Question

I’ve got this short snippet of code. I don’t understand what this construction means. I know this snippet of code reads numbers from input and counts its frequency in an unordered_map. But what is [&]? And what is the meaning of (int x)? What does the input(cin) stand for? I mean the “cin” in parentheses? And how can for_each iterate from input(cin) to empty eof parameter? I don’t understand of this whole construction.

unordered_map<int,int> frequency;
istream_iterator<int> input(cin);
istream_iterator<int> eof;

for_each(input, eof, [&] (int x)
    { frequency[x]++; });

Answer 1

istream_iterator allows you to iteratively extract items from an istream, which you pass in to the constructor. The eof object is explained thus:

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).

for_each 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 cin and increments it (which translates to extracting items) until there is no more input to consume — this makes input compare equal to eof and the loop ends.

The construct [&] (int x) { frequency[x]++; } is an anonymous function; it is simply a shorthand way to write functions inline. Approximately the same effect could be achieved with

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);

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.