C error with pointer and const char[]

char *ptr;
for (ptr= arr; *ptr!= ''; ptr++)
  /* some code*/

const char* ptr;

const char* ptr;

char* ptr;              // Both pointer & pointed-to value are non-const
const char* ptr;        // Pointed-to value is const, pointer is non-const
char* const ptr;        // Pointed-to value is non-const, pointer is const
const char* const ptr;  // Both pointer & pointed-to value are const.

const char* const ptr;