stable_sort


Category: algorithms 
Component type: function 
Prototype
Stable_sort is an overloaded name; there are actually two stable_sort
functions.
template <class RandomAccessIterator>
void stable_sort(RandomAccessIterator first, RandomAccessIterator last);
template <class RandomAccessIterator, class StrictWeakOrdering>
void stable_sort(RandomAccessIterator first, RandomAccessIterator last,
StrictWeakOrdering comp);
Description
Stable_sort is much like sort: it sorts the elements in
[first, last) into ascending order, meaning that if i and j are
any two valid iterators in [first, last) such that i precedes j,
then *j is not less than *i. Stable_sort differs from
sort in two ways. First, stable_sort uses an algorithm that
has different runtime complexity than sort. Second, as the
name suggests, stable_sort is stable: it preserves the relative
ordering of equivalent elements. That is, if x and y are elements
in [first, last) such that x precedes y, and if the two
elements are equivalent (neither x < y nor y < x)
then a postcondition of stable_sort is that x still precedes y.
[1]
The two versions of stable_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 function object comp.
Definition
Defined in the standard header algorithm, and in the nonstandard
backwardcompatibility header algo.h.
Requirements on types
For the first version, the one that takes two arguments:
For the second version, the one that takes three arguments:

RandomAccessIterator is a model of Random Access Iterator.

RandomAccessIterator is mutable.

StrictWeakOrdering is a model of Strict Weak Ordering.

RandomAccessIterator's value type is convertible to
StrictWeakOrdering's argument type.
Preconditions

[first, last) is a valid range.
Complexity
Stable_sort is an adaptive algorithm: it attempts to
allocate a temporary memory buffer, and its runtime complexity depends
on how much memory is available. Worstcase behavior (if no auxiliary memory
is available) is N (log N)^2 comparisons, where N is last 
first, and best case
(if a large enough auxiliary memory buffer is available)
is N (log N). [2]
Example
Sort a sequence of characters, ignoring their case. Note that the
relative order of characters that differ only by case is preserved.
inline bool lt_nocase(char c1, char c2) { return tolower(c1) < tolower(c2); }
int main()
{
char A[] = "fdBeACFDbEac";
const int N = sizeof(A)  1;
stable_sort(A, A+N, lt_nocase);
printf("%s\n", A);
// The printed result is ""AaBbCcdDeEfF".
}
Notes
[1]
Note that two elements may be equivalent without being equal.
One standard example is sorting a sequence of names by last name: if
two people have the same last name but different first names, then
they are equivalent but not equal. This is why stable_sort is
sometimes useful: if you are sorting a sequence of records that have
several different fields, then you may want to sort it by one field
without completely destroying the ordering that you previously
obtained from sorting it by a different field. You might, for
example, sort by first name and then do a stable sort by last name.
[2]
Stable_sort uses the merge sort algorithm; see section
5.2.4 of Knuth. (D. E. Knuth, The Art of Computer
Programming. Volume 3: Sorting and Searching. AddisonWesley, 1975.)
See also
sort,
partial_sort,
partial_sort_copy,
binary_search,
lower_bound,
upper_bound,
less<T>,
StrictWeakOrdering,
LessThan Comparable
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