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stable_partition


Category: algorithms	Component type: function

Prototype

template <class ForwardIterator, class Predicate>
ForwardIterator stable_partition(ForwardIterator first, ForwardIterator last, 
                                 Predicate pred);

Description

Stable_partition is much like partition: it reorders the elements in the range [first, last) based on the function object pred, such that all of the elements that satisfy pred appear before all of the elements that fail to satisfy it. The postcondition is that, for some iterator middle in the range [first, last), pred(*i) is true for every iterator i in the range [first, middle) and false for every iterator i in the range [middle, last). The return value of stable_partition is middle.

Stable_partition differs from partition in that stable_partition is guaranteed to preserve relative order. That is, if x and y are elements in [first, last) such that pred(x) == pred(y), and if x precedes y, then it will still be true after stable_partition is true that x precedes y. [1]

Definition

Defined in the standard header algorithm, and in the nonstandard backward-compatibility header algo.h.

Requirements on types

ForwardIterator is a model of Forward Iterator
Predicate is a model of Predicate
ForwardIterator's value type is convertible to Predicate's argument type.

Preconditions

[first, last) is a valid range.

Complexity

Stable_partition is an adaptive algorithm: it attempts to allocate a temporary memory buffer, and its run-time complexity depends on how much memory is available. Worst-case behavior (if no auxiliary memory is available) is at most N*log(N) swaps, where N is last - first, and best case (if a large enough auxiliary memory buffer is available) is linear in N. In either case, pred is applied exactly N times.

Example

Reorder a sequence so that even numbers precede odd numbers.

int A[] = {1, 2, 3, 4, 5, 6, 7, 8, 9, 10};
const int N = sizeof(A)/sizeof(int);
stable_partition(A, A + N,
                 compose1(bind2nd(equal_to<int>(), 0),
                          bind2nd(modulus<int>(), 2)));
copy(A, A + N, ostream_iterator<int>(cout, " "));
// The output is "2 4 6 8 10 1 3 5 7 9". [1]

Notes

[1] Note that the complexity of stable_partition is greater than that of partition: the guarantee that relative order will be preserved has a significant runtime cost. If this guarantee isn't important to you, you should use partition.