Question

This is C++. It's based on structured data, chapter 11 of Starting out with C++. Please use basic operations. Please read the requirements carefully and make sure it's executable. The first 2 answers that I received are nowhere close. Thank you.

Secret Codes!

Create a program that will accept a message from the user and either
encrypt ordecrypt it with the following algorithms:

To encrypt:

get the character you wish to encrypt

find the index of that character in the alphabet array

add the shift offset to the index

add the increment to the index

"wrap around" the new index so the result is between 0 and 29

find the character at the new index

To decrypt:

get the character you wish to decrypt

find the index of that character in the alphabet array

subtract the shift offset from the index

subtract the increment from the index

"wrap around" the new index so the result is between 0 and 29

find the character at the new index

repeat steps 1-6 for every shift offset

The alphabet is <space>,a-z(lowercase),<period>,<question mark>,
and <comma> in that order.

The increment should be +3 per letter position.

Here is a sample message. Who said it?

f..yqjrnvrpuabefw.j,ddxgcndpxmsdehsomjkcydfygtd.rrp?dgstmvxmf.wo?jxgrneslzifrgzyqv.v

Answer 1

#include <iostream>
using namespace std;

char c[]= {' ', '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', '.', '?'};
int inc = 3;

string encrypt(string text, int s)
{
string result = "";

// traverse text
for (int i=0;i<text.length();i++)
{
if (islower(text[i]))
result += c[((text[i] - 'a' + 1) + s + inc)%29];
else if (text[i] == ' ')
{
result += c[(0 + s + inc)%29];
}
else if (text[i] == '.')
{
result += c[(27 + s + inc)%29];
}
else if (text[i] == '?')
{
result += c[(28 + s + inc)%29];
}
else
{
result += text[i];
}
}

// Return the resulting string
return result;
}

int wrapIndex(int i, int i_max) {
return ((i % i_max) + i_max) % i_max;
}

string decryptHelper(string text, int s)
{
string result = "";

// traverse text
for (int i=0;i<text.length();i++)
{
if (islower(text[i]))
result += c[wrapIndex((text[i] - 'a' + 1) - s - inc, 29)];
else if (text[i] == ' ')
{
result += c[wrapIndex(0 - s - inc, 29)];
}
else if (text[i] == '.')
{
result += c[wrapIndex(27 - s - inc, 29)];
}
else if (text[i] == '?')
{
result += c[wrapIndex(28 - s - inc, 29)];
}
else
{
result += text[i];
}
}

// Return the resulting string
return result;
}

void decrypt(string text)
{
for (int i = 0; i < 28; i++)
{
cout << "Shift s: " << i << " decrypted string: " << decryptHelper(text, i) << endl;
}
}

// Driver program to test the above function
int main()
{
string text="attackatonce";
int s = 4;
cout << "Text : " << text;
cout << "\nShift: " << s;
string cipher = encrypt(text, s);
cout << "\nCipher: " << cipher;

cout << "Possible decryption: " << endl;
decrypt(cipher);

return 0;
}

Output: