Question

I have following code:

#include <iostream>
#include <fstream>
#include <algorithm>
#include <iterator>
#include <vector>
#include <functional>

using namespace std;

typedef istream_iterator<string> is_it;
typedef vector<string>::iterator v_str_it;

int main()
{
    int i = 4;
    ifstream ifstr("1.txt");
    is_it ifstrm(ifstr);
    is_it eof;
    vector<string> v_str(ifstrm, eof);
    v_str_it vsit = v_str.begin();
    while( (vsit = find_if(vsit, v_str.end(),
        bind2nd(equal_to<string>(), i ))) != v_str.end())
    {
        cout << *vsit << endl;
        ++vsit;
    }
    return 0;
}

As far as I understand in find_if(vsit, v_str.end(), bind2nd(equal_to<string>(), i ) i should use const char like "sometext" instead of int i. But how can i find words with length equal to 4 e.g. ? I’m confused and need some advice.

Answer 1

find_if will only return the first item in the sequence that satisfies the predicate.

For this you really want a lambda and if you are using C++11. This will look something like:

[](std::string const& x) { return x.size() == i; }

(Not sure of the exact syntax).

To create a “functor” which is the simplest here you might do:

struct CompareStringLength
{
   int len_;
   explicit CompareStringLength( int len ) : len_(len)
   {
   }

   bool operator()(std::string const& str ) const
   {
      return str.size() == len_;
   }
};

Within your vector you would now use std::find_if( v.begin(), v.end(), CompareStringLength(i) );

to get the first element. To find all of them there is no std::copy_if to copy them into another vector so you’d actually have to create a different predicate that returns the opposite and use remove_copy_if which does exist or write your own copy_if algorithm.