size_t first = 0;
size_t mid   = n/2;
size_t last  = n-1;

printf("first: %c (at %zu)n", a[first], first);
printf("mid:   %c (at %zu)n", a[mid],   mid);
printf("last:  %c (at %zu)n", a[last],  last);

size_t n = strlen(a);