Language: C++

#include<bits/stdc++.h>
using namespace std;
int main(){
  cin>>string a;
  int s= atoi(s.at(0));
  vector<int> d;
  d.push_back(s);
  int f;
  int cnt=0;
  for(int i=1;i<4;i++){
    f =d.at(i-1) + a.at(i);
    d.push_back(f);
    f =d.at(i-1) - a.at(i);
    d.push_back(f);
  }
    for(int i=0;i<4;i++){
    f =d.at(i+2) + a.at(3);
    d.push_back(f);
    cnt++;
      if(f == 7)
        break;
    f =d.at(i+3) - a.at(3);
    d.push_back(f);
    cnt++;
      if(f == 7)
        break;
  }
  vector<string> h;
  int r = 3;
  while(r){
    if(cnt % 2 == 1){
      cnt -= 1;
      cnt = cnt /2;
      h.push_back(1);
    }else{
      cnt = cnt /2;
      h.push_back(0);
    }
    r--;
  }
  reverse(h.begin(),h.end());
  string k;
  int r = 3;
  for(r = 0;r<3;r++){
    k += a.at(r);
    if(h.at(r) == 0)
      k += "+";
    if(h.at(r) == 1)
      k += "-";
  }
  cout << k << a.at(3) << "=7" <<endl;
}
  
  
    

Compiler output:
a.cc: In function 'int main()':
a.cc:4:15: error: expected primary-expression before 'a'
    4 |   cin>>string a;
      |               ^
a.cc:5:17: error: request for member 'at' in 's', which is of non-class type 'int'
    5 |   int s= atoi(s.at(0));
      |                 ^~
a.cc:11:20: error: 'a' was not declared in this scope
   11 |     f =d.at(i-1) + a.at(i);
      |                    ^
a.cc:17:20: error: 'a' was not declared in this scope
   17 |     f =d.at(i+2) + a.at(3);
      |                    ^
a.cc:34:18: error: no matching function for call to 'std::vector<std::__cxx11::basic_string<char> >::push_back(int)'
   34 |       h.push_back(1);
      |       ~~~~~~~~~~~^~~
In file included from /usr/include/c++/14/vector:66,
                 from /usr/include/c++/14/functional:64,
                 from /usr/include/x86_64-linux-gnu/c++/14/bits/stdc++.h:53,
                 from a.cc:1:
/usr/include/c++/14/bits/stl_vector.h:1283:7: note: candidate: 'void std::vector<_Tp, _Alloc>::push_back(const value_type&) [with _Tp = std::__cxx11::basic_string<char>; _Alloc = std::allocator<std::__cxx11::basic_string<char> >; value_type = std::__cxx11::basic_string<char>]'
 1283 |       push_back(const value_type& __x)
      |       ^~~~~~~~~
/usr/include/c++/14/bits/stl_vector.h:1283:35: note:   no known conversion for argument 1 from 'int' to 'const std::vector<std::__cxx11::basic_string<char> >::value_type&' {aka 'const std::__cxx11::basic_string<char>&'}
 1283 |       push_back(const value_type& __x)
      |                 ~~~~~~~~~~~~~~~~~~^~~
/usr/include/c++/14/bits/stl_vector.h:1300:7: note: candidate: 'void std::vector<_Tp, _Alloc>::push_back(value_type&&) [with _Tp = std::__cxx11::basic_string<char>; _Alloc = std::allocator<std::__cxx11::basic_string<char> >; value_type = std::__cxx11::basic_string<char>]'
 1300 |       push_back(value_type&& __x)
      |       ^~~~~~~~~
/usr/include/c++/14/bits/stl_vector.h:1300:30: note:   no known conversion for argument 1 from 'int' to 'std::vector<std::__cxx11::basic_string<char> >::value_type&&' {aka 'std::__cxx11::basic_string<char>&&'}
 1300 |       push_back(value_type&& __x)
      |                 ~~~~~~~~~~~~~^~~
a.cc:43:7: error: redeclaration of 'int r'
   43 |   int r = 3;
      |       ^
a.cc:29:7: note: 'int r' previously declared here
   29 |   int r = 3;
      |       ^
a.cc:45:10: error: 'a' was not declared in this scope
   45 |     k += a.at(r);
      |          ^
a.cc:46:16: error: no match for 'operator==' (operand types are '__gnu_cxx::__alloc_traits<std::allocator<std::__cxx11::basic_string<char> >, std::__cxx11::basic_string<char> >::value_type' {aka 'std::__cxx11::basic_string<char>'} and 'int')
   46 |     if(h.at(r) == 0)
      |        ~~~~~~~ ^~ ~
      |            |      |
      |            |      int
      |            __gnu_cxx::__alloc_traits<std::allocator<std::__cxx11::basic_string<char> >, std::__cxx11::basic_string<char> >::value_type {aka std::__cxx11::basic_string<char>}
In file included from /usr/include/c++/14/regex:68,
                 from /usr/include/x86_64-linux-gnu/c++/14/bits/stdc++.h:181:
/usr/include/c++/14/bits/regex.h:1103:5: note: candidate: 'template<class _BiIter> bool std::__cxx11::operator==(const sub_match<_BiIter>&, const sub_match<_BiIter>&)'
 1103 |     operator==(const sub_match<_BiIter>& __lhs, const sub_match<_BiIter>& __rhs)
      |     ^~~~~~~~
/usr/include/c++/14/bits/regex.h:1103:5: note:   template argument deduction/substitution failed:
a.cc:46:19: note:   '__gnu_cxx::__alloc_traits<std::allocator<std::__cxx11::basic_string<char> >, std::__cxx11::basic_string<char> >::value_type' {aka 'std::__cxx11::basic_string<char>'} is not derived from 'const std::__cxx11::sub_match<_BiIter>'
   46 |     if(h.at(r) == 0)
      |                   ^
/usr/include/c++/14/bits/regex.h:1199:5: note: candidate: 'template<class _Bi_iter, class _Ch_traits, class _Ch_alloc> bool std::__cxx11::operator==(__sub_match_string<_Bi_iter, _Ch_traits, _Ch_alloc>&, const sub_match<_BiIter>&)'
 1199 |     operator==(const __sub_match_string<_Bi_iter, _Ch_traits, _Ch_alloc>& __lhs,
      |     ^~~~~~~~
/usr/include/c++/14/bits/regex.h:1199:5: note:   template argument deduction/substitution failed:
a.cc:46:19: note:   mismatched types 'const std::__cxx11::sub_match<_BiIter>' and 'int'
   46 |     if(h.at(r) == 0)
      |                   ^
/usr/include/c++/14/bits/regex.h:1274:5: note: candidate: 'template<class _Bi_iter, class _Ch_traits, class _Ch_alloc> bool std::__cxx11::operator==(const sub_match<_BiIter>&, __sub_match_string<_Bi_iter, _Ch_traits, _Ch_alloc>&)'
 1274 |     operator==(const sub_match<_Bi_iter>& __lhs,
      |     ^~~~~~~~
/usr/include/c++/14/bits/regex.h:1274:5: note:   template argument deduction/substitution failed:
a.cc:46:19: note:   '__gnu_cxx::__alloc_traits<std::allocator<std::__cxx11::basic_string<char> >, std::__cxx11::basic_string<char> >::value_type' {aka 'std::__cxx11::basic_string<char>'} is not derived from 'const std::__cxx11::sub_match<_BiIter>'
   46 |     if(h.at(r) == 0)
      |                   ^
/usr/include/c++/14/bits/regex.h:1366:5: note: candidate: 'template<class _Bi_iter> bool std::__cxx11::operator==(const typename std::iterator_traits<_Iter>::value_type*, const sub_match<_BiIter>&)'
 1366 |     operator==(typename iterator_traits<_Bi_iter>::value_type const* __lhs,
      |     ^~~~~~~~
/usr/include/c++/14/bits/regex.h:1366:5: note:   template argument deduction/substitution failed:
a.cc:46:19: note:   mismatched types 'const std::__cxx11::sub_match<_BiIter>' and 'int'
   46 |     if(h.at(r) == 0)
      |                   ^
/usr/include/c++/14/bits/regex.h:1441:5: note: candidate: 'template<class _Bi_iter> bool std::__cxx11::operator==(const sub_match<_BiIter>&, const typename std::iterator_traits<_Iter>::value_type*)'
 1441 |     operator==(const sub_match<_Bi_iter>& __lhs,
      |     ^~~~~~~~
/usr/include/c++/14/bits/regex.h:1441:5: note:   template argument deduction/substitution failed:
a.cc:46:19: note:   '__gnu_cxx::__alloc_traits<std::allocator<std::__cxx11::basic_string<char> >, std::__cxx11::basic_string<char> >::value_type' {aka 'std::__cxx11::basic_string<char>'} is not derived from 'const std::__cxx11::sub_match<_BiIter>'
   46 |     if(h.at(r) == 0)
      |                   ^
/usr/include/c++/14/bits/regex.h:1534:5: note: candidate: 'template<class _Bi_iter> bool std::__cxx11::operator==(const typename std::iterator_traits<_Iter>::value_type&, const sub_match<_BiIter>&)'
 1534 |     operator==(typename iterator_traits<_Bi_iter>::value_type const& __lhs,
      |     ^~~~~~~~
/usr/include/c++/14/bits/regex.h:1534:5: note:   template argument deduction/substitution failed:
a.cc:46:19: note:   mismatched types 'const std::__cxx11::sub_match<_BiIter>' and 'int'
   46 |     if(h.at(r) == 0)
      |                   ^
/usr/include/c++/14/bits/regex.h:1613:5: note: candidate: 'template<class _Bi_iter> bool std::__cxx11::operator==(const sub_match<_BiIter>&, const typename std::iterator_traits<_Iter>::value_type&)'
 1613 |     operator==(const sub_match<_Bi_iter>& __lhs,
      |     ^~~~~~~~
/usr/include/c++/14/bits/regex.h:1613:5: note:   template argument deduction/substitution failed:
a.cc:46:19: note:   '__gnu_cxx::__alloc_traits<std::allocator<std::__cxx11::basic_string<char> >, std::__cxx11::basic_string<char> >::value_type' {aka 'std::__cxx11::basic_string<char>'} is not derived from 'const std::__cxx11::sub_match<_BiIter>'
   46 |     if(h.at(r) == 0)
      |                   ^
/usr/include/c++/14/bits/regex.h:2186:5: note: candidate: 'template<class _Bi_iter, class _Alloc> bool std::__cxx11::operator==(const match_results<_BiIter, _Alloc>&, const match_results<_BiIter, _Alloc>&)'
 2186 |     operator==(const match_results<_Bi_iter, _Alloc>& __m1,
      |     ^~~~~~~~
/usr/include/c++/14/bits/regex.h:2186:5: note:   template argument deduction/substitution failed:
a.cc:46:19: note:   '__gnu_cxx::__alloc_traits<std::allocator<std::__cxx11::basic_string<char> >, std::__cxx11::basic_string<char> >::value_type' {aka 'std::__cxx11::basic_string<char>'} is not derived from 'const std::__cxx11::match_results<_BiIter, _Alloc>'
   46 |     if(h.at(r) == 0)
      |                   ^
In file included from /usr/include/c++/14/bits/stl_algobase.h:64,
                 from /usr/include/c++/14/algorithm:60,
                 from /usr/include/x86_64-linux-gnu/c++/14/bits/stdc++.h:51:
/usr/include/c++/14/bits/stl_pair.h:1033:5: note: candidate: 'template<class _T1, class _T2> constexpr bool std::operator==(const pair<_T1, _T2>&, const pair<_T1, _T2>&)'
 1033 |     operator==(const pair<_T1, _T2>& __x, const pair<_T1, _T2>& __y)
      |     ^~~~~~~~
/usr/include/c++/14/bits/stl_pair.h:1033:5: note:   template argument deduction/substitution failed:
a.cc:46:19: note:   '__gnu_cxx::__alloc_traits<std::allocator<std::__cxx11::basic_string<char> >, std::__cxx11::basic_string<char> >::value_type' {aka 'std::__cxx11::basic_string<char>'} is not derived from 'const std::pair<_T1, _T2>'
   46 |     if(h.at(r) == 0)
      |                   ^
In file included from /usr/include/c++/14/bits/stl_algobase.h:67:
/usr/include/c++/14/bits/stl_iterator.h:441:5: note: candidate: 'template<class _Iterator> constexpr bool std::operator==(const reverse_iterator<_Iterator>&, const reverse_iterator<_Iterator>&)'
  441 |     operator==(const reverse_iterator<_Iterator>& __x,
      |     ^~~~~~~~
/usr/include/c++/14/bits/stl_iterator.h:441:5: note:   template argument deduction/substitution failed:
a.cc:46:19: note:   '__gnu_cxx::__alloc_traits<std::allocator<std::__cxx11::basic_string<char> >, std::__cxx11::basic_string<char> >::value_type' {aka 'std::__cxx11::basic_string<char>'} is not derived from 'const std::reverse_iterator<_Iterator>'
   46 |     if(h.at(r) == 0)
      |                   ^
/usr/include/c++/14/bits/stl_iterator.h:486:5: note: candidate: 'template<class _IteratorL, class _IteratorR> constexpr bool std::operator==(const reverse_iterator<_Iterator>&, const reverse_iterator<_IteratorR>&)'
  486 |     operator==(const reverse_iterator<_IteratorL>& __x,
      |     ^~~~~~~~
/usr/include/c++/14/bits/stl_iterator.h:486: