results in stopped working when running reverse string program - c++

Question

I wrote the function to reverse a string in c++ but it results in "stopped working".
#include<iostream>
#include<string.h>
using namespace std;
string reverse(string s1)
{
string s2;
for(int i=0;i<s1.length();i++)s2[i]=s1[s1.length()-i-1];
return s2;
}
int main()
{
string s1,s2;
cin>>s1;
s2=reverse(s1);
cout<<s2;
}
What can be the problem?

Answer 1

The problem is that you accessing string s2 outside its boundaries. You are only allowed to access characters with [] that already exist; attempting to write outside the string causes undefined behaviour.
One possible solution is to pre-allocate s2:
string s2 = s1;
Another option is to count down over s1, and then simply add new characters to the end of s2.

Answer 2

The function declaration has a drawback. First of all in fact it does not reverse a string. It makes a copy of a string in the reverse order.
Also it is more efficient to declare the parameter as constant reference.
For example
std::string reverse_copy( const std::string &s );
Within the function you are using the subscript operator applied to an ampty string
string s2; // the string is empty
for(int i=0;i<s1.length();i++)s2[i]=s1[s1.length()-i-1];
^^^^^
that results in undefined behaviour.
Also instead of the index of type int it is better to use an index of type std::string::size_type.
The function can be written without any explicit loop. For example
std::string reverse_copy( const std::string &s );
{
return std::string( s.rbegin(), s.rend() );
}
If you want to use a loop then the function can look like
std::string reverse_copy( const std::string &s );
{
std::string t;
t.reserve( s.size() );
for ( auto i = s.size(); i != 0; --i ) t.push_back( s[i-1] );
return t;
}
Instead of the statement
t.push_back( s[i-1] );
you can write also
t += s[i-1];
For example
std::string reverse_copy( const std::string &s );
{
std::string t;
t.reserve( s.size() );
for ( auto i = s.size(); i != 0; --i ) t += s[i-1];
return t;
}

Answer 3

you can't index into a string when it's not been formed as yet.
Correct your code as follows and append to s2 rather than index into it
string reverse(string s1)
{
string s2;
for (int i = 0; i<s1.length(); i++)
s2 += s1[s1.length() - i - 1];
return s2;
}

Answer 4

If you're not using standard algorithms, the problem is almost always that you're not using standard algorithms.
#include <iostream>
#include <string>
#include <algorithm>
#include <iterator>
std::string reverse(const std::string& s)
{
std::string result;
result.reserve(s.size());
std::copy(s.rbegin(), s.rend(), std::back_inserter(result));
return result;
}
int main()
{
auto s = std::string("Hello, World");
auto s2 = reverse(s);
std::cout << s << std::endl;
std::cout << s2 << std::endl;
return 0;
}
expected result:
Hello, World
dlroW ,olleH

Answer 5

s2 does not have a well-defined length: you are assuming it has at least the same length as s1. Specifically, the behaviour of s2[i] is undefined.
Consider the C++ standard library-based solution
std::string s2(s1);
std::reverse(s2.begin(), s2.end());
Where conceptually, I'm considering the string as a container of chars.Using standard functions means that someone viewing your code knows exactly what you're doing.