Implementation of Playfair Cipher Algorithm

PlayFair.cpp

/*
 * Name: Syed Owais Ali Chishti
 * Roll no: P14-6011
 * Date: 2/3/2015
 * Task: Implement the Playfair Cipher Algorithm as discussed in the class.
 */

#include <iostream>
#include <string>
#include <cstdlib>
using namespace std;

struct COORD {
    int row;
    int col;
};

/*
 * PROTOTYPE Functions
 */
string onlyUnique(string);
string onlyAlphabet (string);
bool isRepeated(string,char);
bool findCh(string,char);
void printArray();
void fillList(string);
COORD getCoord(char);
string EnDeCryptText(string);
void getUserInput(string&, string&);

const int row = 5;
const int col = 5;
char listArray[row][col];

int main() {		
	string text,key,textProccesed,keyProccesed;
    
    getUserInput(text, key); // Take input from user
    
    textProccesed = onlyAlphabet(text);
    keyProccesed = onlyUnique(key);
    
    fillList(keyProccesed); // Create table for encryption
    
    // Same function encrypt and decrypt text
    string encryptedText = EnDeCryptText(textProccesed);
    string decryptedText = EnDeCryptText(encryptedText);
    
    // Printing
	cout << endl
		 << "Text: " << text << endl	
         << "Key: " << key << endl << endl
         << "Remove Repeated from Key: " << keyProccesed << endl
         << "Only Alphabet: " << textProccesed << endl << endl
         << "Encrypted Text: " << encryptedText << endl
         << "Decrypted Text: " << decryptedText << endl
         << endl << endl
         << "Check: Text and decrypted text is " << ((textProccesed == decryptedText) ? "same" : "not same") << "."
         << endl << endl;
         
         
    // Printing encrypting and decrypting table.
    cout << endl << "Encrypting and Decrypting Table: " << endl;     
	printArray();

	system("pause");
	return 0; 
}

/* 
 * Take valid input from user.
 */
void getUserInput(string &text, string &key) {
     do {
        cout << "Enter string to encrypt: ";
        getline(cin,text);
        string sampleText = onlyAlphabet(text);
        if (sampleText.size() % 2 != 0) {
            cout << "Error: No of alphabet must be in even. TRY AGAIN" << endl << endl;
            continue;
        }
        else {
            break;
        }
    } 
    while (true);
    
    do {
        cout << "Enter key for encryption: ";
        getline(cin,key);
        if (key.size() > 25) {
            cout << "Error: Maximun unique character allowed is 25. TRY AGAIN" << endl;
            continue;
        }
        else {
            break;
        }
    } 
    while (true);
}

/*
 * Encrypt text and decrypt text
 */
string EnDeCryptText(string text) {
    COORD pos1,pos2;
    string encryptText;
    for (int i = 0; i < (int)text.size(); i += 2) {
        pos1 = getCoord(text.c_str()[i]);
        pos2 = getCoord(text.c_str()[i+1]);
        encryptText += listArray[pos2.row][pos1.col];
        encryptText += listArray[pos1.row][pos2.col];
    }
    return encryptText;
}

/*
 * Return coordination of pin location in listArray
 */
COORD getCoord(char pin) {
    COORD pos = {0,0};
    for (int r = 0; r < row; r++) {
		for (int c = 0; c < col; c++) { 
            if (listArray[r][c] == pin) {
                pos.row = r;
                pos.col = c;
                return pos;
            }
        }
    }
    return pos;
}

/*
 * Add key and alphabet logic to array.
 */
void fillList(string key) {
	int firstIndex = 0;
    int initialsForNextLoop[2];
    char ch = 'a';
	for (int r = 0; r < row; r++) {
		for (int c = 0; c < col; c++) {
            if ((r*5)+c < (int)key.size()) {
                initialsForNextLoop[0] = row;                
                initialsForNextLoop[1] = col;                
                listArray[r][c] = key.c_str()[firstIndex++];
            }
            else {
                if (ch <= 'z') {
                    while(findCh(key,ch)) ch++;
                    if (ch == 'j') ch++;
                    listArray[r][c] = ch++;
                }
            }
		}
	}    
}

/*
 * Find character in string
 * TRUE if found
 */
bool findCh(string ch,char pin) {
    
	int size = ch.size();
	while(size--) {
		if (ch[size] == pin) {
			return true;
		}
	}
	return false;
}

/*
 * Tell whether specific character is repeated in string or not
 * TRUE if repeated
 */
bool isRepeated(string ch,char pin) {
	int size = ch.size();
	int timesFound = 0;
	while(size--) {
		if (pin == ch[size]) {
			timesFound++;
		}
	}
	return (timesFound > 1 ? true : false);
}

/*
 * Keep a-z and remove all other characters\
 */
string onlyAlphabet (string text) {
	for (int i = 0; i < (int)text.size(); i++) {
		if (!(
			(text.c_str()[i] >= 'a' && text.c_str()[i] <= 'z')
		   )) 
		{
           text.erase(i,1);
           i--;
		}
        if (text.c_str()[i] == 'j') {
            text.replace(text.find(text.c_str()[i]), 1, "i");
        }
	}
    return text;
} 

/*
 * Remove any repeating alphabet from string
 */
string onlyUnique(string key) {
	int size = key.size();
	while(size--) {
		if(isRepeated(key,key.c_str()[size]) || (key.c_str()[size] >= 'A' && key.c_str()[size] <= 'Z')) {
			key.erase(size,1);
            size--;
		}
        
	}
	return onlyAlphabet(key);
}

/*
 * Print listArray to screen
 */
void printArray(){
	for (int r = 0; r < row; r++) {
		for (int c = 0; c < col; c++) {
			cout << listArray[r][c] << "\t";
		}
		cout << endl;
	}
}

I need to explain this algorithm