在C ++中使用尾部递归函数对列表计数

Question

I am doing assignment for my college. I need to create recursive function.

My list_t interface contain following functions:

List Interface
The file recursive.h defines the type "list_t" and the following operations on lists:
// EFFECTS: returns true if list is empty, false otherwise
bool list_isEmpty​ (const list_t& list);
// EFFECTS: returns an empty list.
list_t list_make​ ();
// EFFECTS: given the list (list) make a new list consisting of
// the new element followed by the elements of the
// original list.
list_t list_make​ (int elt, const list_t& list);
// REQUIRES: list is not empty
// EFFECTS: returns the first element of list
int list_first​ (const list_t& list);
// REQUIRES: list is not empty
// EFFECTS: returns the list containing all but the first element of list
list_t list_rest​ (const list_t& list);
// MODIFIES: cout
// EFFECTS: prints list to cout.
void list_print​ (const list_t& list);

Please note sum function need to be tail recursive and I can't use static or global variable.

until now I have come with this which is gives me wrong answer:

int sum(list_t list) {
    if(list.get_rest_list().is_empty())
    return list.get_first_elt() + sum(list.get_rest_list());
}

Answer 1

Let's rewrite that function with proper indentation:

int sum(list_t list)
{
    if(list.get_rest_list().is_empty())
        return list.get_first_elt() + sum(list.get_rest_list());

    // what to return here?
}

In addition to the flawed logic, you don't have all control paths covered with return statement, which causes indeterminate value to be returned if the condition is not satisfied.

(Not so) Corrected code:

int sum(list_t list)
{
    if(list.get_rest_list().is_empty())
        return list.get_first_elt();

    return list.get_first_elt() + sum(list.get_rest_list());
}

You can rewrite this using ternary operator, if you like.

But what if you pass an empty list_t ? Better do this:

int sum(list_t list)
{
    if(list.is_empty())
        return 0;

    return list.get_first_elt() + sum(list.get_rest_list());
}