merge
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| Category: algorithms |
Component type: function |
Prototype
Merge is an overloaded name: there are actually two merge functions.
template <class InputIterator1, class InputIterator2, class OutputIterator>
OutputIterator merge(InputIterator1 first1, InputIterator1 last1,
InputIterator2 first2, InputIterator2 last2,
OutputIterator result);
template <class InputIterator1, class InputIterator2, class OutputIterator,
class StrictWeakOrdering>
OutputIterator merge(InputIterator1 first1, InputIterator1 last1,
InputIterator2 first2, InputIterator2 last2,
OutputIterator result, StrictWeakOrdering comp);
Description
Merge combines two sorted ranges [first1, last1) and
[first2, last2) into a single sorted range. That is, it copies
elements from [first1, last1) and [first2, last2) into
[result, result + (last1 - first1) + (last2 - first2)) such that
the resulting range is in ascending order.
Merge is stable, meaning both that
the relative order of elements within each input range is preserved,
and that for equivalent [1] elements in both input ranges the element
from the first range precedes the element from the second.
The return value is result + (last1 - first1) + (last2 - first2).
The two versions of merge differ in how elements are compared.
The first version uses operator<. That is, the input ranges and
the output range satisfy the condition that for every pair of
iterators i and j such that i precedes j, *j < *i is false.
The second version uses the function object comp.
That is, the input ranges and the output range satisfy the condition
that for every pair of
iterators i and j such that i precedes j, comp(*j, *i) is false.
Definition
Defined in the standard header algorithm, and in the nonstandard
backward-compatibility header algo.h.
Requirements on types
For the first version:
-
InputIterator1 is a model of Input Iterator.
-
InputIterator2 is a model of Input Iterator.
-
InputIterator1's value type is the same type as InputIterator2's
value type.
-
InputIterator1's value type is a model of LessThan Comparable.
-
The ordering on objects of InputIterator1's value type is a
strict weak ordering, as defined in the LessThan Comparable
requirements.
-
InputIterator1's value type is convertible to a type in OutputIterator's
set of value types.
For the second version:
-
InputIterator1 is a model of Input Iterator.
-
InputIterator2 is a model of Input Iterator.
-
InputIterator1's value type is the same type as InputIterator2's
value type.
-
StrictWeakOrdering is a model of Strict Weak Ordering.
-
InputIterator1's value type is convertible to StrictWeakOrdering's
argument type.
-
InputIterator1's value type is convertible to a type in OutputIterator's
set of value types.
Preconditions
For the first version:
-
[first1, last1) is a valid range.
-
[first1, last1) is in ascending order. That is, for every pair
of iterators i and j in [first1, last1) such that i precedes
j, *j < *i is false.
-
[first2, last2) is a valid range.
-
[first2, last2) is in ascending order. That is, for every pair
of iterators i and j in [first2, last2) such that i precedes
j, *j < *i is false.
-
The ranges [first1, last1) and
[result, result + (last1 - first1) + (last2 - first2)) do not overlap.
-
The ranges [first2, last2) and
[result, result + (last1 - first1) + (last2 - first2)) do not overlap.
-
There is enough space to hold all of the elements being copied.
More formally, the requirement is that
[result, result + (last1 - first1) + (last2 - first2)) is a valid range.
For the second version:
-
[first1, last1) is a valid range.
-
[first1, last1) is in ascending order. That is, for every pair
of iterators i and j in [first1, last1) such that i precedes
j, comp(*j, *i) is false.
-
[first2, last2) is a valid range.
-
[first2, last2) is in ascending order. That is, for every pair
of iterators i and j in [first2, last2) such that i precedes
j, comp(*j, *i) is false.
-
The ranges [first1, last1) and
[result, result + (last1 - first1) + (last2 - first2)) do not overlap.
-
The ranges [first2, last2) and
[result, result + (last1 - first1) + (last2 - first2)) do not overlap.
-
There is enough space to hold all of the elements being copied.
More formally, the requirement is that
[result, result + (last1 - first1) + (last2 - first2)) is a valid range.
Complexity
Linear. No comparisons if both [first1, last1) and [first2, last2)
are empty ranges, otherwise at most (last1 - first1) + (last2 -
first2) - 1 comparisons.
Example
int main()
{
int A1[] = { 1, 3, 5, 7 };
int A2[] = { 2, 4, 6, 8 };
const int N1 = sizeof(A1) / sizeof(int);
const int N2 = sizeof(A2) / sizeof(int);
merge(A1, A1 + N1, A2, A2 + N2,
ostream_iterator<int>(cout, " "));
// The output is "1 2 3 4 5 6 7 8"
}
Notes
[1]
Note that you may use an ordering that is a strict weak ordering
but not a total ordering; that is, there might be values x and y
such that x < y, x > y, and x == y are all false. (See the
LessThan Comparable requirements for a more complete discussion.)
Two elements x and y are equivalent if neither x < y nor
y < x. If you're using a total ordering, however (if you're
using strcmp, for example, or if you're using ordinary arithmetic
comparison on integers), then you can ignore this technical
distinction: for a total ordering, equality and equivalence are
the same.
See also
inplace_merge, set_union, sort
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