Fix basic error: no match for 'operator!=' in 'i != ...

Hello all at Cprogramming. I'm hoping someone can help me with this error. It probably involves changing one line... or I've made a stupid mistake or something. I've never seen the error before though.

Anyways:

Attempt to compile with gcc: C:\>gcc -o ywordf ywordf.cpp

Error:
ywordf.cpp: In function `int main(int, char**)':
ywordf.cpp:30: error: no match for 'operator!=' in 'i != __gnu_norm::multimap<_Key, _Tp, _Compare, _Alloc>::rend() [with _Key = size_t, _Tp = std::string, _Compare = std::less<size_t>, _Alloc = std::allocator<std::pair<const size_t, std::string> >]()'

Code:

Code:

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#include <set>
#include <map>
#include <fstream>
#include <iostream>
#include <iterator>
#include <utility>

using namespace std;

int main(int ac, char** av)
{
if(ac != 2)
{
cout << "Usage: " << av[0] << " filename" << endl;
return 1;
}
cout << "Reading file " << av[1] << endl;
ifstream f(av[1]);

// read and count words
istream_iterator<string> i(f);
multiset<string> s(i, istream_iterator<string>());

// sort by count
multimap<size_t, string> wordstats;
for(multiset<string>::const_iterator i = s.begin(); i != s.end(); i = s.upper_bound(*i))
wordstats.insert( make_pair( s.count(*i), *i ));

// output in decreasing order
for( multimap<size_t, string>::const_reverse_iterator i = wordstats.rbegin(); i != wordstats.rend(); ++i)
cout << " word " << i->second << " found " << i->first << " times " << endl;
}

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The error is in line 28, which is:

Code:

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for( multimap<size_t, string>::const_reverse_iterator i = wordstats.rbegin(); i != wordstats.rend(); ++i)

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'

What on earth am I doing wrong? What do I need to change in this line?

I've never seen this error before, but I believe it has something to do with having or not having a const value in the for(multimap( paramaters.

Help! and Thank You! I would be happy to post the purpose of my code, but its not necessary. People far more experienced than I can tell exactly what I'm doing.

AA.

code formatting please

So for me, changing const_reverse_iterator to just plain reverse_iterator caused the error to go away. Not sure why, exactly.

I have no problem compiling this code on Ubuntu with g++ 4.4.1

looks like problem with your stl version

Quote:

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Originally Posted by mramazing

code formatting please

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Not really sure what you mean...

I use GCC via Cygwin, as well as Visual Studio 2010 to compile my C++. Does that help?

This topic is now resolved.

The code that I posted above can be successfully compiled with g++, via Cygwin on a Windows system. (Thanks for the heads up, vart).

It returns errors with or without const, on Visual Studio, as well as GCC.

Thank you for all your help, vart,tabstop, mramazing.

Looking at the error message, my guess is that because wordstats is non-const, wordstats.rend() returns a reverse iterator to non-const. Now, perhaps the relevant operator!= is a function template with just one template parameter, so the compiler cannot find a matching operator!= since the types of i and the return value of wordstats.rend() are different.

One way out is:

Code:

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// output in decreasing order
for( multimap<size_t, string>::const_reverse_iterator i = wordstats.rbegin(), end = wordstats.rend(); i != end; ++i)
    cout << " word " << i->second << " found " << i->first << " times " << endl;

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which also provides the benefit of not evaluating rend() on each iteration. However, MSVC9 still appears to choke, but on the printing of a const std::string. So, for the time being, it is still better to just use a reverse iterator to non-const (possibly with the caching of rend()).

Quote:

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Originally Posted by aeolusaether

Not really sure what you mean...

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Read this article on indentation.

Quote:

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Originally Posted by laserlight

Looking at the error message, my guess is that because wordstats is non-const, wordstats.rend() returns a reverse iterator to non-const. Now, perhaps the relevant operator!= is a function template with just one template parameter, so the compiler cannot find a matching operator!= since the types of i and the return value of wordstats.rend() are different.

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I didn't see it.

We need a new warning. "This would have worked except for a const mismatch."