Remove duplicates in string algorithm

Question

My homework is remove duplicates in a random string. My idea is use 2 loops to solve the problem.

1st one will scan every character in the string. 2nd one will check that character is duplicated or not. If so, remove the character.

string content = "Blah blah..."

    for (int i = 0; i < content.size(); ++i) {
            char check = content.at(i);
            for (int j = i + 1; j < content.size() - 1; ++j) {
                if (check == content.at(j)) {
                    content.erase(content.begin()+j);

                }
            }
        }

The problem is it doesn't work. It always removes the wrong character. Seems an indices problem but I don't understand why.

A temporary fix is change content.erase(content.begin()+j); to content.erase( remove(content.begin() + i+1, content.end(), check),content.end());

But I think trigger a "remove by value" scan isn't a nice way. I want to do it with 2 loops or fewer.

Any ideas will be appreciated :)

Answer 1

Your loops could look the following way

#include <iostream>
#include <string>

int main() 
{
    std::string s = "Blah blah...";

    std::cout << '\"' << s << '\"' << std::endl;

    for ( std::string::size_type i = 0; i < s.size(); i++ )
    {
        std::string::size_type j = i + 1;
        while ( j < s.size() )
        {
            if ( s[i] == s[j] )
            {
                s.erase( j, 1 );
            }
            else
            {
                ++j;
            }
        }
    }

    std::cout << '\"' << s << '\"' << std::endl;

    return 0;
}

The output is

"Blah blah..."
"Blah b."

There are many other approaches using standard algorithms. For example

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

int main() 
{
    std::string s = "Blah blah...";

    std::cout << '\"' << s << '\"' << std::endl;

    auto last = s.end();

    for ( auto first = s.begin(); first != last; ++first )
    {
        last = std::remove( std::next( first ), last, *first );
    }

    s.erase( last, s.end() );

    std::cout << '\"' << s << '\"' << std::endl;

    return 0;
}

The output is the same as for the previous code example

"Blah blah..."
"Blah b."