打印出未交换的阵列

Question

I'm learning how to make myself a simple XOR encryption. However, to make things more difficult for a person to decipher the message I want to try to swap characters and then swap them back (after the user enters the key for the decryption).

This is my output:

yet ih ssia t se!t !

ey this is a test!!

Does anyone know why it is cutting off the h in the second printout? I'm still fairly new to programming and spent a good hour trying to figure it out.

Here is my code:

#include <cstring>
#include <string>
#include <iostream>
using namespace std;

void doSwap (char &string1, char &string2) {
    char temp;
    temp = string1;
    string1 = string2;
    string2 = temp;
}


int main() {

    string content = "hey this is a test!!";
    string after, after2;
    int i;

        for (i = 0; i < content.size(); i+=2) {
            doSwap(content[i], content[i+2]);
        }
    after = content;

cout << after << "\n";

    for (i = after.size(); i > 0; i -=2) {
        doSwap(after[i], after[i-2]);
    }
    after2 = after;

cout << after2 << "\n";

 }

Answer 1

Your loop indexing is not correct. As @tadman pointed out, you will walk off the string.

The first loop would have to terminate 2 indices shorter than the size, so that when you add 2, it would be 1 less than content.size(). Remember C/C++ is 0 indexed, therefore if size is 10, element 9 is the last index of the string.

for (i = 0; i < content.size()-2; i+=2) {
            doSwap(content[i], content[i+2]);
    }

Similarly the second loop should start 1 short of the size and terminate at 2, so when you index into i-2 , you are indexing no less than 0.

 for (i = after.size()-1; i >= 2; i -=2) {
        doSwap(after[i], after[i-2]);
    }