Getting Input Key for Atbash Cipher

Question

An Atbash cipher is a simple substitution cipher wherein a key is used to determine what letters to swap, for example:

Original alphabet: abcdefghijklmnopqrstuvwxyz
Substitute (key):  zyxwvutsrqponmlkjihgfedcba
Original message:  hello world
Altered message:   svool dliow

In my program, I get the key by printing the original letter, and then getting the letter that is to substitute it, and adding it to a key string, for example:

a = <user input>
b = <user input>
c = <user input>
...

Here is the simple code that I use for it:

std::cout << "Key:\n";
std::string key;

for(int i = 0; i <= 25; i++){
    std::cout << static_cast<char>('a' + i) << " = ";
    char inputChar;

    std::cin >> inputChar;
    key += inputChar;

    std::cin.ignore();
    std::cin.clear();
    std::cout << std::flush;
}

How can I improve this code?

Answer 1

How can I improve this code?

Fix a bug ;). At the moment, one can accidentally enter the same character twice, e.g.:

a = a
b = a
c = a
...
z = a

Unless you want to allow that usage, you probably want to check which characters are already used:

for(int i = 0; i <= 25; i++){
    std::cout << static_cast<char>('a' + i) << " = ";
    char inputChar;

    std::cin >> inputChar;

    if(key.find(inputChar) == std::string::npos){
        key += inputChar;
    } else {
        // yell at user!
    }

    std::cin.ignore();
    std::cin.clear();
    std::cout << std::flush;
}

Also, if you know the size of the std::string in advance, you can speed up append operations (such as +=) with reserve:

std::string key;

key.reserve(26);

// ... rest of your code

Answer 2

For one thing, it is very inadvisable to use the += operator with strings, because it's slow and it requires the string to be relocated in the memory. This isn't much of a problem with higher-level languages like Python, but with C and also Java, it's not a good idea, and it's not good practice. Since you know that there are 26 letters in the alphabet, you can make a length 26 array to store the key instead. Otherwise, the rest of the code looks good! :)