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< div class = "section range_reference_algorithms_introduction" >
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< div class = "titlepage" >< div >< div >< h4 class = "title" >
< a name = "range.reference.algorithms.introduction" ></ a >< a class = "link" href = "introduction.html" title = "Introduction and motivation" > Introduction
and motivation</ a >
</ h4 ></ div ></ div ></ div >
< p >
In its most simple form a < span class = "bold" >< strong > Range Algorithm</ strong ></ span >
(or range-based algorithm) is simply an iterator-based algorithm where
the < span class = "emphasis" >< em > two</ em ></ span > iterator arguments have been replaced by
< span class = "emphasis" >< em > one</ em ></ span > range argument. For example, we may write
</ p >
< p >
</ p >
< pre class = "programlisting" >< span class = "preprocessor" > #include</ span > < span class = "special" > < </ span >< span class = "identifier" > boost</ span >< span class = "special" > /</ span >< span class = "identifier" > range</ span >< span class = "special" > /</ span >< span class = "identifier" > algorithm</ span >< span class = "special" > .</ span >< span class = "identifier" > hpp</ span >< span class = "special" > > </ span >
< span class = "preprocessor" > #include</ span > < span class = "special" > < </ span >< span class = "identifier" > vector</ span >< span class = "special" > > </ span >
< span class = "identifier" > std</ span >< span class = "special" > ::</ span >< span class = "identifier" > vector</ span >< span class = "special" > < </ span >< span class = "keyword" > int</ span >< span class = "special" > > </ span > < span class = "identifier" > vec</ span > < span class = "special" > =</ span > < span class = "special" > ...;</ span >
< span class = "identifier" > boost</ span >< span class = "special" > ::</ span >< span class = "identifier" > sort</ span >< span class = "special" > (</ span >< span class = "identifier" > vec</ span >< span class = "special" > );</ span >
</ pre >
< p >
</ p >
< p >
instead of
</ p >
< p >
</ p >
< pre class = "programlisting" >< span class = "identifier" > std</ span >< span class = "special" > ::</ span >< span class = "identifier" > sort</ span >< span class = "special" > (</ span >< span class = "identifier" > vec</ span >< span class = "special" > .</ span >< span class = "identifier" > begin</ span >< span class = "special" > (),</ span > < span class = "identifier" > vec</ span >< span class = "special" > .</ span >< span class = "identifier" > end</ span >< span class = "special" > ());</ span >
</ pre >
< p >
</ p >
< p >
However, the return type of range algorithms is almost always different
from that of existing iterator-based algorithms.
</ p >
< p >
One group of algorithms, like < code class = "computeroutput" >< span class = "identifier" > boost</ span >< span class = "special" > ::</ span >< span class = "identifier" > sort</ span >< span class = "special" > ()</ span ></ code > , will simply return the same range so
that we can continue to pass the range around and/or further modify it.
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Because of this we may write
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</ p >
< pre class = "programlisting" >< span class = "identifier" > boost</ span >< span class = "special" > :</ span >< span class = "identifier" > unique</ span >< span class = "special" > (</ span >< span class = "identifier" > boost</ span >< span class = "special" > ::</ span >< span class = "identifier" > sort</ span >< span class = "special" > (</ span >< span class = "identifier" > vec</ span >< span class = "special" > ));</ span >
</ pre >
< p >
to first sort the range and then run < code class = "computeroutput" >< span class = "identifier" > unique</ span >< span class = "special" > ()</ span ></ code > on the sorted range.
</ p >
< p >
Algorithms like < code class = "computeroutput" >< span class = "identifier" > boost</ span >< span class = "special" > ::</ span >< span class = "identifier" > unique</ span >< span class = "special" > ()</ span ></ code >
fall into another group of algorithms that return (potentially) narrowed
views of the original range. By default < code class = "computeroutput" >< span class = "identifier" > boost</ span >< span class = "special" > ::</ span >< span class = "identifier" > unique</ span >< span class = "special" > (</ span >< span class = "identifier" > rng</ span >< span class = "special" > )</ span ></ code > returns the range < code class = "computeroutput" >< span class = "special" > [</ span >< span class = "identifier" > boost</ span >< span class = "special" > ::</ span >< span class = "identifier" > begin</ span >< span class = "special" > (</ span >< span class = "identifier" > rng</ span >< span class = "special" > ),</ span > < span class = "identifier" > found</ span >< span class = "special" > )</ span ></ code >
where < code class = "computeroutput" >< span class = "identifier" > found</ span ></ code > denotes the
iterator returned by < code class = "computeroutput" >< span class = "identifier" > std</ span >< span class = "special" > ::</ span >< span class = "identifier" > unique</ span >< span class = "special" > (</ span >< span class = "identifier" > boost</ span >< span class = "special" > ::</ span >< span class = "identifier" > begin</ span >< span class = "special" > (</ span >< span class = "identifier" > rng</ span >< span class = "special" > ),</ span > < span class = "identifier" > boost</ span >< span class = "special" > ::</ span >< span class = "identifier" > end</ span >< span class = "special" > (</ span >< span class = "identifier" > rng</ span >< span class = "special" > ))</ span ></ code >
</ p >
< p >
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Therefore exactly the unique values can be copied by writing
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</ p >
< pre class = "programlisting" >< span class = "identifier" > boost</ span >< span class = "special" > ::</ span >< span class = "identifier" > copy</ span >< span class = "special" > (</ span >< span class = "identifier" > boost</ span >< span class = "special" > ::</ span >< span class = "identifier" > unique</ span >< span class = "special" > (</ span >< span class = "identifier" > boost</ span >< span class = "special" > ::</ span >< span class = "identifier" > sort</ span >< span class = "special" > (</ span >< span class = "identifier" > vec</ span >< span class = "special" > )),</ span >
< span class = "identifier" > std</ span >< span class = "special" > ::</ span >< span class = "identifier" > ostream_iterator</ span >< span class = "special" > < </ span >< span class = "keyword" > int</ span >< span class = "special" > > (</ span >< span class = "identifier" > std</ span >< span class = "special" > ::</ span >< span class = "identifier" > cout</ span >< span class = "special" > ));</ span >
</ pre >
< p >
</ p >
< p >
Algorithms like < code class = "computeroutput" >< span class = "identifier" > boost</ span >< span class = "special" > ::</ span >< span class = "identifier" > unique</ span ></ code > usually return the same range:
< code class = "computeroutput" >< span class = "special" > [</ span >< span class = "identifier" > boost</ span >< span class = "special" > ::</ span >< span class = "identifier" > begin</ span >< span class = "special" > (</ span >< span class = "identifier" > rng</ span >< span class = "special" > ),</ span > < span class = "identifier" > found</ span >< span class = "special" > )</ span ></ code > . However, this behaviour may be changed
by supplying the algorithms with a template argument:
</ p >
< div class = "informaltable" >< table class = "table" >
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< p >
Expression
</ p >
</ th >
< th >
< p >
Return
</ p >
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</ tr ></ thead >
< tbody >
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< p >
< code class = "computeroutput" >< span class = "identifier" > boost</ span >< span class = "special" > ::</ span >< span class = "identifier" > unique</ span >< span class = "special" > < </ span >< span class = "identifier" > boost</ span >< span class = "special" > ::</ span >< span class = "identifier" > return_found</ span >< span class = "special" > > (</ span >< span class = "identifier" > rng</ span >< span class = "special" > )</ span ></ code >
</ p >
</ td >
< td >
< p >
returns a single iterator like < code class = "computeroutput" >< span class = "identifier" > std</ span >< span class = "special" > ::</ span >< span class = "identifier" > unique</ span ></ code >
</ p >
</ td >
</ tr >
< tr >
< td >
< p >
< code class = "computeroutput" >< span class = "identifier" > boost</ span >< span class = "special" > ::</ span >< span class = "identifier" > unique</ span >< span class = "special" > < </ span >< span class = "identifier" > boost</ span >< span class = "special" > ::</ span >< span class = "identifier" > return_begin_found</ span >< span class = "special" > > (</ span >< span class = "identifier" > rng</ span >< span class = "special" > )</ span ></ code >
</ p >
</ td >
< td >
< p >
returns the range < code class = "computeroutput" >< span class = "special" > [</ span >< span class = "identifier" > boost</ span >< span class = "special" > ::</ span >< span class = "identifier" > begin</ span >< span class = "special" > (</ span >< span class = "identifier" > rng</ span >< span class = "special" > ),</ span >
< span class = "identifier" > found</ span >< span class = "special" > )</ span ></ code >
(this is the default)
</ p >
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< code class = "computeroutput" >< span class = "identifier" > boost</ span >< span class = "special" > ::</ span >< span class = "identifier" > unique</ span >< span class = "special" > < </ span >< span class = "identifier" > boost</ span >< span class = "special" > ::</ span >< span class = "identifier" > return_begin_next</ span >< span class = "special" > > (</ span >< span class = "identifier" > rng</ span >< span class = "special" > )</ span ></ code >
</ p >
</ td >
< td >
< p >
returns the range < code class = "computeroutput" >< span class = "special" > [</ span >< span class = "identifier" > boost</ span >< span class = "special" > ::</ span >< span class = "identifier" > begin</ span >< span class = "special" > (</ span >< span class = "identifier" > rng</ span >< span class = "special" > ),</ span >
< span class = "identifier" > boost</ span >< span class = "special" > ::</ span >< span class = "identifier" > next</ span >< span class = "special" > (</ span >< span class = "identifier" > found</ span >< span class = "special" > ))</ span ></ code >
</ p >
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< tr >
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< p >
< code class = "computeroutput" >< span class = "identifier" > boost</ span >< span class = "special" > ::</ span >< span class = "identifier" > unique</ span >< span class = "special" > < </ span >< span class = "identifier" > boost</ span >< span class = "special" > ::</ span >< span class = "identifier" > return_found_end</ span >< span class = "special" > > (</ span >< span class = "identifier" > rng</ span >< span class = "special" > )</ span ></ code >
</ p >
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< td >
< p >
returns the range < code class = "computeroutput" >< span class = "special" > [</ span >< span class = "identifier" > found</ span >< span class = "special" > ,</ span >
< span class = "identifier" > boost</ span >< span class = "special" > ::</ span >< span class = "identifier" > end</ span >< span class = "special" > (</ span >< span class = "identifier" > rng</ span >< span class = "special" > ))</ span ></ code >
</ p >
</ td >
</ tr >
< tr >
< td >
< p >
< code class = "computeroutput" >< span class = "identifier" > boost</ span >< span class = "special" > ::</ span >< span class = "identifier" > unique</ span >< span class = "special" > < </ span >< span class = "identifier" > boost</ span >< span class = "special" > ::</ span >< span class = "identifier" > return_next_end</ span >< span class = "special" > > (</ span >< span class = "identifier" > rng</ span >< span class = "special" > )</ span ></ code >
</ p >
</ td >
< td >
< p >
returns the range < code class = "computeroutput" >< span class = "special" > [</ span >< span class = "identifier" > boost</ span >< span class = "special" > ::</ span >< span class = "identifier" > next</ span >< span class = "special" > (</ span >< span class = "identifier" > found</ span >< span class = "special" > ),</ span >< span class = "identifier" > boost</ span >< span class = "special" > ::</ span >< span class = "identifier" > end</ span >< span class = "special" > (</ span >< span class = "identifier" > rng</ span >< span class = "special" > ))</ span ></ code >
</ p >
</ td >
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< tr >
< td >
< p >
< code class = "computeroutput" >< span class = "identifier" > boost</ span >< span class = "special" > ::</ span >< span class = "identifier" > unique</ span >< span class = "special" > < </ span >< span class = "identifier" > boost</ span >< span class = "special" > ::</ span >< span class = "identifier" > return_begin_end</ span >< span class = "special" > > (</ span >< span class = "identifier" > rng</ span >< span class = "special" > )</ span ></ code >
</ p >
</ td >
< td >
< p >
returns the entire original range.
</ p >
</ td >
</ tr >
</ tbody >
</ table ></ div >
< p >
This functionality has the following advantages:
</ p >
< div class = "orderedlist" >< ol class = "orderedlist" type = "1" >
< li class = "listitem" >
it allows for < span class = "emphasis" >< em >< span class = "bold" >< strong > seamless functional-style
programming</ strong ></ span ></ em ></ span > where you do not need to use named
local variables to store intermediate results
</ li >
< li class = "listitem" >
it is very < span class = "emphasis" >< em >< span class = "bold" >< strong > safe</ strong ></ span ></ em ></ span >
because the algorithm can verify out-of-bounds conditions and handle
tricky conditions that lead to empty ranges
</ li >
</ ol ></ div >
< p >
For example, consider how easy we may erase the duplicates in a sorted
container:
</ p >
< p >
</ p >
< pre class = "programlisting" >< span class = "identifier" > std</ span >< span class = "special" > ::</ span >< span class = "identifier" > vector</ span >< span class = "special" > < </ span >< span class = "keyword" > int</ span >< span class = "special" > > </ span > < span class = "identifier" > vec</ span > < span class = "special" > =</ span > < span class = "special" > ...;</ span >
< span class = "identifier" > boost</ span >< span class = "special" > ::</ span >< span class = "identifier" > erase</ span >< span class = "special" > (</ span >< span class = "identifier" > vec</ span >< span class = "special" > ,</ span > < span class = "identifier" > boost</ span >< span class = "special" > ::</ span >< span class = "identifier" > unique</ span >< span class = "special" > < </ span >< span class = "identifier" > boost</ span >< span class = "special" > ::</ span >< span class = "identifier" > return_found_end</ span >< span class = "special" > > (</ span >< span class = "identifier" > boost</ span >< span class = "special" > ::</ span >< span class = "identifier" > sort</ span >< span class = "special" > (</ span >< span class = "identifier" > vec</ span >< span class = "special" > )));</ span >
</ pre >
< p >
</ p >
< p >
Notice the use of < code class = "computeroutput" >< span class = "identifier" > boost</ span >< span class = "special" > ::</ span >< span class = "identifier" > return_found_end</ span ></ code > .
What if we wanted to erase all the duplicates except one of them? In old-fashined
STL-programming we might write
</ p >
< p >
</ p >
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< pre class = "programlisting" >< span class = "comment" > // assume 'vec' is already sorted</ span >
< span class = "identifier" > std</ span >< span class = "special" > ::</ span >< span class = "identifier" > vector</ span >< span class = "special" > < </ span >< span class = "keyword" > int</ span >< span class = "special" > > ::</ span >< span class = "identifier" > iterator</ span > < span class = "identifier" > i</ span > < span class = "special" > =</ span > < span class = "identifier" > std</ span >< span class = "special" > ::</ span >< span class = "identifier" > unique</ span >< span class = "special" > (</ span >< span class = "identifier" > vec</ span >< span class = "special" > .</ span >< span class = "identifier" > begin</ span >< span class = "special" > (),</ span > < span class = "identifier" > vec</ span >< span class = "special" > .</ span >< span class = "identifier" > end</ span >< span class = "special" > ());</ span >
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< span class = "comment" > // remember this check or you get into problems</ span >
< span class = "keyword" > if</ span > < span class = "special" > (</ span >< span class = "identifier" > i</ span > < span class = "special" > !=</ span > < span class = "identifier" > vec</ span >< span class = "special" > .</ span >< span class = "identifier" > end</ span >< span class = "special" > ())</ span >
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< span class = "special" > ++</ span >< span class = "identifier" > i</ span >< span class = "special" > ;</ span >
< span class = "identifier" > vec</ span >< span class = "special" > .</ span >< span class = "identifier" > erase</ span >< span class = "special" > (</ span >< span class = "identifier" > i</ span >< span class = "special" > ,</ span > < span class = "identifier" > vec</ span >< span class = "special" > .</ span >< span class = "identifier" > end</ span >< span class = "special" > ());</ span >
</ pre >
< p >
</ p >
< p >
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The same task may be accomplished simply with
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</ p >
< pre class = "programlisting" >< span class = "identifier" > boost</ span >< span class = "special" > ::</ span >< span class = "identifier" > erase</ span >< span class = "special" > (</ span >< span class = "identifier" > vec</ span >< span class = "special" > ,</ span > < span class = "identifier" > boost</ span >< span class = "special" > ::</ span >< span class = "identifier" > unique</ span >< span class = "special" > < </ span >< span class = "identifier" > boost</ span >< span class = "special" > ::</ span >< span class = "identifier" > return_next_end</ span >< span class = "special" > > (</ span >< span class = "identifier" > vec</ span >< span class = "special" > ));</ span >
</ pre >
< p >
and there is no need to worry about generating an invalid range. Furthermore,
if the container is complex, calling < code class = "computeroutput" >< span class = "identifier" > vec</ span >< span class = "special" > .</ span >< span class = "identifier" > end</ span >< span class = "special" > ()</ span ></ code > several times will be more expensive
than using a range algorithm.
</ p >
</ div >
< table xmlns:rev = "http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width = "100%" >< tr >
< td align = "left" ></ td >
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< td align = "right" >< div class = "copyright-footer" > Copyright © 2003-2010 Thorsten Ottosen,
Neil Groves< p >
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Distributed under the Boost Software License, Version 1.0. (See accompanying
file LICENSE_1_0.txt or copy at < a href = "http://www.boost.org/LICENSE_1_0.txt" target = "_top" > http://www.boost.org/LICENSE_1_0.txt</ a > )
</ p >
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