C++ copy cin into cout directly but in reverse order - c++

Question

Is there is any similar solution to this command:
using namespace std;
copy(istream_iterator<string>(cin), istream_iterator<string>(),ostream_iterator<string>(cout, "\n"));
-- this command copies everything into cout but I would like to change it to copy the string in reverse order so I have used this:
using namespace std;
reverse_copy(istream_iterator<string>(cin), istream_iterator<string>(),ostream_iterator<string>(cout, "\n"));
-- but this did not even compile. Are there any solutions to this? Thank you

Answer 1

The first two arguments to std::reverse_copy must be Bidirectional Iterator whereas std::istream_iterator is Input Iterator which cannot behave as Bidirectional Iterator. That explains why it doesn't work — it wouldn't even compile.
You've to write your own iterator — or do it manually in a loop — to solve this problem (which is not clear as to what mean by reverse : given foo bar as input, do you want bar foo or oof rab, or rab oof? as many of the commenters say).

Answer 2

You can write a recursive function. For example
#include <iostream>
#include <string>
#include <sstream>
std::ostream & reverse_output( std::istream &is = std::cin,
std::ostream &os = std::cout )
{
std::string s;
if ( is >> s ) reverse_output( is, os ) << s << '\n';
return os;
}
int main()
{
std::istringstream is( "Hello Bobul Mentol" );
reverse_output( is );
}
The program output is
Mentol
Bobul
Hello
Of course instead of the string stream I used for the demonstrative purpose you can use std::cin. In this case the call of the function will look just like
reverse_output();
Otherwise you need to store the input in some container and use it to reverse the inputted data for outputing.
For example
std::vector<std::string> v( std::istream_iterator<std::string>( std::cin ),
std::istream_iterator<std::string>() );
std::reverse_copy( v.begin(), v.end(),
std::ostream_iterator<std::string>( std::cout, "\n" ) );

Answer 3

I have never heard of some standard algorithm which can copy a reversed collection by an input_iterator or even forward_iterator - probably if this exist, it requires at least bidirectional_iterator.
So, you can use the temporary collection to store the values read, like this:
vector<string> tmp;
copy(istream_iterator<string>(cin), istream_iterator<string>(), back_inserter(tmp));
copy(tmp.rbegin(), tmp.rend(), ostream_iterator<string>(cout, "\n"));

Answer 4

There is a general problem here: std::cin is a stream. When it is attached to a file, you could imagine a way to initially find the size and so how to know where the reverse iterator should start. But when it is attached to a terminal, with an imprevisible human being able to type input data at will, at what position should the reverse iterator start? I have no idea of it, and it looks like cin designer had no more - more seriously, cin does not propose a reverse iterator, and it is by design.
If you want to present what has been inputted in cin but in reverse order, you must first specify:
what is the piece of input to reverse: anything until stream is closed, or anything until first end of line, or [put here your own definition]. Once it's done you have the start place for your reverse iterator
what is the unit to be reversed: one character or one word at a time. Once this is done, you know what your reverse iterator should return.
The implementation could use a vector of strings. You consistently accumulate words or single characters in it until what you have defined as the end of the stream. Once you hit the end, you hold a container with bidirectional iterators so copying it in reverse order should be easy.