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copied!<p><a href="http://www.cplusplus.com/reference/std/iterator/istream_iterator/" rel="nofollow"><code>istream_iterator</code></a> allows you to iteratively extract items from an <a href="http://www.cplusplus.com/reference/iostream/istream/" rel="nofollow"><code>istream</code></a>, which you pass in to the constructor. The <code>eof</code> object is explained thus:</p> <blockquote> <p>A special value for this iterator exists: the end-of-stream; When an iterator is set to this value has either reached the end of the stream (operator void* applied to the stream returns false) or has been constructed using its default constructor (without associating it with any basic_istream object).</p> </blockquote> <p><a href="http://www.cplusplus.com/reference/algorithm/for_each/" rel="nofollow"><code>for_each</code></a> is a loop construct that takes iterator #1 and increments it until it becomes equal with iterator #2. Here it takes the iterator that wraps standard input <a href="http://www.cplusplus.com/reference/iostream/cin/" rel="nofollow"><code>cin</code></a> and increments it (which translates to extracting items) until there is no more input to consume -- this makes <code>input</code> compare equal to <code>eof</code> and the loop ends.</p> <p>The construct <code>[&] (int x) { frequency[x]++; }</code> is an <a href="http://en.wikipedia.org/wiki/Anonymous_function#C.2B.2B" rel="nofollow">anonymous function</a>; it is simply a shorthand way to write functions inline. Approximately the same effect could be achieved with</p> <pre><code>unordered_map<int,int> frequency; // this NEEDS to be global now istream_iterator<int> input(cin); istream_iterator<int> eof; void consume(int x) { frequency[x]++; } for_each(input, eof, consume); </code></pre> <p>So in a nutshell: this code reads integers from standard input until all available data is consumed, keeping a count of each integer's appearance frequency in a map.</p>

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