STL containers and algorithm C++

Question

I been stumped on a problem for a while. I can't seem to check a text file with a set of excluded words before inputing it into a map container. I tried many things but it just can't seem to solve it. I'm new to C++ and just started to learn STL and containers.

using namespace std;
//checking I know is wrong but I do not know how to compare the pair with the set.

bool checking(pair<string, int> const & a, set<string> const &b) {
    return a.first != b;
}

void print(pair<string, int> const & a) {cout << a.first << "  " << a.second << endl;}

int main() {

    ifstream in("document.txt");
    ifstream exW("excluded.txt");

    map<string, int> M;
    set<string> words;

    copy(istream_iterator<string>(exW),
         istream_iterator<string>(),
         inserter(words, begin(words)));

    //Need to exlclude certain words before copying into a Map
    // CAN NOT USE FOR LOOP
    //I cant seem to get the predicate right.
    copy_if(istream_iterator<string>(in),
            istream_iterator<string>(),
    [&](const string & s) { M[s]++;},
    checking);

    for_each(begin(M),
             end(M),
             [](pair<string, int> const & a) 
             {
                 cout << a.first << "  " <<  a.second << endl;
             }
    );

    return 0;
}

Any tips or advice word be great!

What exactly are you trying to achieve? You cannot "compare" a string with a set, they represent different concepts altogether. Are you trying to see if the string belongs to the set? — vsoftco
– vsoftco, Commented Nov 24, 2015 at 23:02
@vsoftco I"m trying to read the sample.txt and copy_if not exclude words into a map container. — Nahniv
– Nahniv, Commented Nov 24, 2015 at 23:19
The copy_if predicate needs to return true if you want the string copied, and false if not. — Jonathan Potter
– Jonathan Potter, Commented Nov 24, 2015 at 23:21

vsoftco · Accepted Answer · 2015-11-24 23:58:58Z

2

I'd do it like this, using a lambda expression as your test, so this can get you started:

#include <set>
#include <fstream>
#include <iostream>
#include <algorithm>
#include <iterator>

using namespace std;

int main() 
{
    ifstream in("document.txt");
    ifstream exW("excluded.txt");

    set<string> words{istream_iterator<string>(exW),{}}; // here we store the excluded words

    copy_if(istream_iterator<string>(in),
            istream_iterator<string>(), // can also use just {} instead
            ostream_iterator<string>(std::cout," "), // output to std::cout
            [&words](const std::string& word) // this is how the predicate should look
            {
                return words.find(word) == words.end(); // true if not found
            }
            );
}

Note that I output directly to std::cout in the std::copy_if. You can of course use an iterator into some container instead (your std::map for example). Also remark that the predicate takes a std::string as an input (that's what you verify) and checks whether it belongs to the std::set of excluded words, returning a bool. Also words needs to be captured inside the lambda. I capture it by reference so you don't end up with an additional copy.

edited Nov 24, 2015 at 23:58

answered Nov 24, 2015 at 23:25

vsoftco

56.9k12 gold badges150 silver badges269 bronze badges

Sign up to request clarification or add additional context in comments.

2 Comments

Nahniv Over a year ago

@vsofto Thanks vsoft! Is there any source you recommend for learning and understand about stl containers and iterators better?

vsoftco Over a year ago

@Nahniv stackoverflow.com/questions/388242/… The ultimate standard library book (IMO) is The C++ Standard Library by Nicolai Josuttis.

Vlad from Moscow · Accepted Answer · 2015-11-25 00:11:46Z

If you need to use a standard algorithm instead of a loop then I can suggest to use standard algorithm std::accumulate declared in header <numeric>

Here is a demonstrative program. Instead of the files I am using string streams.

#include <iostream>
#include <set>
#include <map>
#include <string>
#include <sstream>
#include <numeric>
#include <iterator>

int main( void )
{
    std::istringstream exclude( "two four six" );
    std::set<std::string> words( ( std::istream_iterator<std::string>( exclude ) ),
                                 std::istream_iterator<std::string>() ); 

    for ( const auto &t : words ) std::cout << t << ' ';
    std::cout << std::endl;

    std::cout << std::endl;

    std::map<std::string, int> m;

    std::istringstream include( "one two three four five six five four one one" );

    std::accumulate( std::istream_iterator<std::string>( include ),
                     std::istream_iterator<std::string>(),
                     &m,
                     [&]( std::map<std::string, int> *acc, const std::string &t )
                     {
                         if ( !words.count( t ) ) ++( *acc )[t];
                         return acc;
                     } );

    for ( const auto &p : m ) std::cout << p.first << '\t' << p.second << std::endl;                     
}

The program output is

four six two 

five    2
one 3
three   1

For readability of the program the lambda definition can be placed outside the algorithm call. For example

auto add_if_not_in_set = [&]( std::map<std::string, int> *acc, const std::string &t )
{
    if ( !words.count( t ) ) ++( *acc )[t];
    return acc;
};

//...

std::accumulate( std::istream_iterator<std::string>( include ),
                 std::istream_iterator<std::string>(),
                 &m, add_if_not_in_set );

Or as @T.C. pointed out a more simplified approach is to use standard algorithm std::for_each

For example

#include <iostream>
#include <set>
#include <map>
#include <string>
#include <sstream>
#include <algorithm>
#include <iterator>

int main( void )
{
    std::istringstream exclude( "two four six" );
    std::set<std::string> words( ( std::istream_iterator<std::string>( exclude ) ),
                                 std::istream_iterator<std::string>() ); 

    for ( const auto &t : words ) std::cout << t << ' ';
    std::cout << std::endl;

    std::cout << std::endl;

    std::map<std::string, int> m;


    std::istringstream include( "one two three four five six five four one one" );

    std::for_each( std::istream_iterator<std::string>( include ),
                   std::istream_iterator<std::string>(),
                   [&m, &words]( const std::string &s )
                   {
                       if ( !words.count( s ) ) ++m[s];
                   } );

    for ( const auto &p : m ) std::cout << p.first << '\t' << p.second << std::endl;                     
}

Usually the same task can be done in several ways using different algorithms.:)

Why are you using accumulate and passing an accumulator that doesn't get changed ever? std::for_each( std::istream_iterator<std::string>( include ), std::istream_iterator<std::string>(), [&]( const std::string &t ) { if ( !words.count( t ) ) ++m[t]; } ); is shorter and arguably easier to understand.
@T.C. I did not use std::for_each because I thought that it is the same as the range-based for loop.:) So I thought about some other algorithm.:)

Collectives™ on Stack Overflow

STL containers and algorithm C++

2 Answers 2

2 Comments

2 Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

2 Comments

2 Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related