Remove duplicate vectors from vector of vectors - c++

Question

I am trying to implement the solution to the problem found at Link.
Here is my snippet of code
bool compareVec(vector<int> a, vector<int> b) {
return std::equal(a.begin(), a.end(), b.begin());
}
vector<vector<int> > ans;
ans.erase(std::remove_if(ans.begin(), ans.end(), compareVec), ans.end());
I am getting the following errors
/usr/include/c++/4.8/bits/stl_algo.h: In instantiation of
'_RandomAccessIterator std::__find_if(_RandomAccessIterator,
_RandomAccessIterator, _Predicate, std::random_access_iterator_tag) [with
_RandomAccessIterator = __gnu_cxx::__normal_iterator<std::vector<int>*,
std::vector<std::vector<int> > >; _Predicate = bool (*)(std::vector<int>,
std::vector<int>)]':
/usr/include/c++/4.8/bits/stl_algo.h:4465:41: required from '_IIter
std::find_if(_IIter, _IIter, _Predicate) [with _IIter =
__gnu_cxx::__normal_iterator<std::vector<int>*, std::vector<std::vector<int>
> >; _Predicate = bool (*)(std::vector<int>, std::vector<int>)]'
/usr/include/c++/4.8/bits/stl_algo.h:1144:64: required from '_FIter
std::remove_if(_FIter, _FIter, _Predicate) [with _FIter =
__gnu_cxx::__normal_iterator<std::vector<int>*, std::vector<std::vector<int>
> >; _Predicate = bool (*)(std::vector<int>, std::vector<int>)]'
solution.cpp:40:64: required from here
/usr/include/c++/4.8/bits/stl_algo.h:214:23: error: too few arguments to
function
if (__pred(*__first))
^
/usr/include/c++/4.8/bits/stl_algo.h:218:23: error: too few arguments to
function
if (__pred(*__first))
^
/usr/include/c++/4.8/bits/stl_algo.h:222:23: error: too few arguments to
function
if (__pred(*__first))
^
Can anyone help me out in debugging this?
Thanks in advance
EDIT
The elements of vector are sorted and all these vectors are also sorted.
Unique also gives an error. I am unable to figure out why?
Why is the example given in the link I provided, not helpful here?

Answer 1

std::remove_if requires a unary predicate. You pass a binary predicate, which causes your errors (/usr/include/c++/4.8/bits/stl_algo.h:222:23: error: too few arguments to function → your function takes two arguments, not one).
Further, std::remove_if does its removals with no consideration of other elements (which is why it accepts a unary predicate), so it isn't actually what you're looking for.
What you want to use is std::unique, which does require the compareVec you've implemented. However, std::vector already provides the operator== overload, so that implementation is actually redundant! Also, you say that you get an error when using std::unique. Try passing your parameters as const&.
Thus, when your outer vector and inner vectors are already sorted, the solution is as it'd be for any other vector of sorted elements:
outer.erase(std::unique(outer.begin(), outer.end()), outer.end());

Answer 2

Okay, since this is not marked with C++11, I will use a functor instead of a lambda.
The first problem you have is that remove_if takes a UnaryPredicate, which means it should only accept a single argument.
The second issue is also related to your understanding of remove_if. After you fix compareVec to only accept one argument, you're left wondering how you could possibly compare all elements against each other.
You could approach this one of two ways:
Sort your vector of vectors (< operator is defined lexicographically for vector) and then use std::unique (Examples) (More examples).
In the link you provided (same as the one I just linked to), notice that they sort first, and you do not.
Or, if there's no clear definition of < for your elements, only ==, you could perform an O(N2) lookup/erase on each subsequent item (shown below):
Comparison functor (could make as a lambda in C++11 and greater)
struct CompareVec
{
CompareVec(const std::vector<int>& _in) : compare_against(_in){}
bool operator()(const std::vector<int>& rhs) const
{
return compare_against == rhs;
};
const std::vector<int>& compare_against;
};
To be used like so:
for (size_t i = 0; i < ans.size(); ++i)
{
CompareVec comparator(ans[i]);
ans.erase(std::remove_if(ans.begin()+i+1, ans.end(), comparator), ans.end());
}
Live Demo (Compiled in C++11 for want of initializing test vectors with initializer lists)
Edit
In C++11 we can get rid of the CompareVec functor and replace it with a lambda:
for (size_t i = 0; i < ans.size(); ++i)
{
ans.erase(std::remove_if(ans.begin()+i+1, ans.end(),
[&ans, &i](const std::vector<int>& _rhs)
{
return ans[i] == _rhs;
}) , ans.end());
}
Demo2