vector <int> v1(6);
//some procedure to fill the vector v1 with ints.
set <int> s(v1);
vector <int> v2(s)
Here 'v2' will contain the same elements as 'v1' but sorted in ascending order.what will be the time complexity of this sort process. set s stores ints in sorted form .
Copying the data from the vector to the set will be slower, because it will involve creating a data structure on the heap (typically a red-black tree), while sorting can be done in-place (effectively using the stack as a temporary data store).
#include <iostream>
#include <vector>
#include <set>
size_t gAllocs;
size_t gDeallocs;
void * operator new ( size_t sz ) { ++gAllocs; return std::malloc ( sz ); }
void operator delete ( void *pt ) { ++gDeallocs; return std::free ( pt ); }
int main () {
gAllocs = gDeallocs = 0;
std::vector<int> v { 8, 6, 7, 5, 3, 0, 9 };
std::cout << "Allocations = " << gAllocs << "; Deallocations = " << gDeallocs << std::endl;
std::set<int> s(v.begin(), v.end());
std::cout << "Allocations = " << gAllocs << "; Deallocations = " << gDeallocs << std::endl;
std::sort ( v.begin(), v.end ());
std::cout << "Allocations = " << gAllocs << "; Deallocations = " << gDeallocs << std::endl;
return 0;
}
On my system (clang, libc++, Mac OS 10.8), this prints:
$ ./a.out
Allocations = 1; Deallocations = 0
Allocations = 8; Deallocations = 0
Allocations = 8; Deallocations = 0
Building the set takes 7 memory allocations (one per entry). Sorting the vector takes none.
If there are no duplicates in v1
std::sort(v1.begin(), v1.end());
will be much faster
If duplicates in v1 is too large, following will be faster
std::set<int> s( v1.begin(), v1.end() );
v2.assign( s.begin(), s.end() );
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