Removing elements of 2D vector - c++

Question

I have a 2D vector and I need to remove blocks that do not contain sufficient values:
typedef vector<double> iniMatrix;
bool hasInsufficientEnergy() {
return true;
}
vector<double> Audio::filter(vector<iniMatrix>&blocks, double sumThres, double ZeroThres)
{
vector<double> totalEnergy;
vector<double> totalZeroCross;
double totalSum = sumThres * blocks.size();
double totalZero = ZeroThres * blocks.size();
vector<iniMatrix> blockked;
for(unsigned i=0; (i < blocks.size()); i++)
{
totalEnergy.push_back(abs(this->energy(blocks[i])));
totalZeroCross.push_back(zerocross(blocks[i]));
if(!totalEnergy[i] > totalSum || totalZeroCross[i] < ZeroThres)
{
hasInsufficientEnergy();
}else{
//hasInsufficientEnergy();
}
iniMatrix::iterator erase_after = remove_if(blocks[i].begin(), blocks[i].end(),
&hasInsufficientEnergy);
}
}
The problem is erase_after and comes up with an error message:
In function ‘_OutputIterator std::remove_copy_if(_InputIterator, _InputIterator,
_OutputIterator, _Predicate) [with _InputIterator = __gnu_cxx::__normal_iterator<double*,
std::vector<double, std::allocator<double> > >, _OutputIterator =
__gnu_cxx::__normal_iterator<double*, std::vector<double, std::allocator<double> > >,
_Predicate = bool]’:
/usr/include/c++/4.2.1/bits/stl_algo.h:1302: instantiated from ‘_ForwardIterator
std::remove_if(_ForwardIterator, _ForwardIterator, _Predicate) [with _ForwardIterator =
__gnu_cxx::__normal_iterator<double*, std::vector<double, std::allocator<double> > >,
_Predicate = bool]’
Audio.cpp:105: instantiated from here
/usr/include/c++/4.2.1/bits/stl_algo.h:1227: error: ‘__pred’ cannot be used as a function
Anyone have any ideas to where I am going wrong?

Answer 1

The third argument to std::remove_if is a predicate function (or any callable entity, for that matter) that takes a single element and returns true if it needs to be removed and false if not. The element of a vector of vectors (or "2D vector", as you call it) is a vector, vector<double> in your case. That's what the argument to your predicate should be:
bool HasInsufficientEnergy(const vector<double>& elem)
{
// herein is the code that tests elem and decides
// whether it should be removed or not
}
Then your filter() method shouldn't contain much more than this
void Audio::filter(vector<iniMatrix>&blocks)
{
auto it = std::remove_if(blocks.begin(), blocks.end(), &HasInsufficientEnergy);
blocks.erase(it, blocks.end());
}
If your predicate needs aditional arguments, then implement it as a class that takes them as constructor parameters. You need to overload operator(), too, so you have a callable functor.
If your compiler supports C++11, then learn how to use lambda functions as they are very useful for exactly this task.

Answer 2

true or false aren't function objects. If you want to use remove_if you must give it a pointer to function or function object, which takes a member of the collection as parameter and returns whether to remove it or not. Look at the example in std::remove_if
AFAICS, you could replace the remove_if/erase into the if and simplify it to:
for(auto i=blocks.begin(); i != blocks.end(); )
{
totalEnergy.push_back(abs(this->energy(*i)));
totalZeroCross.push_back(zerocross(*i));
if(!totalEnergy.rbegin() > totalSum || totalZeroCross.rbegin() < ZeroThres)
{
i = blocks.erase(i);
} else {
++i;
}
}

Answer 3

remove_if, and similar algorithms, expect a function, not a value, i.e. you should pass a function pointer or functor that an double const & and returns a bool
bool hasInsufficientEnergy(double const & element) {
// return true if it should be removed, false otherwise
}
and then
iniMatrix::iterator erase_after = remove_if(blocks[i].begin(), blocks[i].end(),
&hasInsufficientEnergy);
will work