View Compilation Error

0_0_17541906_27160.cpp: In function 'int main()':
0_0_17541906_27160.cpp:12:19: error: 'strlen' was not declared in this scope
   int len=strlen(a);
                   ^
0_0_17541906_27160.cpp:17:4: error: 'cout' was not declared in this scope
    cout<<a[i];
    ^
0_0_17541906_27160.cpp:19:3: error: 'cout' was not declared in this scope
   cout<<endl;
   ^
0_0_17541906_27160.cpp:19:9: error: 'endl' was not declared in this scope
   cout<<endl;
         ^