View Compilation Error

0_0_34390022_10553.cpp: In function 'int main()':
0_0_34390022_10553.cpp:25:24: error: 'scanf_s' was not declared in this scope
  while (scanf_s("%s", a) != EOF)
                        ^
0_0_34390022_10553.cpp:28:31: error: 'strlen' was not declared in this scope
   for (int i = 0; i < strlen(a); i++)
                               ^
0_0_34390022_10553.cpp:37:30: error: 'strlen' was not declared in this scope
   for (int i = b; i <strlen(a); i++)
                              ^