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c++ std::mismatch problem with vector<string>

Posted on 2013-12-13
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Last Modified: 2013-12-14
ref: std::mismatch
http://www.cplusplus.com/reference/algorithm/mismatch/?kw=mismatch

In this reference is sample code that uses std::vector<int>. When I modify the program to use std::vector<std::string>, the mismatch() returns two bad pointers in the pair. I added a std::equal() and it appears to work correctly. I also modified the sample program for  set_intersection to use std::vector<std::string>,  and it worked ok.
http://www.cplusplus.com/reference/algorithm/set_intersection/

How do you fix this program using std::mismatch? I am actually using std::set<std::string> so I don't need to sort it. (I would like to keep this in the non-C++11 domain as the OS is older and does not support this.)


// mismatch algorithm example
#include <iostream>     // std::cout
#include <algorithm>    // std::mismatch
#include <vector>       // std::vector
#include <string>
#include <utility>      // std::pair

bool mypredicate (std::string i, std::string j) {
  return (i==j);
}

int main () {

  bool isSame=false;
  std::string myStrs[] = {"10","20","80","320","1024"};   // myStrs: 10 20 80 320 1024
  std::vector<std::string> myvector(myStrs, myStrs + 5);
  std::vector<std::string> myvector1(myvector.begin(), myvector.end());

  // std::equal works, but std::mismatch returns bad pointers
  isSame = std::equal(myvector.begin(), myvector.end(), myvector1.begin());
  std::pair< std::vector<std::string>::iterator,std::vector<std::string>::iterator> mypair;
  mypair = std::mismatch (myvector.begin(), myvector.end(), myvector1.begin());
  std::cout << "First mismatching elements: " << *mypair.first;
  std::cout << " and " << *mypair.second << '\n';

  ++mypair.first; ++mypair.second;

  // using predicate comparison:
  mypair = std::mismatch (myvector.begin(), myvector.end(), myvector1.begin(), mypredicate);
  std::cout << "Second mismatching elements: " << *mypair.first;
  std::cout << " and " << *mypair.second << '\n';

  return 0;
}

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Question by:phoffric
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by:phoffric
ID: 39718244
Update: Although the intersection program worked with vector<string>, it did not compile with set<string>. But that can be another question. In case you are interested, the compiler error is on the std::set_intersection line. But when I make v (in line 12) a std::vector<std::string>, then it compiles, but gives an exception.
#include <iostream>     // std::cout
#include <algorithm>    // std::set_intersection, std::sort
#include <set>       // std::set
#include <vector>
#include <string>

int main () {
  std::string first[] = {"5","10","15","20","25"};
  std::string second[] = {"50","40","30","20","10"};
  std::set<std::string> f1(first, first+5);
  std::set<std::string> f2(second, second+5);
  std::set<std::string> v;
//  std::set<std::string>::iterator it;

  std::set_intersection (f1.begin(), f1.end(), f2.begin(), f2.end(), v.begin());

The compiler error is:
 error C2678: binary '=' : no operator found which takes a left-hand operand of type 'const std::basic_string<_Elem,_Traits,_Ax>' (or there is no acceptable conversion)

or

/usr/lib/gcc/i686-pc-cygwin/4.7.3/include/c++/bits/stl_algo.h: In instantiation of ‘_OIter std::set_intersection(_IIter1, _IIter1, _IIter2, _IIter2, _OIter) [with _IIter1 = std::_Rb_tree_const_iterator<std::basic_string<char> >; _IIter2 = std::_Rb_tree_const_iterator<std::basic_string<char> >; _OIter = std::_Rb_tree_const_iterator<std::basic_string<char> >]’:
set_intersection.cpp:17:79:   required from here
/usr/lib/gcc/i686-pc-cygwin/4.7.3/include/c++/bits/stl_algo.h:5914:6: error: passing ‘const std::basic_string<char>’ as ‘this’ argument of ‘std::basic_string<_CharT, _Traits, _Alloc>& std::basic_string<_CharT, _Traits, _Alloc>::operator=(const std::basic_string<_CharT, _Traits, _Alloc>&) [with _CharT = char; _Traits = std::char_traits<char>; _Alloc = std::allocator<char>; std::basic_string<_CharT, _Traits, _Alloc> = std::basic_string<char>]’ discards qualifiers [-fpermissive]

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LVL 32

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by:phoffric
ID: 39718291
UPDATE #2: Got intersection program in my previous post to work by making line 12 std::vector<std::string> v(10);
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Accepted Solution

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phoffric earned 0 total points
ID: 39719118
I added this code to the OP and now it works with vector<string>
  if (mypair.first == myvector.end()) {
     std::cout << "mypair.first == myvector.end()" << std::endl;
     exit(0);
  }

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I will try using set<string> to see if that works.

Update: After changing to set<string> and testing for the no-mismatch case, the program does not crash. The bad ptr that I referred to in the OP was actually myvector.end() which cannot be dereferenced.
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