noreturn noreturn - 1 year ago 98
C++ Question

Caesar Cipher deleting Data

I am wondering if someone can help me trouble shoot my code. It seems to be working all properly but when I go to run it that halfway through it starts to delete my data even though I never call anything to delete it.

So for example my file goes from:

=======================
Rotated by 11 positions
=======================
Lnwypeyw lala sqz xa na lnwypeyw eb pdau sqz pwa w eppa
na pea bn znawec.
-- F. L. IyAru**
to
=======================
Rotated by 12 positions
=======================
Moxzqzx mbmb tra yb ob moxzqzx c qebv tra qxb x qqb
ob qb co aobxd.
-- G. M. JzBsv


Here is my code, I have tried going through it a few times and it all makes sense logically and have no idea while it is losing data.

#include <iostream>
#include <cctype>
#include <fstream>

using namespace std;

bool isUpper(char character){

//this will test and see if it's upper or lowercase
bool upper;

if(character>= 'A' && character <= 'Z'){
upper=true;
}
else {
upper= false;
}

return upper;
}

bool isLower(char character){

//this will test and see if it's upper or lowercase
bool lower;

if(character >= 'a' && character <= 'z'){
lower=true;
}

else {
lower= false;
}

return lower;
}

//Actual function that will rotate the character
char rotate(char character, int offset) {

char next_letter;

//Changes it if it's a lower case
if (isLower(character)) {

next_letter = character + offset;

if (next_letter > 'z'){
next_letter = (next_letter - 26);
return next_letter;
}

return next_letter;
}

else if(isUpper(character)) {

next_letter = character + offset;

if (next_letter > 'Z'){
next_letter = (next_letter - 26);
return next_letter;
}

return next_letter;
}

else {
return character;
}
}


int main() {

//variables for program
char character = 'a';
int offset = 0;

while(offset < 26){

//opens the file
ifstream fin;
fin.open("secretMessage.txt");

if(!fin.good()) {
cout << "Please check your file name!!!";
return 0;
}

//report for reading
cout << "=======================" << endl
<< "Rotated by " << offset << " positions" << endl
<< "=======================" << endl;

//Reads until it's at the end of the file
while ((! fin.eof())){
character = fin.get();

cout << rotate(character,offset);

}

//makes it run 26 times
++offset;
fin.close();
cout << endl << endl;
}
//Closes the file output.
return 0;
}

Answer Source

In this logic:

    if (next_letter > 'z') {
        next_letter = (next_letter - 26);
        return next_letter;
    }

you didn't think of what happens if char is signed. Test it:

assert(rotate('z', 0) == 'z');
assert(rotate('z', 1) == 'a');
assert(rotate('z', 2) == 'b');
assert(rotate('z', 3) == 'c');
assert(rotate('z', 4) == 'd');
assert(rotate('z', 5) == 'e');

Fine so far, but now:

assert(rotate('z', 6) == 'f'); // WHOOPS

Ascii 'z'+5 is 127, 'z'+6 is 128. But, z+=6 results in it being negative, so the check > 'z' fails.

Suggested simplifications:

bool isUpper(char ch) { return (ch >= 'A' && ch <= 'Z'); }
bool isLower(char ch) { return (ch >= 'a' && ch <= 'z'); }

// Actual function that will rotate the character
char rotate(char ch, int offset) {
    if (isLower(ch)) return (ch - 'a' + offset) % 26 + 'a';
    if (isUpper(ch)) return (ch - 'A' + offset) % 26 + 'A';
    return ch;
}

Since you include <cctype>, you might as well use std::islower and std::isupper.

Live On Coliru

#include <cctype>
#include <fstream>
#include <iostream>
#include <algorithm>

// Actual function that will rotate the character
char rotate(char ch, int offset) {
    if (std::islower(ch)) return (ch - 'a' + offset) % 26 + 'a';
    if (std::isupper(ch)) return (ch - 'A' + offset) % 26 + 'A';
    return ch;
}

std::string readFile(std::string const& fname) {
    std::ifstream fin;
    fin.exceptions(std::ios::failbit);
    fin.open(fname);
    return { std::istreambuf_iterator<char>(fin), {} };
}

int main() {

    auto const original = readFile("main.cpp");

    // makes it run 26 times
    for (int offset = 0; offset < 26; ++offset) {

        std::cout 
            << "\n"
            << "=======================\n"
            << "Rotated by " << offset << " positions\n"
            << "=======================\n";

        // Reads until it's at the end of the file
        std::transform(
                original.begin(), original.end(),
                std::ostreambuf_iterator<char>(std::cout),
                [offset](char ch) { return rotate(ch, offset); });
    }
}

BONUS

Actually, you can do without the complexity in main by doing in-place rotate-by-1 26 times:

Live On Coliru

auto text = readFile("main.cpp");

for (int offset = 0; offset < 26; ++offset) {
    std::cout 
        << "\n"
        << "=======================\n"
        << "Rotated by " << offset << " positions\n"
        << "=======================\n";

    std::cout << text;
    for (auto& ch : text) ch = rotate(ch, 1);
}
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