replace() not changing characters in a string to the intended characters they are supposed to be replaced with

user_16177991 0
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I am creating a program in C++ that encrypts text using the Caesar Cipher it allows the user to pick the offset that is used to encrypt at the moment i have on written it for offset 1 but when i use replace()function as part of the STL, rather than replacing them with the specified characters they should be replaced to it replaces them all with the same letter

#include <iostream>
#include <string>
#include <algorithm>

using namespace std;

int main()
{
    int Offset;
    string Message;

    cout << "What Would You Like To Offset By" << endl;
    cin >> Offset;

    cout << "Please Enter The Text You Would Like To Encrypt" << endl;
    cin >> Message;

    switch(Offset)
    {
        case 1:
        {
            replace(Message.begin(), Message.end(), 'a', 'b');
            replace(Message.begin(), Message.end(), 'A', 'B');
            replace(Message.begin(), Message.end(), 'b', 'c');
            replace(Message.begin(), Message.end(), 'B', 'C');
            replace(Message.begin(), Message.end(), 'c', 'd');
            replace(Message.begin(), Message.end(), 'C', 'D'); 
            replace(Message.begin(), Message.end(), 'd', 'e');
            replace(Message.begin(), Message.end(), 'D', 'E');
            replace(Message.begin(), Message.end(), 'e', 'f');
            replace(Message.begin(), Message.end(), 'E', 'F');
            replace(Message.begin(), Message.end(), 'f', 'g');
            replace(Message.begin(), Message.end(), 'F', 'G');
            replace(Message.begin(), Message.end(), 'g', 'h');
            replace(Message.begin(), Message.end(), 'G', 'H');
            replace(Message.begin(), Message.end(), 'h', 'i');
            replace(Message.begin(), Message.end(), 'H', 'I');
            replace(Message.begin(), Message.end(), 'i', 'j');
            replace(Message.begin(), Message.end(), 'I', 'J');
            replace(Message.begin(), Message.end(), 'j', 'k');
            replace(Message.begin(), Message.end(), 'J', 'K');
            replace(Message.begin(), Message.end(), 'k', 'l');
            replace(Message.begin(), Message.end(), 'K', 'L');
            replace(Message.begin(), Message.end(), 'l', 'm');
            replace(Message.begin(), Message.end(), 'L', 'M');
            replace(Message.begin(), Message.end(), 'm', 'n');
            replace(Message.begin(), Message.end(), 'M', 'N');
            replace(Message.begin(), Message.end(), 'n', 'o');
            replace(Message.begin(), Message.end(), 'N', 'O');
            replace(Message.begin(), Message.end(), 'o', 'p');
            replace(Message.begin(), Message.end(), 'O', 'P');
            replace(Message.begin(), Message.end(), 'p', 'q');
            replace(Message.begin(), Message.end(), 'P', 'Q');
            replace(Message.begin(), Message.end(), 'q', 'r');
            replace(Message.begin(), Message.end(), 'Q', 'R');
            replace(Message.begin(), Message.end(), 'r', 's');
            replace(Message.begin(), Message.end(), 'R', 'S');
            replace(Message.begin(), Message.end(), 's', 't');
            replace(Message.begin(), Message.end(), 'S', 'T');
            replace(Message.begin(), Message.end(), 't', 'u');
            replace(Message.begin(), Message.end(), 'T', 'U');
            replace(Message.begin(), Message.end(), 'u', 'v');
            replace(Message.begin(), Message.end(), 'U', 'V');
            replace(Message.begin(), Message.end(), 'v', 'w');
            replace(Message.begin(), Message.end(), 'V', 'W');
            replace(Message.begin(), Message.end(), 'w', 'x');
            replace(Message.begin(), Message.end(), 'W', 'X');
            replace(Message.begin(), Message.end(), 'x', 'y');
            replace(Message.begin(), Message.end(), 'X', 'Y');
            replace(Message.begin(), Message.end(), 'y', 'z');
            replace(Message.begin(), Message.end(), 'Y', 'Z');
            replace(Message.begin(), Message.end(), 'z', 'a');
            replace(Message.begin(), Message.end(), 'Z', 'A');

            cout << Message << endl;
            break;
        }
    }
}
c++
2 Answers

The Golden Rule Of Computer Programming states: "Your computer always does exactly what you tell it to do, instead of what you want it to do".

Now let's explore what you told your computer to do.

replace(Message.begin(), Message.end(), 'a', 'b');

You told your computer to replace every occurrence of the letter 'a' with the letter 'b'. Your computer will do exactly that. Two statements later:

replace(Message.begin(), Message.end(), 'b', 'c');

Here, you told your computer to replace every occurrence of the letter 'b' with the letter 'c'. Your computer will do exactly that. This includes both letter 'b' that were in the original text, as well as all letters that were originally 'a' but are now 'b'. That's because, earlier, you told your computer to change all a-s to b-s, and now you have a bunch of bs, and now all of them are indistinguishable from the other.

In this manner, if you work out, on paper, what is the result of everything you told your computer to do, it becomes obvious why the resulting string always winds up with the same letter (two letters, actually, both uppercase and lowercase).

Your obvious goal here is to replace each letter by the next one, rotated. The correct approach will be fundamentally different, but this explains why the text gets replaced "all with the same letter".

user_39433120

your text made me curious, so I tried another approach:

Taking the cin as string; Converting chars to (ASCII + offset) % 128 and then cout as chars again. If you're interested, i can provide the code, but don't want to take you the chance, to solve it yourself, (in one of the countless possible ways) :)

user_165530220
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