] in increasing order 
    void selectionSort(int a[], int n) {
     int i = 0, spos; 
     while (i < n) {
        spos = findSmallest(a, n, i, n-1);
        swap(a, i, spos);
        i = i+1; 
    The main program should look as follows:
    const int size = 10;
    int main() {
        int x[size];
        getArray(x, size);
        printArray(x, size);
        cout << "The sorted array is: ";
        char answer = 'n';
        while (answer != 'y') {
            cout <<"Do you think using functions makes programming easier and more fun? (y/n)";
            cin >> answer; 
    	if (answer =='y') {
    	   cout << "great! you can go to the next problem..:)\n";
    	} else {
    	  cout << "then..look again at today's code for selection sort..";
    	  cout << "and compare it with the code that was all in main()..\n";
        return 1;

    Characters and ASCII codes

    In C++ (and in general in any programming language) each character is represented as an integer value. The standard scheme is called ASCII. ASCII is an international standard for representing characters. The ASCII code of a character is an integer between 0 and 255. However, only numbers 32 to 126 have been assigned so far o printable characters. The remaining numbers are either unnassigned or are used for control characters such as tab and return.

    Take a look at the following programs char1.cpp and char2.cpp (in EXAMPLES directory on collaboration).

    //The following program gives you the ascii code of a character that you type
    #include < iostream.h >
    int main() {
      char c; 
      char again = 'y';
      while (again == 'y') {
        cout << "Type in a character and I'll tell you its ASCII code: ";
        cin >> c;
        cout << "you typed char=" << c << " it's ascii code is " << (int)c << endl;
        cout << "again? (y/n)" << endl;
        cin >> again;
      return 1;
    //The following program gives you the ascii conversion table for the numerical values 32-126
    #include < iostream.h >
    int main() {
      int i = 32;
      while (i <= 126) { 
        cout << "code=" << i << " char=" << (char)i << endl;
        i = i+1; 
      return 1;
    Before running them, try to understand what they do. Then load them in Xcode and run them.

    Cryptology using Caesar's Cypher

    Cryptology is the science of "secret codes". Messages are encoded before they are sent out for thr purpose of keeping their content secret if they are intercepted by the wrong parties, and they are decoded when they are received to retrieve the original information.

    The most famous instances of cryptology occur in military history, beginning with Julius caesar of the Roman Empire, who developed the Caesar Cypher, and certainly including the German Enigma code cracked by the Allies during World War II (by Alan Turing, a British mathematician).

    Transmitting information securely has taken a modern turn with electronic commerce on the Internet and concerns over protection of consumer credit card information and other personal data.

    A Caesar cypher, also called a shift cypher, involves shifting each character in the message to another character some fixed distance farther along in the alphabet. Specifically, let s be some integer between 1 and 25 that represents the amount of the shift. Each letter in the message is encoded as the letter that is s units farther along in the alphabet, with the last s letters of the alphabet shifted in a cycle to the first s letters.

    For example, if s=3, then A is encoded as D, B is encoded as E, X is encoded as A, and Z is encoded as C. Decoding a message requires knowing s. For example, knowing that s=3, the code word DUPB is decoded as ARMY.

  2. Write a program that reads a text from the user and encodes all lower case characters (leave all other characters unchanged) using Caesar cypher with s=3, then decodes the encoded message. The goal is to obtain the original message. Use the following skeleton for your program (crypt.cpp from the EXAMPLES directory on collaboration). The body of the main functions, encode() and decode(), is missing.

    // Program skeleton to solve the cryptography problem
    #include < iostream.h >
    //function: read characters from the user into array a[], until the user
    //enters a period. Count how many characters were entered in total
    //including the period, and return this value.
    int getCharArray(char a[]) {
        int i; 
        char c = ' '; //some dummy initial value
        cout << "Enter a text vertically; end it with a period:" << endl;
        while (c != '.') { 
            cin >> c; 
            a[i] = c; 
            i = i+1; 
        return i;
    //input: array a[] of n characters given
    //function: print the array
    void printCharArray(char a[], int n) {
        int i = 0;
        while (i < n) { 
    	cout << a[i]; 
        cout << endl; 
    //input: array a[] of n characters 
    //function: encode the n characters of a[] using the Caesar cypher and
    //write them in order into b[].  That is, translate each character to
    //the one that is 3 characters beyond it in the alphabet.
    void encode(char a[], int n, char b[]) {
    //fill in 
    //input: array a[] of n characters (assumed encrypted with Caesar's cyper s=3)
    //function: decode the n characters of a[] into b[] using the Caesar
    //cypher. That is, translate each character into the one that is 3
    //characters before it in the alphabet
    void decode(char a[], int n, char b[]) {
    //fill in
    //input: character arrays a[] and b[] of size n 
    //function: return 1 if a[] and b[] are equal, that is, every
    //character matches; return 0 otherwise
    int areEqual(char a[], char b[], int n) {
      //fill in
    int main () {
      char original[50], cypher[50], decypher[50];
      int length;
      //read in the text that you want to encode
      length = getCharArray(original);
      //print out the text
      cout << "text length = " << length << endl; 
      cout << "This is the text that you entered: ";	
      printCharArray(original, length);
      //encode the text 
      encode(original, length, cypher);
      //print out the encrypted text 	
      cout << "This is the encrypted text: ";
      printCharArray(cypher, length);
      //decode it
      decode(cypher, length, decypher);
      //print out decoded text
      cout << "This is the decrypted text: ";
      printCharArray(decypher, length);
      //check to see of you decrypted correctly
      if (areEqual(original, decypher, length) == 1) {
        cout << "Great, you're done" << endl;
      } else {
        cout << "OOPS! The original text and the decrypted text do not match.."<< endl;
      return 1;

    Hint: Do you actually need to know the ASCII code of a character in order to encode it? Keep in mind that ASCII codes of characters that are consecutive in the alphabet are consecutive! Check this out:

    char c = 'a';
    char x = c+1;
    cout << x;
    x = c+3;
    cout << x;
    In order to test whether a character mychar is between b and d, you can test whether
    (mychar >= 'b' && 'mychar <= 'd')
    If your solution is elegant, it will not use ASCII codes!

    Extra credit: Assume the lower case characters have been shifted with an unknown amount s. Write a decode function that tries all possible values of s and picks out the good one.

What to turn in:

To submit the algorithms first rename (Xcode->File->Rename) the file main.cpp, then drag it to the Desktop, then drag it into the csci107bf04->drop-box. The names that you choose should be your loginname followed by problem number. For instance if I were to submit the file for the first problem i would rename it as ltoma1.cpp.

The drop-box has been set up such that you don't have read access (or you could copy the solutions of your colleagues), only write access. Therefore you will not be able to see whether your file has been submitted or not. Check with me in class to make sure your files got submitted.

If you work in a team submit only one file per team.

Make sure you include your name on the top line of all your .cpp programs!!