How to use different comparators for priority queue depending on conditions
I'm working on an assignment where I have a priority queue and I want it to work like this:
if(field == '0')
priority_queue<record_t*,vector<record_t*>, CompareRecordID > pq;
else if(field == '1')
priority_queue<record_t*,vector<record_t*>, CompareRecordNum > pq;
else if(field == '2')
priority_queue<record_t*,vector<record_t*>, CompareRecordStr > pq;
else if(field == '3')
priority_queue<record_t*,vector<record_t*>, CompareRecordNumStr > pq;
Where record_t is:
typedef struct {
unsigned int recid;
unsigned int num;
char str[STR_LENGTH];
bool valid; // if set, then this record is valid
int blockID; //The block the record belongs too -> Used only for minheap
} record_t;
This means that depending on the function argument field, the queue will "sort" another record_t field. However, I cannot declare the queue inside the if statement, as this will obviously give me a "pq was not declared in this scope" error. What can I do?
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You can use a constructor std::priority_queue
that takes a comparator object as a parameter. Then you can copy its configurable comparator a little like this:
#include <vector>
#include <queue>
#include <cstring>
#include <iostream>
const int STR_LENGTH = 20;
struct record_t
{
unsigned int recid;
unsigned int num;
char str[STR_LENGTH];
bool valid; // if set, then this record is valid
int blockID; //The block the record belongs too -> Used only for minheap
};
// switchable priority comparator
struct CompareRecord
{
int field;
CompareRecord(int field = 0): field(field) {}
bool operator() (const record_t* lhs, const record_t* rhs) const
{
switch(field)
{
case 0: return lhs->recid < rhs->recid;
case 1: return lhs->num < rhs->num;
case 2: return std::strcmp(lhs->str, rhs->str) < 0;
}
return true;
}
};
int main()
{
// physical records
std::vector<record_t> records;
record_t r;
r.recid = 1;
r.num = 1;
std::strcpy(r.str, "banana");
records.push_back(r);
r.recid = 2;
r.num = 4;
std::strcpy(r.str, "orange");
records.push_back(r);
r.recid = 3;
r.num = 2;
std::strcpy(r.str, "apple");
records.push_back(r);
// input priority type: 0, 1 or 2
int field;
std::cout << "Sort type [0, 1, 2]: " << std::flush;
std::cin >> field;
std::cout << '\n';
// build priority view
CompareRecord cmp(field);
std::priority_queue<record_t*, std::vector<record_t*>, CompareRecord> pq(cmp);
for(auto& r: records)
pq.push(&r);
while(!pq.empty())
{
std::cout << "rec: " << pq.top()->recid << '\n';
std::cout << "num: " << pq.top()->num << '\n';
std::cout << "str: " << pq.top()->str << '\n';
std::cout << '\n';
pq.pop();
}
}
Output:
Sort type [0, 1, 2]: 0
rec: 3
num: 2
str: apple
rec: 2
num: 4
str: orange
rec: 1
num: 1
str: banana
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The priority_queue value can take a comparator as a constructor argument.
std::priority_queue<record_t*,vector<record_t*>, CompareRecord > pq((CompareRecord(field)));
You just need to define the CompareRecord comparator appropriately. An easy way to do this would be:
struct CompareRecord{
char type;
CompareRecord(char type):type(type){}
bool operator()(const record_t* lhs, const record_t* rhs){
switch(type){
case '1': return lhs->recid < rhs->recid;
.. and so forth.
}
}
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