prev_permutation(3C++) - prev_permutation(3C++)
Standard C++ Library Copyright 1998, Rogue Wave Software, Inc.
NAMEprev_permutation
- Generates successive permutations of a sequence based on an ordering
function.
SYNOPSIS
#include <algorithm>
template <class BidirectionalIterator>
bool prev_permutation (BidirectionalIterator first,
BidirectionalIterator last);
template <;class BidirectionalIterator, class Compare>
bool prev_permutation (BidirectionalIterator first,
BidirectionalIterator last,
Compare comp);
DESCRIPTION
The permutation-generating algorithms (next_permutation and prev_permu‐
tation) assume that the set of all permutations of the elements in a
sequence is lexicographically sorted with respect to operator< or comp.
For example, if a sequence includes the integers 1 2 3, that sequence
has six permutations. In order from first to last, they are: 1 2 3, 1 3
2, 2 1 3, 2 3 1, 3 1 2, and 3 2 1.
The prev_permutation algorithm takes a sequence defined by the range
[first, last) and transforms it into its previous permutation, if pos‐
sible. If such a permutation does exist, the algorithm completes the
transformation and returns true. If the permutation does not exist,
prev_permutation returns false, and transforms the permutation into its
"last" permutation. prev_permutation does the transformation according
to the lexicographical ordering defined by either operator< (the
default used in the first version of the algorithm) or comp (which is
user-supplied in the second version of the algorithm).
For example, if the sequence defined by [first, last) contains the
integers 1 2 3 (in that order), there is not a "previous permutation."
Therefore, the algorithm transforms the sequence into its last permuta‐
tion (3 2 1) and returns false.
COMPLEXITY
At most (last - first)/2 swaps are performed.
EXAMPLE
//
// permute.cpp
//
#include <numeric> //for accumulate
#include <vector> //for vector
#include <functional> //for less
#include <iostream>
using namespace std;
int main()
{
//Initialize a vector using an array of ints
int a1[] = {0,0,0,0,1,0,0,0,0,0};
char a2[] = "abcdefghji";
//Create the initial set and copies for permuting
vector<int> m1(a1, a1+10);
vector<int> prev_m1((size_t)10), next_m1((size_t)10);
vector<char> m2(a2, a2+10);
vector<char> prev_m2((size_t)10), next_m2((size_t)10);
copy(m1.begin(), m1.end(), prev_m1.begin());
copy(m1.begin(), m1.end(), next_m1.begin());
copy(m2.begin(), m2.end(), prev_m2.begin());
copy(m2.begin(), m2.end(), next_m2.begin());
//Create permutations
prev_permutation(prev_m1.begin(),
prev_m1.end(),less<int>());
next_permutation(next_m1.begin(),
next_m1.end(),less<int>());
prev_permutation(prev_m2.begin(),
prev_m2.end(),less<int>());
next_permutation(next_m2.begin(),
next_m2.end(),less<int>());
//Output results
cout << "Example 1: " << endl << " ";
cout << "Original values: ";
copy(m1.begin(),m1.end(),
ostream_iterator<int,char>(cout," "));
cout << endl << " ";
cout << "Previous permutation: ";
copy(prev_m1.begin(),prev_m1.end(),
ostream_iterator<int,char>(cout," "));
cout << endl<< " ";
cout << "Next Permutation: ";
copy(next_m1.begin(),next_m1.end(),
ostream_iterator<int,char>(cout," "));
cout << endl << endl;
cout << "Example 2: " << endl << " ";
cout << "Original values: ";
copy(m2.begin(),m2.end(),
ostream_iterator<char,char>(cout," "));
cout << endl << " ";
cout << "Previous Permutation: ";
copy(prev_m2.begin(),prev_m2.end(),
ostream_iterator<char,char>(cout," "));
cout << endl << " ";
cout << "Next Permutation: ";
copy(next_m2.begin(),next_m2.end(),
ostream_iterator<char,char>(cout," "));
cout << endl << endl;
return 0;
}
Program OutputExample 1:
Original values: 0 0 0 0 1 0 0 0 0 0
Previous permutation: 0 0 0 0 0 1 0 0 0 0
Next Permutation: 0 0 0 1 0 0 0 0 0 0
Example 2:
Original values: a b c d e f g h j i
Previous Permutation: a b c d e f g h i j
Next Permutation: a b c d e f g i h j
WARNINGS
If your compiler does not support default template parameters, then you
always need to supply the Allocator template argument. For instance,
you need to write:
vector<int, allocator<int> >
instead of:
vector<int>
If your compiler does not support namespaces, then you do not need the
using declaration for std.
SEE ALSO
next_permutation
Rogue Wave Software 02 Apr 1998 prev_permutation(3C++)
