foo.cc: In function ‘int main()’:
foo.cc:8:18: warning: comparison of integer expressions of different signedness: ‘int’ and ‘size_t’ {aka ‘long unsigned int’} [-Wsign-compare]
    8 |     for(int i=0;i<strlen(a);i++)
      |                 ~^~~~~~~~~~
foo.cc:13:24: warning: array subscript has type ‘char’ [-Wchar-subscripts]
   13 |                 num[a[i]]++;
      |                     ~~~^