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| <TITLE>rotate(), rotate_copy()</TITLE> |
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| <H2>rotate(), rotate_copy()</H2> |
| <P><B>Library:</B> <A HREF="2-9.html">Algorithms</A></P> |
| |
| <PRE><HR><B><I>Function</I></B><HR></PRE> |
| |
| <UL> |
| <LI><A HREF="#sec1">Local Index</A></LI> |
| <LI><A HREF="#sec2">Summary</A></LI> |
| <LI><A HREF="#sec3">Synopsis</A></LI> |
| <LI><A HREF="#sec4">Description</A></LI> |
| <LI><A HREF="#sec5">Complexity</A></LI> |
| <LI><A HREF="#sec6">Example</A></LI> |
| <LI><A HREF="#sec7">Standards Conformance</A></LI> |
| </UL> |
| <A NAME="sec1"><H3>Local Index</H3></A> |
| No Entries |
| <A NAME="sec2"><H3>Summary</H3></A> |
| <P>An algorithm that rotates the given sequence left by swapping the segment that contains elements from <SAMP>start</SAMP> through <SAMP>mid-1</SAMP> with the segment that contains the elements from <SAMP>mid</SAMP> through <SAMP>finish</SAMP></P> |
| <A NAME="sec3"><H3>Synopsis</H3></A> |
| |
| <PRE>#include <algorithm> |
| |
| namespace std { |
| template <class ForwardIterator> |
| void rotate(ForwardIterator start, |
| ForwardIterator mid, |
| ForwardIterator finish); |
| |
| template <class ForwardIterator, class OutputIterator> |
| OutputIterator rotate_copy(ForwardIterator start, |
| ForwardIterator mid, |
| ForwardIterator finish, |
| OutputIterator result); |
| } |
| </PRE> |
| <A NAME="sec4"><H3>Description</H3></A> |
| <P>The <SAMP>rotate()</SAMP> algorithm takes three iterator arguments: <SAMP>start</SAMP>, which defines the beginning of a sequence; <SAMP>finish</SAMP>, which defines the end of the sequence; and <SAMP>mid</SAMP>, which defines a point within the sequence. <SAMP>rotate()</SAMP> swaps the segment that contains elements from <SAMP>start</SAMP> through <SAMP>mid-1</SAMP> with the segment that contains the elements from <SAMP>mid</SAMP> through <SAMP>finish</SAMP>. After <SAMP>rotate()</SAMP> has been applied, the element that was in position <SAMP>mid</SAMP>, is in position<SAMP> start</SAMP>, and the other elements in that segment are in the same order relative to each other. Similarly, the element that was in position <SAMP>start</SAMP> is now in position <SAMP>finish-mid +1</SAMP>. An example illustrates how <SAMP>rotate()</SAMP> works:</P> |
| <P>Say that we have the sequence:</P> |
| <P><SAMP> 2 4 6 8 1 3 5 </SAMP></P> |
| <P>If we call <SAMP>rotate()</SAMP> with <SAMP>mid</SAMP> pointing to <SAMP>5</SAMP>, the two segments are</P> |
| <P><SAMP> 2 4 6 8 and 1 3 5 </SAMP></P> |
| <P>After we apply <SAMP>rotate()</SAMP>, the new sequence is:</P> |
| <P><SAMP> 1 3 5 2 4 6 8</SAMP></P> |
| <P>Note that the element that was in the fifth position is now in the first position, and the element that was in the first position is in position 4 (<SAMP>finish - start + 1</SAMP>, or 8 - 5 +1 =4).</P> |
| <P>The formal description of this algorithms is: for each non-negative integer <SAMP>i < (finish - start)</SAMP>, <SAMP>rotate()</SAMP> places the element from the position <SAMP>start + i</SAMP> into position <SAMP>start + (i + (finish - mid)) % (finish - start)</SAMP>.<SAMP> [start, mid)</SAMP> and <SAMP>[mid, finish)</SAMP> are valid ranges. </P> |
| <P><SAMP><A HREF="rotate.html">rotate_copy()</A></SAMP> rotates the elements as described above, but instead of swapping elements within the same sequence, it copies the result of the rotation to a sequence specified by <SAMP>result</SAMP>. <SAMP>rotate_copy()</SAMP> copies the range <SAMP>[start, finish)</SAMP> to the range <SAMP>[result, result + (finish - start))</SAMP> such that for each non- negative integer <SAMP>i < (finish - start)</SAMP> the following assignment takes place: </P> |
| |
| <UL><PRE> |
| *(result + (i + (finish - mid)) % (finish - start)) = |
| *(start + i). |
| </PRE></UL> |
| <P>The ranges <SAMP>[start, finish)</SAMP> and <SAMP>[result, result, + (finish - start)) </SAMP>may not overlap. </P> |
| <A NAME="sec5"><H3>Complexity</H3></A> |
| <P>For <SAMP>rotate()</SAMP>, at most <SAMP>finish - start </SAMP>swaps are performed.</P> |
| <P>For <SAMP><A HREF="rotate.html">rotate_copy()</A></SAMP>, <SAMP>finish - start</SAMP> assignments are performed.</P> |
| <A NAME="sec6"><H3>Example</H3></A> |
| |
| <UL><PRE>// |
| // rotate.cpp |
| // |
| |
| #include <algorithm> // for rotate |
| #include <iostream> // for cout, endl |
| #include <vector> // for vector |
| |
| |
| int main () |
| { |
| typedef std::vector<int, std::allocator<int> > Vector; |
| |
| // Initialize a vector with an array of integers. |
| const Vector::value_type arr[] = { 1, 2, 3, 4, 5, |
| 6, 7, 8, 9, 10}; |
| Vector v (arr + 0, arr + sizeof arr / sizeof *arr); |
| |
| typedef std::ostream_iterator<int, char, |
| std::char_traits<char> > |
| Iter; |
| |
| // Print out elements in original (sorted) order. |
| std::cout << "Elements before rotate: \n "; |
| std::copy (v.begin (), v.end (), Iter (std::cout, " ")); |
| |
| // Rotate the elements. |
| std::rotate (v.begin (), v.begin () + 4, v.end ()); |
| |
| // Print out the rotated elements. |
| std::cout << "\n\nElements after rotate: \n "; |
| std::copy (v.begin (), v.end (), Iter (std::cout, " ")); |
| std::cout << std::endl; |
| |
| return 0; |
| } |
| |
| |
| Program Output: |
| </PRE></UL> |
| <UL><PRE>Elements before rotate: |
| 1 2 3 4 5 6 7 8 9 10 |
| |
| Elements after rotate: |
| 5 6 7 8 9 10 1 2 3 4 |
| </PRE></UL> |
| <A NAME="sec7"><H3>Standards Conformance</H3></A> |
| <P><I>ISO/IEC 14882:1998 -- International Standard for Information Systems -- Programming Language C++, Section 25.2.10</I></P> |
| |
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