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| Category: algorithms | | Component type: function |
Prototype
Partial_sort is an overloaded name; there are actually two partial_sort functions.
template <class RandomAccessIterator>
void partial_sort(RandomAccessIterator first,
RandomAccessIterator middle,
RandomAccessIterator last);
template <class RandomAccessIterator, class StrictWeakOrdering>
void partial_sort(RandomAccessIterator first,
RandomAccessIterator middle,
RandomAccessIterator last,
StrictWeakOrdering comp);
Description
Partial_sort rearranges the elements in the range [first, last) so that they are partially in ascending order. Specifically, it places the smallest middle - first elements, sorted in ascending order, into the range [first, middle). The remaining last - middle elements are placed, in an unspecified order, into the range [middle, last). [1] [2]
The two versions of partial_sort differ in how they define whether one element is less than another. The first version compares objects using operator<, and the second compares objects using a functors comp.
The postcondition for the first version of partial_sort is as follows. If i and j are any two valid iterators in the range [first, middle) such that i precedes j, and if k is a valid iterator in the range [middle, last), then *j < *i and *k < *i will both be false. The corresponding postcondition for the second version of partial_sort is that comp(*j, *i) and comp(*k, *i) are both false. Informally, this postcondition means that the first middle - first elements are in ascending order and that none of the elements in [middle, last) is less than any of the elements in [first, middle).
Definition
Defined in the standard header algorithm, and in the nonstandard backward-compatibility header algo.h.
Requirements on types
For the first version:
-
RandomAccessIterator is a model of RandomAccessIterator.
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RandomAccessIterator is mutable.
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RandomAccessIterator's value type is LessThanComparable.
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The ordering relation on
RandomAccessIterator's value type is a strict weak ordering, as defined in the LessThanComparable requirements.
For the second version:
-
RandomAccessIterator is a model of RandomAccessIterator.
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RandomAccessIterator is mutable.
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StrictWeakOrdering is a model of StrictWeakOrdering.
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RandomAccessIterator's value type is convertible to StrictWeakOrdering's argument type.
Preconditions
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[first, middle) is a valid range.
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[middle, last) is a valid range.
(It follows from these two conditions that [first, last) is a valid range.)
Complexity
Approximately (last - first) * log(middle - first) comparisons.
Example
int A[] = {7, 2, 6, 11, 9, 3, 12, 10, 8, 4, 1, 5};
const int N = sizeof(A) / sizeof(int);
partial_sort(A, A + 5, A + N);
copy(A, A + N, ostream_iterator<int>(cout, " "));
Notes
[1] Note that the elements in the range [first, middle) will be the same (ignoring, for the moment, equivalent elements) as if you had sorted the entire range using sort(first, last). The reason for using partial_sort in preference to sort is simply efficiency: a partial sort, in general, takes less time.
[2] partial_sort(first, last, last) has the effect of sorting the entire range [first, last), just like sort(first, last). They use different algorithms, however: sort uses the introsort algorithm (a variant of quicksort), and partial_sort uses heapsort. See section 5.2.3 of Knuth (D. E. Knuth, The Art of Computer Programming. Volume 3: Sorting and Searching. Addison-Wesley, 1975.), and J. W. J. Williams (CACM 7, 347, 1964). Both heapsort and introsort have complexity of order N log(N), but introsort is usually faster by a factor of 2 to 5.
See also
partial_sort_copy, sort, stable_sort, binary_search, lower_bound, upper_bound, less<T>, StrictWeakOrdering, LessThanComparable
