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2. Linked Data Structures

In our example application, we need to record the names of all the people that enroll in a course. We have already established that it is more efficient to use dynamic allocation to record their names. Therefore, for each name we know starting at what memory location we have copied it. We call this memory location the address of the name.

Now, in order to record the names of all the students in the course, what should we do? We could use an array that's big enough to accommodate the largest number of students we can reasonably expect. For each student, we'd record his/her name as illustrated earlier, and store the corresponding address in the next available cell of the array.

This array might have to be quite big. And what if we wanted to do the same for all courses offered at the university. We'd have to allocate very large arrays for each course, even though most of them would not be at full capacity; therefore, large portions of each array would remain used, yet would consume memory.

Let's take a look at what our example might look like. For concreteness we suppose that the name JOE starts at memory location 1 and MARYJANE at memory location 7.

Note the order in which names (or rather the indices of their memory location) are entered into the array. The first student is in the first position; the next student is in the next position, etc... When we consult this array, we rely on the expectation that the next student can be found in the next location. We depend on the fact that these locations are contiguous. In other words, the logical relation next is represented by the physical order of the locations in the array.

With a linked representation, this logical relation is represented explicitly. Along with each student entry, we store the address of the next student entry:

The first student entry, that of JOE, begins at location 1. It starts with the address of the next student entry, which happens to be MARYJANE at location 8. The entry for MARYJANE is linked to the next entry (not shown in the diagram) that begins, say, at location 23 for example.

Each student entry is linked to the next one. What happens when there is no next one? How do we recognize the end of a chain? We use the invalid address 0 for this purpose. In C, this invalid address is represented by the constant NULL (not to be confused with the NUL character '\0', although they both happen to be defined as 0). Thus, if there were no further entries after MARYJANE, we would have:

You can see that this method is much more economical than to statically allocate a large array. Each student entry can be stored anywhere in memory, and we can examine in turn all the students enrolled in a course by following the links until we find one whose value is NULL.

Furthermore, we can similarly envision a linked data structure of all courses offered at the university. Each course would be represented by a pair of positions (dynamically allocated): the first would point to the first student in the first course (see above), and the second would point to the next course. For example, we might have something like this:

2.1. Conclusion

The combination of dynamic allocation and linked data structures is extremely attractive because it utilizes memory with very high efficiency. There is a price to be paid however.