function template
<algorithm>
std::prev_permutation
| default (1) |
template <class BidirectionalIterator>
bool prev_permutation (BidirectionalIterator first,
BidirectionalIterator last );
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| custom (2) |
template <class BidirectionalIterator, class Compare>
bool prev_permutation (BidirectionalIterator first,
BidirectionalIterator last, Compare comp); |
|---|
Transform range to previous permutation
Rearranges the elements in the range [first,last) into the previous lexicographically-ordered permutation.
A permutation is each one of the N! possible arrangements the elements can take (where N is the number of elements in the range). Different permutations can be ordered according to how they compare lexicographicaly to each other; The first such-sorted possible permutation (the one that would compare lexicographically smaller to all other permutations) is the one which has all its elements sorted in ascending order, and the largest has all its elements sorted in descending order.
The comparisons of individual elements are performed using either operator< for the first version, or comp for the second.
If the function can determine the previous permutation, it rearranges the elements as such and returns true. If that was not possible (because it is already at the lowest possible permutation), it rearranges the elements according to the last permutation (sorted in descending order) and returns false.
Parameters
- first, last
- Bidirectional iterators to the initial and final positions of the sequence. The range used is
[first,last), which contains all the elements between first and last, including the element pointed by first but not the element pointed by last.
BidirectionalIterator shall point to a type for which swap is properly defined.
- comp
- Binary function that accepts two arguments of the type pointed by BidirectionalIterator, and returns a value convertible to
bool. The value returned indicates whether the first argument is considered to go before the second in the specific strict weak ordering it defines.
The function shall not modify any of its arguments.
This can either be a function pointer or a function object.
Return value
true if the function could rearrange the object as a lexicographicaly smaller permutation.
Otherwise, the function returns false to indicate that the arrangement is not less than the previous, but the largest possible (sorted in descending order).
Example
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// next_permutation example
#include <iostream> // std::cout
#include <algorithm> // std::next_permutation, std::sort, std::reverse
int main () {
int myints[] = {1,2,3};
std::sort (myints,myints+3);
std::reverse (myints,myints+3);
std::cout << "The 3! possible permutations with 3 elements:\n";
do {
std::cout << myints[0] << ' ' << myints[1] << ' ' << myints[2] << '\n';
} while ( std::prev_permutation(myints,myints+3) );
std::cout << "After loop: " << myints[0] << ' ' << myints[1] << ' ' << myints[2] << '\n';
return 0;
}
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Output:
3 2 1
3 1 2
2 3 1
2 1 3
1 3 2
1 2 3
After loop: 3 2 1
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Complexity
Up to linear in half the distance between first and last (in terms of actual swaps).
Data races
The objects in the range [first,last) are modified.
Exceptions
Throws if any element swap throws or if any operation on an iterator throws.
Note that invalid arguments cause undefined behavior.