throwing an instance with no reason - c++

Question

I have a hard time to debbug this program.
The purpose of the program is to covert a string in a following way.
aaabccddd → abccddd → abddd → abd
Delete the two adjacent element, if they are the same alphabet.
Keep doing this several times, until got a result that cant do it anymore.
If the result dont have any alphabet print out Empty String.
I face a serious problem, my program throwing an instant of 'std::length_error'.
And i check my code. I cant problem in it. Can someone please tell me what happened,
and how to fix it.
This is code:
#include <iostream>
#include <string>
using namespace std;
int main(){
string data;
cin >> data;
for(int i = 0; i < data.size() - 1; i++){
if(data[i] == data[i+1]){
data.erase(data.begin()+i, data.begin()+i+2);
i = -1;
}
}
if(data.size() == 0){
cout << "Empty String";
}else{
cout << data;
}
return 0;
}
There are some imformation about the program.
This is input:
aa
This is execution result:
aa
terminate called after throwing an instance of 'std::length_error'
what(): basic_string::_S_create
--------------------------------
Process exited after 94.29 seconds with return value 3
but sometimes the program can run successfully and
This is input:
aaabccddd
This is execution result:
aaabccddd
abd
--------------------------------
Process exited after 11.27 seconds with return value 0
I dont know what happened. Can someone give me some advice?

Answer 1

When you want to iterate over a container and want to erase something inside the loop, use the following pattern:
for (auto i = container.begin(); i != container.end();) {
if (...some condition...) {
// erase:
i = container.erase(i);
} else {
// keep:
++i;
}
}
The erase() function will return a valid iterator to the item after the range you just deleted.
You do need to use proper iterators for this to work. So in your case, your code should look like:
#include <iostream>
#include <string>
using namespace std;
int main() {
string data;
cin >> data;
for (auto i = data.begin(); i < data.end() - 1;) {
if (i[0] == i[1]) {
i = data.erase(i, i + 2);
} else {
++i;
}
}
if (data.empty()) {
cout << "Empty String\n";
} else {
cout << data << "\n";
}
return 0;
}