Help with Simple encryption using c-style strings

I have been struggling with this piece of code for a few days now, i'm new to c++ and wanted to master this bit.

Here is the entire code. Ive marked where the code is missing.

Any ideas or suggestions are greatly appreciated...thanks...

Code:

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#include <iostream>

using namespace std;
using namespace std;

const int alphaLen = 'Z'-'A'+1;

char upper( char c )
{
        if ( c>='a' && c<='z' )
                c=c+'A'-'a';

        return c;
}


/*
  cipherGenerate implicitly returns the generated cipher alphabet via
  char cipherAlphabet [] parameter.

  The function can assume that the associated cipherAlphabet array
  is sufficently dimensioned to correctly store the cipher alphabet.


*/

void cipherGenerate(const char secretphrase[], char cipherAlphabet[])
{
 
  cipherAlphabet = "ABCDEFGHIJKLMNOPQRSTUVWXYZ";
  char str[20]; //not sure how to get length of cipherAlphabet and secretphrase //together?

  strcat(str, secretphrase);
  strcat(str, cipherAlphabet);
  puts(str);
  cipherAlphabet = str;

  for (int i=0; i < strlen(cipherAlphabet); i++) {
    for(int j=0; j < strlen(cipherAlphabet); j++) {
      if(cipherAlphabet[i]=cipherAlphabet[j])
  }
    }

  // MISSING CODE

} //End of function cipherGenerate

/*
  encryptMessage implicitly returns the encryted message via
  char encryptedMessage [] parameter.

  The function can assume that the associated encryptedMessage array
  is sufficently dimensioned to correctly store the encrypted message.

*/

void encryptMessage(const char secretPhrase[],
                    const char message[],
                    char encryptedMessage[] )
{
  char cipherAlphabet[alphaLen];
  cipherGenerate(secretPhrase, cipherAlphabet);


  // MISSING CODE

} // end of function encryptMessage


/*
  decryptMessage implicitly returns the decryted message via
  char decryptedMessage [] parameter.

  The function can assume that the associated decryptedMessage array
  is sufficently dimensioned to correctly store the decrypted message.

*/

void decryptMessage(const char secretPhrase[],
                    const char encryptedMessage[],
                    char decryptedMessage[] )
{
  char cipherAlphabet[alphaLen];
  cipherGenerate(secretPhrase, cipherAlphabet);

  //MISSING CODE

} // end function decryptMessage


int main(int argc, char * argv[])
{
        char cipherAlphabet[alphaLen+1];

        if ( argc<3 )
        {
                cout << argv[0] << " \"secret phrase\" \"message\"" << endl;
                return 1;
        }

        cipherGenerate(argv[1], cipherAlphabet);
        cipherAlphabet[alphaLen] = '\0';

        cout << "The cipher alphabet is -" << cipherAlphabet << endl;

        char * encryptedMessage = new char[strlen(argv[2])+1];
        char * decryptedMessage = new char[strlen(argv[2])+1];
        encryptMessage(argv[1], argv[2], encryptedMessage );

        cout << "The encrypted message is -" << encryptedMessage<<endl;

        decryptMessage(argv[1], encryptedMessage, decryptedMessage );
        cout << "The decrypted message is -" << decryptedMessage<<endl;

        delete encryptedMessage;
}

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I read the code and I was a little confused. Anyway there were some lines that seemed as error to me:

The line

Code:

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cipherAlphabet = "ABCDEFGHIJKLMNOPQRSTUVWXYZ";

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is wrong. cipherAlphabet is not a modifiable lvalue. You can initialize it in main(), where it is declared or make it local to cipherGenerate().

The line

Code:

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cipherAlphabet = str;

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is wrong. cipherAlphabet is not a modifiable lvalue. Use strcpy().

The line

Code:

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if(cipherAlphabet[i]=cipherAlphabet[j])

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is wrong. Expected == . Although the first is legal it is an assignment, not a comparison.

What is this? What do you expect it to do?

Code:

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for (int i=0; i < strlen(cipherAlphabet); i++) {
    for(int j=0; j < strlen(cipherAlphabet); j++) {
      if(cipherAlphabet[i]=cipherAlphabet[j])
    }
}

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What do you expect the missing codes to do?
What do you expect your program to do?