Follow Techotopia on Twitter

On-line Guides
All Guides
eBook Store
iOS / Android
Linux for Beginners
Office Productivity
Linux Installation
Linux Security
Linux Utilities
Linux Virtualization
Linux Kernel
System/Network Admin
Programming
Scripting Languages
Development Tools
Web Development
GUI Toolkits/Desktop
Databases
Mail Systems
openSolaris
Eclipse Documentation
Techotopia.com
Virtuatopia.com

How To Guides
Virtualization
General System Admin
Linux Security
Linux Filesystems
Web Servers
Graphics & Desktop
PC Hardware
Windows
Problem Solutions
Privacy Policy

  




 

 

Thinking in C++ Vol 2 - Practical Programming
Prev Home Next

Priority queues

When you push( ) an object onto a priority_queue, that object is sorted into the queue according to a comparison function or function object. (You can allow the default less template to supply this, or you can provide one of your own.) The priority_queue ensures that when you look at the top( ) element, it will be the one with the highest priority. When you re done with it, you call pop( ) to remove it and bring the next one into place. Thus, the priority_queue has nearly the same interface as a stack, but it behaves differently.

Like stack and queue, priority_queue is an adaptor that is built on top of one of the basic sequences the default sequence being vector.

It s trivial to make a priority_queue that works with ints:

//: C07:PriorityQueue1.cpp
#include <cstdlib>
#include <ctime>
#include <iostream>
#include <queue>
using namespace std;
 
int main() {
priority_queue<int> pqi;
srand(time(0)); // Seed the random number generator
for(int i = 0; i < 100; i++)
pqi.push(rand() % 25);
while(!pqi.empty()) {
cout << pqi.top() << ' ';
pqi.pop();
}
} ///:~
 

This pushes into the priority_queue 100 random values from 0 to 24. When you run this program you ll see that duplicates are allowed, and the highest values appear first. To show how you can change the ordering by providing your own function or function object, the following program gives lower-valued numbers the highest priority:

//: C07:PriorityQueue2.cpp
// Changing the priority.
#include <cstdlib>
#include <ctime>
#include <functional>
#include <iostream>
#include <queue>
using namespace std;
 
int main() {
priority_queue<int, vector<int>, greater<int> > pqi;
srand(time(0));
for(int i = 0; i < 100; i++)
pqi.push(rand() % 25);
while(!pqi.empty()) {
cout << pqi.top() << ' ';
pqi.pop();
}
} ///:~
 

A more interesting problem is a to-do list, where each object contains a string and a primary and secondary priority value:

//: C07:PriorityQueue3.cpp
// A more complex use of priority_queue.
#include <iostream>
#include <queue>
#include <string>
using namespace std;
 
class ToDoItem {
char primary;
int secondary;
string item;
public:
ToDoItem(string td, char pri = 'A', int sec = 1)
: primary(pri), secondary(sec), item(td) {}
friend bool operator<(
const ToDoItem& x, const ToDoItem& y) {
if(x.primary > y.primary)
return true;
if(x.primary == y.primary)
if(x.secondary > y.secondary)
return true;
return false;
}
friend ostream&
operator<<(ostream& os, const ToDoItem& td) {
return os << td.primary << td.secondary
<< ": " << td.item;
}
};
 
int main() {
priority_queue<ToDoItem> toDoList;
toDoList.push(ToDoItem("Empty trash", 'C', 4));
toDoList.push(ToDoItem("Feed dog", 'A', 2));
toDoList.push(ToDoItem("Feed bird", 'B', 7));
toDoList.push(ToDoItem("Mow lawn", 'C', 3));
toDoList.push(ToDoItem("Water lawn", 'A', 1));
toDoList.push(ToDoItem("Feed cat", 'B', 1));
while(!toDoList.empty()) {
cout << toDoList.top() << endl;
toDoList.pop();
}
} ///:~
 

The ToDoItem s operator< must be a nonmember function for it to work with less< >. Other than that, everything happens automatically. The output is

A1: Water lawn
A2: Feed dog
B1: Feed cat
B7: Feed bird
C3: Mow lawn
C4: Empty trash
 

You cannot iterate through a priority_queue, but it s possible to simulate the behavior of a priority_queue using a vector, thus allowing you access to that vector. You can do this by looking at the implementation of priority_queue, which uses make_heap( ), push_heap( ), and pop_heap( ). (These are the soul of the priority_queue in fact you could say that the heap is the priority queue and that priority_queue is just a wrapper around it.) This turns out to be reasonably straightforward, but you might think that a shortcut is possible. Since the container used by priority_queue is protected (and has the identifier, according to the Standard C++ specification, named c), you can inherit a new class that provides access to the underlying implementation:

//: C07:PriorityQueue4.cpp
// Manipulating the underlying implementation.
#include <algorithm>
#include <cstdlib>
#include <ctime>
#include <iostream>
#include <iterator>
#include <queue>
using namespace std;
 
class PQI : public priority_queue<int> {
public:
vector<int>& impl() { return c; }
};
 
int main() {
PQI pqi;
srand(time(0));
for(int i = 0; i < 100; i++)
pqi.push(rand() % 25);
copy(pqi.impl().begin(), pqi.impl().end(),
ostream_iterator<int>(cout, " "));
cout << endl;
while(!pqi.empty()) {
cout << pqi.top() << ' ';
pqi.pop();
}
} ///:~
 

However, if you run this program, you ll discover that the vector doesn t contain the items in the descending order that you get when you call pop( ), the order that you want from the priority queue. It would seem that if you want to create a vector that is a priority queue, you have to do it by hand, like this:

//: C07:PriorityQueue5.cpp
// Building your own priority queue.
#include <algorithm>
#include <cstdlib>
#include <ctime>
#include <iostream>
#include <iterator>
#include <queue>
using namespace std;
 
template<class T, class Compare>
class PQV : public vector<T> {
Compare comp;
public:
PQV(Compare cmp = Compare()) : comp(cmp) {
make_heap(this->begin(),this->end(), comp);
}
const T& top() { return this->front(); }
void push(const T& x) {
this->push_back(x);
push_heap(this->begin(),this->end(), comp);
}
void pop() {
pop_heap(this->begin(),this->end(), comp);
this->pop_back();
}
};
 
int main() {
PQV< int, less<int> > pqi;
srand(time(0));
for(int i = 0; i < 100; i++)
pqi.push(rand() % 25);
copy(pqi.begin(), pqi.end(),
ostream_iterator<int>(cout, " "));
cout << endl;
while(!pqi.empty()) {
cout << pqi.top() << ' ';
pqi.pop();
}
} ///:~
 

But this program behaves in the same way as the previous one! What you are seeing in the underlying vector is called a heap. This heap data structure represents the tree of the priority queue (stored in the linear structure of the vector), but when you iterate through it, you do not get a linear priority-queue order. You might think that you can simply call sort_heap( ), but that only works once, and then you don t have a heap anymore, but instead a sorted list. This means that to go back to using it as a heap, the user must remember to call make_heap( ) first. This can be encapsulated into your custom priority queue:

//: C07:PriorityQueue6.cpp
#include <algorithm>
#include <cstdlib>
#include <ctime>
#include <iostream>
#include <iterator>
#include <queue>
using namespace std;
 
template<class T, class Compare>
class PQV : public vector<T> {
Compare comp;
bool sorted;
void assureHeap() {
if(sorted) {
// Turn it back into a heap:
make_heap(this->begin(),this->end(), comp);
sorted = false;
}
}
public:
PQV(Compare cmp = Compare()) : comp(cmp) {
make_heap(this->begin(),this->end(), comp);
sorted = false;
}
const T& top() {
assureHeap();
return this->front();
}
void push(const T& x) {
assureHeap();
this->push_back(x); // Put it at the end
// Re-adjust the heap:
push_heap(this->begin(),this->end(), comp);
}
void pop() {
assureHeap();
// Move the top element to the last position:
pop_heap(this->begin(),this->end(), comp);
this->pop_back();// Remove that element
}
void sort() {
if(!sorted) {
sort_heap(this->begin(),this->end(), comp);
reverse(this->begin(),this->end());
sorted = true;
}
}
};
 
int main() {
PQV< int, less<int> > pqi;
srand(time(0));
for(int i = 0; i < 100; i++) {
pqi.push(rand() % 25);
copy(pqi.begin(), pqi.end(),
ostream_iterator<int>(cout, " "));
cout << "\n----- << endl;
}
pqi.sort();
copy(pqi.begin(), pqi.end(),
ostream_iterator<int>(cout, " "));
cout << "\n----- << endl;
while(!pqi.empty()) {
cout << pqi.top() << ' ';
pqi.pop();
}
} ///:~
 

If sorted is true, the vector is not organized as a heap but instead as a sorted sequence. The assureHeap( ) function guarantees that it s put back into heap form before performing any heap operations on it. The first for loop in main( ) now has the additional quality that it displays the heap as it is being built.

In the previous two programs we had to introduce a seemingly extraneous usage of the this-> prefix. Although some compilers do not require it, the standard definition of C++ does. Note that the class PQV derives from vector<T>, therefore begin( ) and end( ), inherited from vector<T>, are dependent names.[110] Compilers can t look up names from dependent base classes in the definition of a template (vector, in this case) because for a given instantiation an explicitly specialized version of the template might be used that does not have a given member. The special naming requirement guarantees that you won t end up calling a base class member in some cases and possibly a function from an enclosing scope (such as a global one) in other cases. The compiler has no way of knowing that a call to begin( ) is dependent, so we must give it a clue with a this‑> qualification.[111] This tells the compiler that begin( ) is in the scope of PQV, so it waits until an instance of PQV is fully instantiated. If this qualifying prefix is left out, the compiler will attempt an early lookup for the names begin and end (at template definition time, and will fail to find them because there are no such names declared in enclosing lexical scopes in this example). In the code above, however, the compiler waits until the point of instantiation of pqi, and then finds the correct specializations of begin( ) and end( ) in vector<int>.

The only drawback to this solution is that the user must remember to call sort( ) before viewing it as a sorted sequence (although one could conceivably redefine all the member functions that produce iterators so that they guarantee sorting). Another solution is to create a priority queue that is not a vector, but will build you a vector whenever you want one:

//: C07:PriorityQueue7.cpp
// A priority queue that will hand you a vector.
#include <algorithm>
#include <cstdlib>
#include <ctime>
#include <iostream>
#include <iterator>
#include <queue>
#include <vector>
using namespace std;
 
template<class T, class Compare> class PQV {
vector<T> v;
Compare comp;
public:
// Don't need to call make_heap(); it's empty:
PQV(Compare cmp = Compare()) : comp(cmp) {}
void push(const T& x) {
v.push_back(x); // Put it at the end
// Re-adjust the heap:
push_heap(v.begin(), v.end(), comp);
}
void pop() {
// Move the top element to the last position:
pop_heap(v.begin(), v.end(), comp);
v.pop_back(); // Remove that element
}
const T& top() { return v.front(); }
bool empty() const { return v.empty(); }
int size() const { return v.size(); }
typedef vector<T> TVec;
TVec getVector() {
TVec r(v.begin(), v.end());
// It s already a heap
sort_heap(r.begin(), r.end(), comp);
// Put it into priority-queue order:
reverse(r.begin(), r.end());
return r;
}
};
 
int main() {
PQV<int, less<int> > pqi;
srand(time(0));
for(int i = 0; i < 100; i++)
pqi.push(rand() % 25);
const vector<int>& v = pqi.getVector();
copy(v.begin(), v.end(),
ostream_iterator<int>(cout, " "));
cout << "\n----------- << endl;
while(!pqi.empty()) {
cout << pqi.top() << ' ';
pqi.pop();
}
} ///:~
 

The PQV class template follows the same form as the STL s priority_queue, but has the additional member getVector( ), which creates a new vector that s a copy of the one in PQV (which means that it s already a heap). It then sorts that copy (leaving PQV s vector untouched), and reverses the order so that traversing the new vector produces the same effect as popping the elements from the priority queue.

You may observe that the approach of deriving from priority_queue used in PriorityQueue4.cpp could be used with the above technique to produce more succinct code:

//: C07:PriorityQueue8.cpp
// A more compact version of PriorityQueue7.cpp.
#include <algorithm>
#include <cstdlib>
#include <ctime>
#include <iostream>
#include <iterator>
#include <queue>
using namespace std;
 
template<class T> class PQV : public priority_queue<T> {
public:
typedef vector<T> TVec;
TVec getVector() {
TVec r(this->c.begin(),this->c.end());
// c is already a heap
sort_heap(r.begin(), r.end(), this->comp);
// Put it into priority-queue order:
reverse(r.begin(), r.end());
return r;
}
};
 
int main() {
PQV<int> pqi;
srand(time(0));
for(int i = 0; i < 100; i++)
pqi.push(rand() % 25);
const vector<int>& v = pqi.getVector();
copy(v.begin(), v.end(),
ostream_iterator<int>(cout, " "));
cout << "\n----------- << endl;
while(!pqi.empty()) {
cout << pqi.top() << ' ';
pqi.pop();
}
} ///:~
 

The brevity of this solution makes it the simplest and most desirable, plus it s guaranteed that the user will not have a vector in the unsorted state. The only potential problem is that the getVector( ) member function returns the vector<T> by value, which might cause some overhead issues with complex values of the parameter type T.

Thinking in C++ Vol 2 - Practical Programming
Prev Home Next

 
 
   Reproduced courtesy of Bruce Eckel, MindView, Inc. Design by Interspire