c++ How to search if struct element in vector is equal? - c++

Question

A class named SparseMatrix has a vector of Nodes struct. I want to overload the += operator so that if the i and j members of a Node instance are the same then the value of that node will be added to This. How can i accomplish this using methods from the algorithm library?
I tried using find_if to pass to a function but it act only on one iterator:
class SparseMatrix
{
public:
SparseMatrix(int numRow,int numCol, std::vector<double> fill);
SparseMatrix(int numRow,int numCol);
SparseMatrix();
// assignment operations
bool operator==(const SparseMatrix &other) const;
bool operator!=(const SparseMatrix &other) const;
void operator-() const;
// compound operations
SparseMatrix& operator+=(const SparseMatrix &other);
SparseMatrix& operator*=(const SparseMatrix &other);
// binary operations
const SparseMatrix operator+(const SparseMatrix &other) const;
const SparseMatrix operator*(const SparseMatrix &other) const;
friend std::ostream& operator<<(std::ostream& output, const SparseMatrix sparseMatrix);
bool trace(double& result) const;
bool det(double& result) const;
SparseMatrix transpose();
~SparseMatrix(){};
protected:
vector<Node> _matrix;
int _numCol, _numRow;
};
typedef struct Node {
int i;
int j;
double value;
static bool samePosition(const Node& other)
{
return ((i == other.i) && (j == other.j));
}
} Node;
SparseMatrix& SparseMatrix::operator+=(const SparseMatrix &other)
{
vector<Node>::iterator itThis;
for (vector<Node>::iterator itOther = other._matrix.begin(); itOther != other._matrix.end(); ++itOther)
{
// find if already exists a value in the same matrix position
itThis = find_if(_matrix.begin(), _matrix.end(), Node::samePosition);
// if exists add value to position, else instantiate new Node with value & position
}
return *this;
}
Basically, I want Node::samePosition() to pass two parameters - the current iterator passed by find_if and itOther so it can check if they are equal.
EDIT: I have separated the samePosition function and now want to pass to it two parameters using find_if:
typedef struct Node {
int i;
int j;
double value;
} Node;
static bool SparseMatrix::samePosition(const Node& first, const Node& other)
{
return ((first.i == other.i) && (first.j == other.j));
}
SparseMatrix& SparseMatrix::operator+=(const SparseMatrix &other)
{
vector<Node>::iterator itThis;
for (vector<Node>::iterator itOther = other._matrix.begin(); itOther != other._matrix.end(); ++itOther)
{
itThis = find_if(_matrix.begin(), _matrix.end(), SparseMatrix::samePosition("call what here?",itOther));
}
return *this;
}

Answer 1

You are trying to use
static bool SparseMatrix::samePosition(const Node& first, const Node& other)
{
return ((first.i == other.i) && (first.j == other.j));
}
which is a standalone function. All its data has to be supplied by the caller, but find_if knows nothing about the Node you want to compare against the entire list.
Instead you should use a functor, which is an object that can hold some data, and also implements operator()() so that it can be called like a function.
struct position_finder
{
const Node needle;
position_finder( const Node& sought ) : needle(sought) {}
bool operator()( const Node& haystack ) const
{
return ((needle.i == haystack.i) && (needle.j == haystack.j));
// or return samePosition(needle, haystack)
}
};
and then you pass the sought Node when constructing the functor, so it gets stored for later use:
itThis = find_if(_matrix.begin(), _matrix.end(), position_finder(*itOther));
C++11 makes this all a whole lot easier, since a lambda will cause the compiler to generate that struct for you:
itThis = find_if(_matrix.begin(), _matrix.end(), [itOther](Node& arg){ return ((itOther->i == arg.i) && (itOther->j == arg.j)); });