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<Title>unique</Title>
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<H1>unique</H1>
<Table CellPadding=0 CellSpacing=0 width=100%>
<TR>
<TD Align=left><Img src = "algorithms.gif" Alt="" WIDTH = "194" HEIGHT = "38" ></TD>
<TD Align=right><Img src = "function.gif" Alt="" WIDTH = "194" HEIGHT = "38" ></TD>
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<TR>
<TD Align=left VAlign=top><b>Category</b>: algorithms</TD>
<TD Align=right VAlign=top><b>Component type</b>: function</TD>
</TR>
</Table>
<h3>Prototype</h3>
<tt>Unique</tt> is an overloaded name; there are actually two <tt>unique</tt>
functions.
<pre>
template <class <A href="ForwardIterator.html">ForwardIterator</A>>
ForwardIterator unique(ForwardIterator first, ForwardIterator last);
template <class <A href="ForwardIterator.html">ForwardIterator</A>, class <A href="BinaryPredicate.html">BinaryPredicate</A>>
ForwardIterator unique(ForwardIterator first, ForwardIterator last,
BinaryPredicate binary_pred);
</pre>
<h3>Description</h3>
Every time a consecutive group of duplicate elements appears in the
range <tt>[first, last)</tt>, the algorithm <tt>unique</tt> removes all but the
first element. That is, <tt>unique</tt> returns an iterator
<tt>new_last</tt> such that the range <tt>[first, new_last)</tt> contains no
two consecutive elements that are duplicates. <A href="#1">[1]</A>
The iterators in the range <tt>[new_last, last)</tt> are all still
dereferenceable, but the elements that they point to are unspecified.
<tt>Unique</tt> is stable, meaning that the relative order of elements that
are not removed is unchanged.
<P>
The reason there are two different versions of <tt>unique</tt> is that there
are two different definitions of what it means for a consecutive group
of elements to be duplicates. In the first version, the test is
simple equality: the elements in a range <tt>[f, l)</tt> are duplicates if,
for every iterator <tt>i</tt> in the range, either <tt>i == f</tt> or else <tt>*i == *(i-1)</tt>.
In the second, the test is an arbitrary <A href="BinaryPredicate.html">Binary Predicate</A>
<tt>binary_pred</tt>: the elements in <tt>[f, l)</tt> are duplicates if, for every
iterator <tt>i</tt> in the range, either <tt>i == f</tt> or else
<tt>binary_pred(*i, *(i-1))</tt> is <tt>true</tt>. <A href="#2">[2]</A>
<h3>Definition</h3>
Defined in the standard header <A href="algorithm">algorithm</A>, and in the nonstandard
backward-compatibility header <A href="algo.h">algo.h</A>.
<h3>Requirements on types</h3>
For the first version:
<UL>
<LI>
<tt>ForwardIterator</tt> is a model of <A href="ForwardIterator.html">Forward Iterator</A>.
<LI>
<tt>ForwardIterator</tt> is mutable.
<LI>
<tt>ForwardIterator</tt>'s value type is <A href="EqualityComparable.html">Equality Comparable</A>.
</UL>
For the second version:
<UL>
<LI>
<tt>ForwardIterator</tt> is a model of <A href="ForwardIterator.html">Forward Iterator</A>.
<LI>
<tt>ForwardIterator</tt> is mutable.
<LI>
<tt>BinaryPredicate</tt> is a model of <A href="BinaryPredicate.html">Binary Predicate</A>. <A href="#3">[3]</A>
<LI>
<tt>ForwardIterator</tt>'s value type is convertible to <tt>BinaryPredicate</tt>'s
first argument type and to <tt>BinaryPredicate</tt>'s second argument type.
</UL>
<h3>Preconditions</h3>
<UL>
<LI>
<tt>[first, last)</tt> is a valid range.
</UL>
<h3>Complexity</h3>
Linear. Exactly <tt>(last - first) - 1</tt> applications of <tt>operator==</tt>
(in the case of the first version of <tt>unique</tt>) or of <tt>binary_pred</tt>
(in the case of the second version).
<h3>Example</h3>
Remove duplicates from consecutive groups of equal <tt>int</tt>s.
<pre>
<A href="Vector.html">vector</A><int> V;
V.push_back(1);
V.push_back(3);
V.push_back(3);
V.push_back(3);
V.push_back(2);
V.push_back(2);
V.push_back(1);
<A href="Vector.html">vector</A><int>::iterator new_end = unique(V.begin(), V.end());
<A href="copy.html">copy</A>(V.begin(), new_end, <A href="ostream_iterator.html">ostream_iterator</A><int>(cout, " "));
// The output it "1 3 2 1".
</pre>
<P>
Remove all duplicates from a vector of <tt>char</tt>s, ignoring case. First
sort the vector, then remove duplicates from consecutive groups.
<pre>
inline bool eq_nocase(char c1, char c2) { return tolower(c1) == tolower(c2); }
inline bool lt_nocase(char c1, char c2) { return tolower(c1) < tolower(c2); }
int main()
{
const char init[] = "The Standard Template Library";
<A href="Vector.html">vector</A><char> V(init, init + sizeof(init));
<A href="sort.html">sort</A>(V.begin(), V.end(), lt_nocase);
<A href="copy.html">copy</A>(V.begin(), V.end(), <A href="ostream_iterator.html">ostream_iterator</A><char>(cout));
cout << endl;
<A href="Vector.html">vector</A><char>::iterator new_end = unique(V.begin(), V.end(), eq_nocase);
<A href="copy.html">copy</A>(V.begin(), new_end, <A href="ostream_iterator.html">ostream_iterator</A><char>(cout));
cout << endl;
}
// The output is:
// aaaabddeeehiLlmnprrrStTtTy
// abdehiLmnprSty
</pre>
<h3>Notes</h3>
<P><A name="1">[1]</A>
Note that the meaning of "removal" is somewhat subtle. <tt>Unique</tt>,
like <tt><A href="remove.html">remove</A></tt>, does not destroy any iterators and does not change
the distance between <tt>first</tt> and <tt>last</tt>. (There's no way that it
could do anything of the sort.) So, for example, if <tt>V</tt> is a
<A href="Vector.html">vector</A>, <tt>remove(V.begin(), V.end(), 0)</tt> does not change
<tt>V.size()</tt>: <tt>V</tt> will contain just as many elements as it did before.
<tt>Unique</tt> returns an iterator that points to the end of the resulting
range after elements have been removed from it; it follows that the
elements after that iterator are of no interest. If you are operating
on a <A href="Sequence.html">Sequence</A>, you may wish to use the <A href="Sequence.html">Sequence</A>'s <tt>erase</tt>
member function to discard those elements entirely.
<P><A name="2">[2]</A>
Strictly speaking, the first version of <tt>unique</tt> is redundant:
you can achieve the same functionality by using an object of class
<tt><A href="equal_to.html">equal_to</A></tt> as the <A href="BinaryPredicate.html">Binary Predicate</A> argument. The first version
is provided strictly for the sake of convenience: testing for equality
is an important special case.
<P><A name="3">[3]</A>
<tt>BinaryPredicate</tt> is not required to be an equivalence
relation. You should be cautious, though, about using <tt>unique</tt> with a
<A href="BinaryPredicate.html">Binary Predicate</A> that is not an equivalence relation: you could
easily get unexpected results.
<h3>See also</h3>
<tt><A href="BinaryPredicate.html">Binary Predicate</A></tt>, <tt><A href="remove.html">remove</A></tt>, <tt><A href="remove_if.html">remove_if</A></tt>, <tt><A href="unique_copy.html">unique_copy</A></tt>,
<tt><A href="adjacent_find.html">adjacent_find</A></tt>,
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