Encrypting a string but receiving an infinite loop - c++

Question

Problem:
I was trying to encrypt a std::string password with a single rule:
Add "0" before and after a vowel
So that bAnanASplit becomes b0A0n0a0n0A0Spl0i0t.
However, I got stuck in an infinite loop.
Here is the code:
const std::string VOWELS = "AEIOUaeiou";
std::string pass = "bAnanASplit";
//Add zeroes before and after vowels
for (int i = 0; i < pass.length(); ++i)
{
i = pass.find_first_of(VOWELS, i);
std::cout << pass << "\n";
if(i != std::string::npos)
{
std::cout << pass[i] << ": " << i << "\n";
pass.insert(pass.begin() + i++, '0');
pass.insert(pass.begin() + ++i, '0');
}
}
...And the result:
bAnanASplit
A: 1
b0A0nanASplit
a: 5
b0A0n0a0nASplit
A: 9
b0A0n0a0n0A0Split
i: 15
b0A0n0a0n0A0Spl0i0t
b0A0n0a0n0A0Spl0i0t
A: 2
b00A00n0a0n0A0Spl0i0t
a: 8
b00A00n00a00n0A0Spl0i0t
A: 14
b00A00n00a00n00A00Spl0i0t
i: 22
b00A00n00a00n00A00Spl00i00t
b00A00n00a00n00A00Spl00i00t
...
Any help? This sure seems strange.
Edit: All the answers were useful, and therefore I have accepted the one which I think best answers the question. However, the best way to solve the problem is shown in this answer.

Answer 1

Never, ever, modify the collection/container you are iterating upon!
Saves you a lot of trouble that way.
Let's start with your code and generate a new string with vowels surrounded by 0.
const std::string VOWELS = "AEIOUaeiou";
std::string pass = "bAnanASplit", replacement;
//Add zeroes before and after vowels
for (auto ch : pass)
{
if(VOWELS.find(ch) != std::string::npos)
replacement += '0' + ch + '0';
else
replacement += ch;
}
And there you have it!

Answer 2

As the OP seems to look for the exact reason for the misbehavior, I thought to add another answer as the existing answers do not show the exact issue.
The reason for the unexpected behavior is visible in following lines.
for (int i = 0; i < pass.length(); ++i)
{
i = pass.find_first_of(VOWELS, i);
...
Problem 1:
The loop counter i is an int (i.e. a signed int). But std::string::find_first_of returns std::string::npos if there's no match. This is usually the maximum number representable by an unsigned long. Assigning a huge unsigned value to a shorter signed variable will store a totally unexpected value (assuming you are not aware of that). In this case, i will becomes -1 in most platforms (try int k = std::string::npos; and print k if you need to be sure). i = -1 is valid state for the loop condition i < pass.length(), so the next iteration will be allowed.
Problem 2:
Closely related to the above problem, same variable i is used to define the start position for the find operation. But, as explained, i will not represent the index of the character as you would expect.
Solution:
Storing a malformed value can be solved by using the proper data type. In the current scenario, best options would be using std::string::size_type as this is always guaranteed to work (most probably this will be equal to size_t everywhere). To make the program work with the given logic, you will also have to use a different variable to store the find result.
However, a better solution would be using a std::stringstream for building the string. This will perform better than modifying a string by inserting characters in the middle.
e.g.
#include <iostream>
#include <sstream>
int main() {
using namespace std;
const string VOWELS = "AEIOUaeiou";
const string pass = "bAnanASplit";
stringstream ss;
for (const char pas : pass) {
if (VOWELS.find(pas) == std::string::npos) {
ss << pas;
} else {
ss << '0' << pas << '0';
}
}
cout << pass << "\n";
cout << ss.str() << endl;
}

Answer 3

You are not exiting the loop in case i becomes std::string::npos. So, the i value is changed to some unexpected value (likely something like -1) when it gets to the position of last i or 0 after i(here I am referring to i of split). This is because i is an signed integer but in this case find_first_of() returns std::string::npos which is largest value that can be held by a size_t. In that case the terminating condition i < pass.length() may hold true and the loop continues. So, I am recommending following changes in your code -
for (size_t i = 0; i < pass.length(); ++i)
{
i = pass.find_first_of(VOWELS, i);
if(i == std::string::npos)
break;
pass.insert(pass.begin() + i++, '0');
pass.insert(pass.begin() + ++i, '0');
}
On the same note if (i != std::String::npos) does not do what you are expecting it to do.
But then again it better not to modify the container while you are iterating over it which #Tanveer mentioned in his answer