View Compilation Error

391158_0_0_8536.cpp: In function `std::string add(std::string, std::string)':
391158_0_0_8536.cpp:29: error: `i' undeclared (first use this function)
391158_0_0_8536.cpp:29: error: (Each undeclared identifier is reported only once for each function it appears in.)
391158_0_0_8536.cpp:29: error: `REP' undeclared (first use this function)
391158_0_0_8536.cpp:30: error: expected `;' before '{' token