I'm a complete newbie at C++. I want to create my own predicate. But the part with bool operator seems to be wrong (at least in my humble opinion). Could someone give me a hint? I don't want to change the overall structure of this idea, I'm just sure I don't understand some details about operator () implementation or something related to classes in c++.
#include <iostream>
#include <vector>
class Predicate
{
private:
int number = 0;
public:
Predicate() = default;
Predicate(const int number)
{
this->number = number;
}
bool operator()(int value) const
{
Predicate *pred = new Predicate();
bool result = pred->operator()(value);
return result;
}
};
class Even : public Predicate
{
bool operator()(int value) const
{
return value % 2 == 0;
}
};
class Negative : public Predicate
{
bool operator()(int value) const
{
return value < 0;
}
};
int count(const std::vector<int> &elements, const Predicate &predicate)
{
int count = 0;
for (int index = 0; index < elements.size(); ++index)
{
if (predicate(elements[index]))
{
++count;
}
}
return count;
}
int main()
{
const std::vector<int> elements{-7, 12, -11, 2, 9, -4, -6, 5, 23, -1};
std::cout << count(elements, Even()) << " " << count(elements, Negative()) << std::endl;
}
What you need is:
define Predicate as an abstract type,
implements different versions of it.
Predicate as an abstract type:
class Predicate {
public:
virtual bool operator(int v) const = 0;
};
Implementing (realising) a given Predicate:
class IsNegative : public Predicate { // means IsNegatives are Predicates
public:
virtual bool operator(int v) const { return v<0; } // realisation of the operator
};
Related
class
{
public:
void func(const int val, const bool flag)
{
if(flag)
{
while(!lower.empty() && val <= lower.top())
{
// do a bunch of stuff with lower
}
}
else
{
while(!higher.empty() && val >= higher.top())
{
// do a bunch of stuff with higher, but it's the same stuff as would've done
// for lower
}
}
}
private:
std::stack<int> lower;
std::stack<int> higher;
}
I'm trying to figure out a better way to write the clauses because currently, I have a lot of duplicate code in both. The only difference is one clause operates on lower and the other on higher and the <= in the first clause is changed to >= higher in the second one.
I could wrap the clause in a helper function and call it in each clause (and pass in the lower and higher as an argument), e.g.,
class
{
public:
void func(const int val, const bool flag)
{
if(flag)
{
helper(lower, comparer);
}
else
{
helper(lower, comparer);
}
}
void helper(std::stack<int> &st)
{
// do a bunch of stuff with st
}
private:
std::stack<int> lower;
std::stack<int> higher;
}
I'm not sure if this is a good idea and if it is, I'm not sure how to get around the >= vs. <=. I'm hoping for suggestions on my design!
You can do something like the following:
class
{
public:
void func(const int val, const bool flag)
{
std::stack<int> *st;
bool (*compare)(int, int);
if (flag)
{
st = &lower;
compare = [](int a, int b){ return a <= b; };
}
else
{
st = &higher;
compare = [](int a, int b){ return a >= b; };
}
while (!st->empty() && compare(val, st->top()))
{
// do a bunch of stuff with *st
}
}
private:
std::stack<int> lower;
std::stack<int> higher;
}
Alternatively, using a helper would certainly work, too:
class
{
public:
void func(const int val, const bool flag)
{
if (flag)
func_helper(lower, val, std::less_equal{});
else
func_helper(higher, val, std::greater_equal{});
}
private:
std::stack<int> lower;
std::stack<int> higher;
template<typename Comparer>
void func_helper(stack<int> &st, const int val, Comparer compare)
{
while (!st.empty() && compare(val, st.top()))
{
// do a bunch of stuff with st
}
}
}
How about something like this
class
{
public:
void func(const int val, const bool flag)
{
int sign = 1;
std::stack<int>* higher_or_lower = &higher;
if(flag)
{
higher_or_lower = &lower;
sign = -1;
}
while(!higher_or_lower->empty() && sign*val >= sign*higher_or_lower->top())
{
// do a bunch of stuff with higher_or_lower
}
}
private:
std::stack<int> lower;
std::stack<int> higher;
}
The higher_or_lower covers both stacks and the sign takes care of less than vs. greater than.
Or a bit more compact:
class C
{
public:
void func(const int val, const bool flag)
{
const std::stack<int>* st[] = {&lower, &higher};
bool (*compare[])(int, int) = { [](int a, int b) { return a <= b; } , [](int a, int b) { return a >= b; } };
while (!st[flag]->empty() && compare[flag](val, st[flag]->top()))
{
// do a bunch of stuff with *st
}
}
private:
std::stack<int> lower;
std::stack<int> higher;
};
I'm attempting to get a basic constant forward-iterator to work in C++.
namespace Rcpp {
class SparseMatrix {
public:
IntegerVector i, p;
NumericVector x;
int begin_col(int j) { return p[j]; };
int end_col(int j) { return p[j + 1]; };
class iterator {
public:
int index;
iterator(SparseMatrix& g) : parent(g) {}
iterator(int ind) { index = ind; }; // ERROR!
bool operator!=(int x) const { return index != x; };
iterator operator++(int) { ++index; return (*this); };
int row() { return parent.i[index]; };
double value() { return parent.x[index]; };
private:
SparseMatrix& parent;
};
};
}
My intention is to use the iterator in contexts similar to the following:
// sum of values in column 7
Rcpp::SparseMatrix A(nrow, ncol, fill::random);
double sum = 0;
for(Rcpp::SparseMatrix::iterator it = A.begin_col(7); it != A.end_col(7); it++)
sum += it.value();
Two questions:
The compiler throws an error on the line indicated above: uninitialized reference member in 'class Rcpp::SparseMatrix&' [-fpermissive]. How can this be fixed?
How might double value() { return parent.x[index]; }; be re-worked to return a pointer to the value rather than a copy of the value?
A little context on the SparseMatrix class: like a dgCMatrix in R, this object of class SparseMatrix consists of three vectors:
i holds row pointers for every element in x
p gives indices in i which correspond to the start of each column
x contains non-zero values
Thanks to #Evg, here's the solution:
namespace Rcpp {
class SparseMatrix {
public:
IntegerVector i, p;
NumericVector x;
class iterator {
public:
int index;
iterator(SparseMatrix& g, int ind) : parent(g) { index = ind; }
bool operator!=(iterator x) const { return index != x.index; };
iterator& operator++() { ++index; return (*this); };
int row() { return parent.i[index]; };
double& value() { return parent.x[index]; };
private:
SparseMatrix& parent;
};
iterator begin_col(int j) { return iterator(*this, p[j]); };
iterator end_col(int j) { return iterator(*this, p[j + 1]); };
};
}
And it can be used as follows, for instance, to calculate colSums:
//[[Rcpp::export]]
Rcpp::NumericVector Rcpp_colSums(Rcpp::SparseMatrix& A) {
Rcpp::NumericVector sums(A.cols());
for (int i = 0; i < A.cols(); ++i)
for (Rcpp::SparseMatrix::iterator it = A.begin_col(i); it != A.end_col(i); it++)
sums(i) += it.value();
return sums;
}
And, the above function is faster than RcppArmadillo, RcppEigen, and R::Matrix equivalents when microbenchmarked from R!
Edit:
The above syntax is inspired by Armadillo. I've come to realize that a slightly different syntax (which involves fewer constructions) gives an iterator similar to Eigen:
class col_iterator {
public:
col_iterator(SparseMatrix& ptr, int col) : ptr(ptr) { indx = ptr.p[col]; max_index = ptr.p[col + 1]; }
operator bool() const { return (indx != max_index); }
col_iterator& operator++() { ++indx; return *this; }
const double& value() const { return ptr.x[indx]; }
int row() const { return ptr.i[indx]; }
private:
SparseMatrix& ptr;
int indx, max_index;
};
Which can then be used like this:
int col = 0;
for (Rcpp::SparseMatrix::col_iterator it(A, col); it; ++it)
Rprintf("row: %3d, value: %10.2e", it.row(), it.value());
I have a assignment where I'm suppose to build template using these specifications.
ISet is a container that holds values of a certain where order doesn't matter and
which does not allow duplicates (or multiples).
A dynamically allocated array of type T should be used as an internal data structure for the Set.
The Set should inherit from the ISet interface below - this must not be modified:
template <typename T>
class ISet
{
public:
virtual bool insert (T element) = 0;
virtual bool remove (T element) = 0;
virtual int size () const = 0;
};
• insert (T element): adds elements to the set and returns true provided that
the element is not already present in the quantity (in which case the element is not added and false is returned).
• remove (T element): removes elements from the set and returns true.
If the element is missing in the quantity, false returns.
• size (): returns the number of elements in the set.
In addition to the member functions, you must implement constructor, destructor, copy constructor
and assignment operator.
And so far have I come up with this code:
#pragma once
#include <string>
#include <iostream>
using namespace std;
template <class T>
class ISet
{
public:
virtual bool insert(T element) = 0;
virtual bool remove(T element) = 0;
virtual int size() const = 0;
};
#pragma once
#include "ISet.h"
template <class T>
class Set : public ISet<T>
{
public:
Set(string name);
~Set();
Set(const Set &origin);
//Set& operator=(const Set &origin);
bool insert(T element);
bool remove(T element);
int size()const;
private:
string name;
T *arr;
int cap, nrOfElement;
};
template<class T>
Set<T>::Set(string name)
{
this->name = name;
this->cap = 10;
this->nrOfElement = 0;
this->arr = new T[this->cap];
}
template<class T>
Set<T>::~Set()
{
delete[] arr;
}
template<class T>
Set<T>::Set(const Set & origin)
{
this->nrOfElement = origin.nrOfElement;
this->cap = origin.cap;
arr = new T*[cap];
for (int i = 0; i < nrOfElement; i++)
{
arr[i] = origin.arr[i];
}
}
template<class T>
bool Set<T>::insert(T element)
{
bool found = false;
if (nrOfElement == 0)
{
this->arr[0] = element;
this->nrOfElement++;
}
else
{
for (int i = 0; i < this->nrOfElement; i++)
{
if (this->arr[i] == element)
{
i = this->nrOfElement;
found = true;
}
}
if (found == false)
{
this->arr[nrOfElement++] = element;
}
}
return found;
}
template<class T>
bool Set<T>::remove(T element)
{
bool removed = false;
for (int i = 0; i < this->nrOfElement; i++)
{
if (this->arr[i] == element)
{
this->arr[i] = this->arr[nrOfElement];
nrOfElement--;
removed = true;
}
}
return removed;
}
template<class T>
int Set<T>::size() const
{
return this->nrOfElement;
}
And my problems starts when I start to test this code by adding the different data-type we are suppose to test the template against.
#include "Set.h"
#include "ISet.h"
#include "Runner.h"
int main()
{
_CrtSetDbgFlag(_CRTDBG_ALLOC_MEM_DF | _CRTDBG_LEAK_CHECK_DF);
Set<string> test("test");
test.insert("lol");
cout << test.size();
test.remove("lol");
cout << test.size();
Set<Runner> test2("test");
getchar();
return 0;
}
Getting the error saying that "No operator found which takes a left-hand operand type of 'Runner'. So I have to create a operator== that handles this but don't know?
Runner class looks like this:
#pragma once
#include "Competitor.h"
#include <string>
using namespace std;
class Runner : public Competitor
{
public:
Runner();
Runner(string firstName, string lastName, int startNr);
~Runner();
void addResult(int resultTime);
int getResult() const;
string toString() const;
Runner *clone() const;
private:
int resultTime;
};
#include "Runner.h"
Runner::Runner()
{
this->resultTime = 0;
}
Runner::Runner(string firstName, string lastName, int startNr) : Competitor(firstName, lastName, startNr)
{
this->resultTime = 0;
}
Runner::~Runner()
{
}
void Runner::addResult(int resultTime)
{
this->resultTime = resultTime;
}
int Runner::getResult() const
{
return this->resultTime;
}
string Runner::toString() const
{
return (to_string(this->resultTime) + " sec");
}
Runner * Runner::clone() const
{
return new Runner(*this);
}
How do I build a operator== that will work for this?
You need to add operator== to the Runner class:
bool operator==(const Runner& other) const;
I have a class that contains a tree structure implemented by a vector< vector< Node > > where Node contains a bunch of attributes exposed via getters/setters.
class Tree
{
vector< vector< Node > > mGrid;
printTree(std::ostream& output = std::cout);
};
class Node
{
double property1 { return mProp1; }
double property2 { return mProp2; }
};
printTree() is currently hardwired to use property tstep:
void Tree::printTree( ostream& output )
{
...
for (unsigned t = 0; t < mGrid.size(); ++t)
{
toPrint = "";
for (unsigned state = 0; state < mGrid[t].size(); ++state)
{
toPrint += to_string_with_precision( mGrid[t][state].tstep(), 1 );
...
Is there some slick / convenient / object-oriented way of generalizing this function so that it can print out any of Node's properties (rather than only spitting out the hardwired tstep() property or essentially doing the same thing via if/then statements).
I've done things like this in C using function pointers, but this is C++ and the C++ FAQ says not to mess with pointers to member functions.
You might want template function:
class Tree
{
vector< vector< Node > > mGrid;
public:
template <typename F>
void ForEachNode(F&& f) {
int i = 0;
for (auto& v : mGrid) {
int j = 0;
for (auto& node : v) {
f(node, i, j);
++j;
}
++i;
}
}
};
Then you may do
void printTreeProp1(Tree& tree) {
tree.ForEachNode([](const Node& node, int i, int j) {
if (i != 0 && j == 0) {
std::cout << std::endl;
}
std::cout << node.property1() << " ";
});
}
1st op all you loops are ignoring the first element. vector is zero based and you are using ++t and ++state which increases the values on top of the loop. That means you are never accessing the 0th element (mGrid[0] and mGrid[t][0]).2nd, you did noy include the definition of tstep(), so we don't know what you are getting back. Assuming you want to print each dimension of your 2 dimension array, I think you have to break it to peaces. Something like this:
class Node
{
protected:
double mProp1;
double mProp2;
public:
double GetProp1(void) {return mProp1;}
double GetProp2(void) {return mProp2;}
String tStep(void) {return L"";} // add your code here
};
class NodeRow : public std::vector<Node>
{
public:
void Print(std::ostream& output)
{
iterator i;
String tStr;
for(i = begin(); i != end(); i++)
tStr += /*to_string_with_precision(*/i->tStep()/*, 1)*/;
output << tStr.c_str() << L"\r\n";
}
};
class Node2D : public std::vector<NodeRow>
{
public:
void Print(std::ostream& output = std::cout)
{
iterator i;
for(i = begin(); i != end(); i++)
i->Print(output);
}
};
Say I have a simple vector class, vec:
#include <iostream>
#include <stdlib.h>
class vec {
public:
vec() {}
// Constructor.
vec(int n) {
len = n;
data = new double[len];
}
// Destructor.
~vec() { delete [] data; }
// Accessor.
double & operator[](int i) const {
check_index(i);
return data[i];
}
// Other methods...
// ....
protected:
int len;
double * data;
void check_index(int i) const {
if(i < 0 || i >= len) {
std::cerr << "Bad access.\n";
exit(1);
}
}
};
Now suppose I have a special type of vector with sparse structure, e.g., where every even-index is zero. Call this oddvec. Instances of oddvec should be declared just as with the vec class, but underneath, the memory use should be efficient since only half the data is non-zero.
The accessor for the oddvec class should return 0 if the index is even, and return the odd-index element (stored sequentially) otherwise. There a couple problems with this:
The double & return type is violated if the index is even, since the constant value, 0, is returned.
It's not clear to me how to handle the situation when an even index element is used as an lvalue. E.g., v[0] = 3.0 should not be allowed in the oddvec class, but is perfectly acceptable in the vector class. We can't simply throw an error when even indexes are used, because even indexes are fine as long as the intention is as an rvalue.
How do I design the accessor function for the oddvec class, while both keeping the memory storage efficient and inheriting all the methods from the parent?
Non-working example of oddvec:
class oddvec : public vec {
public:
// Constructor.
oddvec(int n) {
len = n;
data = new double[len/2];
}
// Accessor (doesn't work!)
double & operator[](int i) const {
check_index(i);
if (i%2 == 0)
return 0;
else
return data[(i-1)/2];
}
};
Upon compilation:
main.cpp: In member function ‘double& oddvec::operator[](int) const’:
main.cpp:49:20: error: invalid initialization of non-const reference of type ‘double&’ from an rvalue of type ‘double’
return 0;
Working example using proxy classes:
I have implemented a proxy class as suggested in the answer below.
proxies.h
#ifndef PROXIES_H
#define PROXIES_H
#include <iostream>
#include <stdlib.h>
class proxy {
public:
proxy(int i, double v, double * d) {
index = i;
value = v;
data = d;
}
void operator=(double rhs) {
data[index] = rhs;
}
friend std::ostream & operator<<(std::ostream & outs, const proxy & p) {
outs << p.value;
return outs;
}
protected:
int index;
double value;
double * data;
};
class oddproxy : public proxy {
public:
oddproxy(int i, int v, double * d) : proxy(i, v, d) {}
void operator=(double rhs) {
if (index%2 == 0) {
std::cerr << "Even entries of oddvec are not assignable.\n";
exit(1);
}
data[index/2] = rhs;
}
};
#endif
vectors.h
#ifndef VECTORS_H
#define VECTORS_H
#include "proxies.h"
class vec {
public:
vec() {}
// Constructor.
vec(int n) {
len = n;
data = new double[len];
}
// Destructor.
~vec() { delete [] data; }
// Accessor.
proxy operator[](int i) const {
check_index(i);
return proxy(i, data[i], data);
}
inline int length() const { return len; }
// Other methods...
// ....
protected:
int len;
double * data;
void check_index(int i) const {
if(i < 0 || i >= len) {
std::cerr << "Bad access.\n";
exit(1);
}
}
};
class oddvec : public vec {
public:
// Constructor.
oddvec(int n) {
len = n;
data = new double[len/2];
}
// Accessor.
oddproxy operator[](int i) const {
check_index(i);
return oddproxy(i, (i%2 == 0) ? 0 : data[i/2], data);
}
};
#endif
main.cpp
#include <iostream>
#include "vectors.h"
int main () {
int N = 5;
vec V(N);
oddvec O(N);
for(int i=0; i < V.length(); i++) {
V[i] = i;
if(i%2 != 0) {
O[i] = i;
}
}
for(int i=0; i < O.length(); i++) {
std::cout << "V[" << i << "]=" << V[i] << ", "
<< "O[" << i << "]=" << O[i] << "\n";
}
O[0] = 13;
return 0;
}
output
V[0]=0, O[0]=0
V[1]=1, O[1]=1
V[2]=2, O[2]=0
V[3]=3, O[3]=3
V[4]=4, O[4]=0
Even entries of oddvec are not assignable.
You can use proxy object to do this.
simple sample code:
#include <iostream>
#include <vector>
using namespace std;
class very_odd_vector{
public:
class only_odd_proxy;
friend class only_odd_proxy;
only_odd_proxy operator [](int index);
int operator [](int index)const{return index%2==0?0:content[index/2];}
unsigned int size()const{return content.size()*2;}
private:
vector<int> content{1,3,5,7,9};
};
class very_odd_vector::only_odd_proxy{
public:
only_odd_proxy(very_odd_vector& vec,int index):vec(vec),index(index){}
operator int(){return index%2==0 ? 0 : vec.content[index/2];}
only_odd_proxy& operator =(int value){
if(index%2==0)
cout << "BAD OPERATION";//any error you want
else
vec.content[index/2] = value;
return *this;
}
private:
very_odd_vector& vec;
int index;
};
auto very_odd_vector::operator [](int index)->only_odd_proxy{return only_odd_proxy(*this,index);}
int main(){
very_odd_vector v;
cout << "reading value\n";
for(int i=0;i<v.size();++i)
cout << v[i] <<'\n';
cout << "writting value\n";
for(int i=0;i<v.size();++i){
cout << i << ':';
v[i]=10;
cout << '\n';
}
cout << "reading value\n";
for(int i=0;i<v.size();++i)
cout << v[i] <<'\n';
}
Edit for updated part of question :
I think this class will fit your need more.
//Both base and inherit class return this class
class maybe_readonly_proxy {
public:
maybe_readonly_proxy(double* data, bool readonly):readonly(readonly),data(data){}
maybe_readonly_proxy& operator=(double rhs) {
if(readonly){/*whatever error*/}
else {*data = rhs;}
return *this;
}
operator double()const{return *data;}
private:
bool readonly;
double * data;
};
You may need a variable to contain readonly (0 in this case) value, or modify the operator double() the check readonly state
Or just implement get and set method separately and do not use this proxy may be another choice.