Language: C++

#include<iostream>
#include<cstdio>
#include<cctype>
using namespace std;

int main(){
	char ch;
	while(1){
	scanf("%c",&ch);
	if(islower(ch))cout << (char)toupper(ch);
	else if (isupper(ch))cout << (char)tolower(ch);
	else if(ch == '\n')break;
????????????else cout << ch;
	}
	cout << "\n";
	return 0;
}

Compiler output:
a.cc: In function 'int main()':
a.cc:13:1: error: expected primary-expression before '?' token
   13 | ????????????else cout << ch;
      | ^
a.cc:13:2: error: expected primary-expression before '?' token
   13 | ????????????else cout << ch;
      |  ^
a.cc:13:3: error: expected primary-expression before '?' token
   13 | ????????????else cout << ch;
      |   ^
a.cc:13:4: error: expected primary-expression before '?' token
   13 | ????????????else cout << ch;
      |    ^
a.cc:13:5: error: expected primary-expression before '?' token
   13 | ????????????else cout << ch;
      |     ^
a.cc:13:6: error: expected primary-expression before '?' token
   13 | ????????????else cout << ch;
      |      ^
a.cc:13:7: error: expected primary-expression before '?' token
   13 | ????????????else cout << ch;
      |       ^
a.cc:13:8: error: expected primary-expression before '?' token
   13 | ????????????else cout << ch;
      |        ^
a.cc:13:9: error: expected primary-expression before '?' token
   13 | ????????????else cout << ch;
      |         ^
a.cc:13:10: error: expected primary-expression before '?' token
   13 | ????????????else cout << ch;
      |          ^
a.cc:13:11: error: expected primary-expression before '?' token
   13 | ????????????else cout << ch;
      |           ^
a.cc:13:12: error: expected primary-expression before '?' token
   13 | ????????????else cout << ch;
      |            ^
a.cc:13:13: error: expected primary-expression before 'else'
   13 | ????????????else cout << ch;
      |             ^~~~
a.cc:13:13: error: expected ':' before 'else'
   13 | ????????????else cout << ch;
      |             ^~~~
      |             :
a.cc:13:13: error: expected primary-expression before 'else'
   13 | ????????????else cout << ch;
      |             ^~~~
a.cc:13:13: error: expected ':' before 'else'
   13 | ????????????else cout << ch;
      |             ^~~~
      |             :
a.cc:13:13: error: expected primary-expression before 'else'
   13 | ????????????else cout << ch;
      |             ^~~~
a.cc:13:13: error: expected ':' before 'else'
   13 | ????????????else cout << ch;
      |             ^~~~
      |             :
a.cc:13:13: error: expected primary-expression before 'else'
   13 | ????????????else cout << ch;
      |             ^~~~
a.cc:13:13: error: expected ':' before 'else'
   13 | ????????????else cout << ch;
      |             ^~~~
      |             :
a.cc:13:13: error: expected primary-expression before 'else'
   13 | ????????????else cout << ch;
      |             ^~~~
a.cc:13:13: error: expected ':' before 'else'
   13 | ????????????else cout << ch;
      |             ^~~~
      |             :
a.cc:13:13: error: expected primary-expression before 'else'
   13 | ????????????else cout << ch;
      |             ^~~~
a.cc:13:13: error: expected ':' before 'else'
   13 | ????????????else cout << ch;
      |             ^~~~
      |             :
a.cc:13:13: error: expected primary-expression before 'else'
   13 | ????????????else cout << ch;
      |             ^~~~
a.cc:13:13: error: expected ':' before 'else'
   13 | ????????????else cout << ch;
      |             ^~~~
      |             :
a.cc:13:13: error: expected primary-expression before 'else'
   13 | ????????????else cout << ch;
      |             ^~~~
a.cc:13:13: error: expected ':' before 'else'
   13 | ????????????else cout << ch;
      |             ^~~~
      |             :
a.cc:13:13: error: expected primary-expression before 'else'
   13 | ????????????else cout << ch;
      |             ^~~~
a.cc:13:13: error: expected ':' before 'else'
   13 | ????????????else cout << ch;
      |             ^~~~
      |             :
a.cc:13:13: error: expected primary-expression before 'else'
   13 | ????????????else cout << ch;
      |             ^~~~
a.cc:13:13: error: expected ':' before 'else'
   13 | ????????????else cout << ch;
      |             ^~~~
      |             :
a.cc:13:13: error: expected primary-expression before 'else'
   13 | ????????????else cout << ch;
      |             ^~~~
a.cc:13:13: error: expected ':' before 'else'
   13 | ????????????else cout << ch;
      |             ^~~~
      |             :
a.cc:13:13: error: expected primary-expression before 'else'
   13 | ????????????else cout << ch;
      |             ^~~~
a.cc:13:13: error: expected ':' before 'else'
   13 | ????????????else cout << ch;
      |             ^~~~
      |             :
a.cc:13:13: error: expected primary-expression before 'else'
   13 | ????????????else cout << ch;
      |             ^~~~