C++ empty and array index - c++

Question

Is it possible to do something like:
string word = "Hello";
word[3] = null;
if(word[3] == null){/.../}
in C++, basically making an array element empty. For example if I wanted to remove the duplicate characters from the array I'd set them to null first and then shifted the array to the left every time I found an array index that contained null.
If this is not possible what's a good way of doing something like this in C++ ?

Answer 1

If you want to remove adjacent duplicate characters, you can do this:
std::string::iterator new_end = std::unique(word.begin(), word.end());
word.erase(new_end, word.end());
If you want to mark arbitrary characters for removal, you can skip the marking and just provide the appropriate predicate to std::remove_if:
new_end = std::remove_if(word.begin(), word.end(), IsDuplicate);
word.erase(new_end, word.end());
However, I can't think of an appropriate predicate to use here that doesn't exhibit undefined behavior. I would just write my own algorithm:
template<typename IteratorT>
IteratorT RemoveDuplicates(IteratorT first, IteratorT last)
{
typedef typename std::iterator_traits<IteratorT>::value_type
ValueT;
std::map<ValueT, int> counts;
for (auto scan=first; scan!=last; ++scan)
{
++counts[*scan];
if(counts[*scan] == 1)
{
*first = std::move(*scan);
++first;
}
}
return first;
}
Or, if you don't care about the order of the elements, you could simply sort it, then use the first solution.

Answer 2

This is possible, since a single element of a string is an element within a char-array and thus representable as pointer, i. e. you can retrieve the address of the element. Therefore you can set word[3] = null. Your if-construct is valid but the compiler prints a warning, this is because NULL is only a pointer constant. Alternatives would be: if (!word[3]) or if(word[3] == 0).
But in any case you should consider using STL algorithms for removing duplicates.

Answer 3

I think you should take a look at the algorithm in the STL.
You are not very specific about what you want to remove but maybe this helps:
std::string string_with_dup("AABBCCDD");
std::string string_without_dup;
std::cout << string_with_dup << std::endl;
// with copy
std::unique_copy(string_with_dup.begin(), string_with_dup.end(), std::back_inserter(string_without_dup));
std::cout << string_without_dup << std::endl;
// or inplace
string_with_dup.erase(std::unique(string_with_dup.begin(), string_with_dup.end()), string_with_dup.end());
std::cout << string_with_dup << std::endl;

Answer 4

If you want to remove all duplicates (not only the adjacent ones, you should use the erase-remove idiom with something like this
#include <iostream>
#include <map>
#include <string>
#include <algorithm>
using namespace std;
struct is_repeated {
is_repeated( map<char,int>& x ) :r(&x) {};
map<char,int>* r;
bool operator()( char c ) {
(*r)[c]++;
if( (*r)[c] > 1 )
return true;
return false;
}
};
int main (int argc, char**argv)
{
map<char,int> counter_map;
string v = "hello hello hello hello hello hello hello";
cout << v << endl;
is_repeated counter(counter_map);
v.erase( remove_if(v.begin(), v.end(), counter ), v.end() );
cout << v << endl;
}
outputs (as of this):
hello hello hello hello hello hello hello
helo