Language: C++

#include <bits/stdc++.h>
using namespace std;
int main(){
  vector<char> s(5);
  cin >> s[0] >> s[1] >> s[2]>> s[3];
  int a=s[0]-'0',b=s[1]-'0',c=s[2]-'0',d=s[3]-'0';
  if(a+b+c+d==7); cout << s[0]+'+'s[1]+'+'+s[2]+'+'+s[3]+'=7';
  else if(a+b+c-d==7); cout << s[0]+'+'s[1]+'+'+s[2]+'-'+s[3]+'=7';
  else if(a+b-c+d==7); cout << s[0]+'+'s[1]+'-'+s[2]+'+'+s[3]+'=7';
  else if(a+b-c-d==7); cout << s[0]+'+'s[1]+'-'+s[2]+'-'+s[3]+'=7';
  else if(a-b+c+d==7); cout << s[0]+'-'s[1]+'+'+s[2]+'+'+s[3]+'=7';
  else if(a-b+c-d==7); cout << s[0]+'-'s[1]+'+'+s[2]+'-'+s[3]+'=7';
  else if(a-b-c+d==7); cout << s[0]+'-'s[1]+'-'+s[2]+'+'+s[3]+'=7';
  else if(a-b-c-d==7); cout << s[0]+'-'s[1]+'-'+s[2]+'-'+s[3]+'=7';

Compiler output:
a.cc:7:58: warning: multi-character character constant [-Wmultichar]
    7 |   if(a+b+c+d==7); cout << s[0]+'+'s[1]+'+'+s[2]+'+'+s[3]+'=7';
      |                                                          ^~~~
a.cc:8:63: warning: multi-character character constant [-Wmultichar]
    8 |   else if(a+b+c-d==7); cout << s[0]+'+'s[1]+'+'+s[2]+'-'+s[3]+'=7';
      |                                                               ^~~~
a.cc:9:63: warning: multi-character character constant [-Wmultichar]
    9 |   else if(a+b-c+d==7); cout << s[0]+'+'s[1]+'-'+s[2]+'+'+s[3]+'=7';
      |                                                               ^~~~
a.cc:10:63: warning: multi-character character constant [-Wmultichar]
   10 |   else if(a+b-c-d==7); cout << s[0]+'+'s[1]+'-'+s[2]+'-'+s[3]+'=7';
      |                                                               ^~~~
a.cc:11:63: warning: multi-character character constant [-Wmultichar]
   11 |   else if(a-b+c+d==7); cout << s[0]+'-'s[1]+'+'+s[2]+'+'+s[3]+'=7';
      |                                                               ^~~~
a.cc:12:63: warning: multi-character character constant [-Wmultichar]
   12 |   else if(a-b+c-d==7); cout << s[0]+'-'s[1]+'+'+s[2]+'-'+s[3]+'=7';
      |                                                               ^~~~
a.cc:13:63: warning: multi-character character constant [-Wmultichar]
   13 |   else if(a-b-c+d==7); cout << s[0]+'-'s[1]+'-'+s[2]+'+'+s[3]+'=7';
      |                                                               ^~~~
a.cc:14:63: warning: multi-character character constant [-Wmultichar]
   14 |   else if(a-b-c-d==7); cout << s[0]+'-'s[1]+'-'+s[2]+'-'+s[3]+'=7';
      |                                                               ^~~~
a.cc: In function 'int main()':
a.cc:7:32: error: unable to find character literal operator 'operator""s' with 'char' argument
    7 |   if(a+b+c+d==7); cout << s[0]+'+'s[1]+'+'+s[2]+'+'+s[3]+'=7';
      |                                ^~~~
a.cc:8:3: error: 'else' without a previous 'if'
    8 |   else if(a+b+c-d==7); cout << s[0]+'+'s[1]+'+'+s[2]+'-'+s[3]+'=7';
      |   ^~~~
a.cc:8:37: error: unable to find character literal operator 'operator""s' with 'char' argument
    8 |   else if(a+b+c-d==7); cout << s[0]+'+'s[1]+'+'+s[2]+'-'+s[3]+'=7';
      |                                     ^~~~
a.cc:9:3: error: 'else' without a previous 'if'
    9 |   else if(a+b-c+d==7); cout << s[0]+'+'s[1]+'-'+s[2]+'+'+s[3]+'=7';
      |   ^~~~
a.cc:9:37: error: unable to find character literal operator 'operator""s' with 'char' argument
    9 |   else if(a+b-c+d==7); cout << s[0]+'+'s[1]+'-'+s[2]+'+'+s[3]+'=7';
      |                                     ^~~~
a.cc:10:3: error: 'else' without a previous 'if'
   10 |   else if(a+b-c-d==7); cout << s[0]+'+'s[1]+'-'+s[2]+'-'+s[3]+'=7';
      |   ^~~~
a.cc:10:37: error: unable to find character literal operator 'operator""s' with 'char' argument
   10 |   else if(a+b-c-d==7); cout << s[0]+'+'s[1]+'-'+s[2]+'-'+s[3]+'=7';
      |                                     ^~~~
a.cc:11:3: error: 'else' without a previous 'if'
   11 |   else if(a-b+c+d==7); cout << s[0]+'-'s[1]+'+'+s[2]+'+'+s[3]+'=7';
      |   ^~~~
a.cc:11:37: error: unable to find character literal operator 'operator""s' with 'char' argument
   11 |   else if(a-b+c+d==7); cout << s[0]+'-'s[1]+'+'+s[2]+'+'+s[3]+'=7';
      |                                     ^~~~
a.cc:12:3: error: 'else' without a previous 'if'
   12 |   else if(a-b+c-d==7); cout << s[0]+'-'s[1]+'+'+s[2]+'-'+s[3]+'=7';
      |   ^~~~
a.cc:12:37: error: unable to find character literal operator 'operator""s' with 'char' argument
   12 |   else if(a-b+c-d==7); cout << s[0]+'-'s[1]+'+'+s[2]+'-'+s[3]+'=7';
      |                                     ^~~~
a.cc:13:3: error: 'else' without a previous 'if'
   13 |   else if(a-b-c+d==7); cout << s[0]+'-'s[1]+'-'+s[2]+'+'+s[3]+'=7';
      |   ^~~~
a.cc:13:37: error: unable to find character literal operator 'operator""s' with 'char' argument
   13 |   else if(a-b-c+d==7); cout << s[0]+'-'s[1]+'-'+s[2]+'+'+s[3]+'=7';
      |                                     ^~~~
a.cc:14:3: error: 'else' without a previous 'if'
   14 |   else if(a-b-c-d==7); cout << s[0]+'-'s[1]+'-'+s[2]+'-'+s[3]+'=7';
      |   ^~~~
a.cc:14:37: error: unable to find character literal operator 'operator""s' with 'char' argument
   14 |   else if(a-b-c-d==7); cout << s[0]+'-'s[1]+'-'+s[2]+'-'+s[3]+'=7';
      |                                     ^~~~
a.cc:14:68: error: expected '}' at end of input
   14 |   else if(a-b-c-d==7); cout << s[0]+'-'s[1]+'-'+s[2]+'-'+s[3]+'=7';
      |                                                                    ^
a.cc:3:11: note: to match this '{'
    3 | int main(){
      |           ^