View Compilation Error

0_0_27900252_20144.cpp: In function 'int main()':
0_0_27900252_20144.cpp:24:7: error: 'len' was not declared in this scope
    nb[len - 1 - i] = a[i] - '0';
       ^
0_0_27900252_20144.cpp:24:17: error: 'i' was not declared in this scope
    nb[len - 1 - i] = a[i] - '0';
                 ^