partition


Category: algorithms 
Component type: function 
Prototype
template <class ForwardIterator, class Predicate>
ForwardIterator partition(ForwardIterator first,
ForwardIterator last, Predicate pred)
Description
Partition reorders the elements in the range [first, last) based
on the function object pred, such that
the elements that satisfy pred precede 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). [1]
The return value of partition is middle.
Definition
Defined in the standard header algorithm, and in the nonstandard
backwardcompatibility 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
Linear. Exactly last  first applications of pred, and at most
(last  first)/2 swaps.
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);
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 "10 2 8 4 6 5 7 3 9 1". [1]
Notes
[1]
The relative order of elements in these two blocks is not necessarily
the same as it was in the original sequence. A different algorithm,
stable_partition, does guarantee to preserve the relative order.
See also
stable_partition, Predicate, function object
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