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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 iterator arguments have been replaced by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" >
< colgroup >
< col >
< col >
< / colgroup >
< thead > < tr >
< th >
< p >
Expression
< / p >
< / th >
< th >
< p >
Return
< / p >
< / th >
< / tr > < / thead >
< tbody >
< 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_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 >
< / 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_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 >
< / 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_found_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" > 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 >
< / 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_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 >
< / div > < / td >
< / tr > < / table >
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