The next group of assignments and control statements are included within a "for" loop so we can build our list fast once it is defined. We will go through the loop a number of times equal to the constant "RECORDS" defined at the beginning of our program. Each time through, we allocate memory, fill the first three fields with nonsense, and fill the pointers. The pointer in the last record is given the address of this new record because the "prior" pointer is pointing to the prior record. Thus "prior->next" is given the address of the new record we have just filled.
The pointer in the new record is assigned the value "NULL", and the pointer "prior" is given the address of this new record because the next time we create a record, this one will be the prior one at that time. That may sound confusing but it really does make sense if you spend some time studying it.
When we have gone through the "for" loop 6 times, we will have a list of 7 structures including the one we generated prior to the loop. The list will have the following characteristics.
1. "start" points to the first structure in the list.
2. Each structure contains a pointer to the next structure.
3. The last structure has a pointer that points to NULL and can be used to detect the end
as shown below.
start->struct1 name breed age point->struct2 name breed age
point->struct3 name breed age point-> . . . . struct7 name breed age
point->NULL
It should be clear to you, if you understand the above structure, that it is not possible to simply jump into the middle of the structure and change a few values. The only way to get to the third structure is by starting at the beginning and working your way down through the structure one record at a time. Although this may seem like a large price to pay for the convenience of putting so much data outside of the program area, it is actually a very good way to store some kinds of data.
Aword processor would be a good application for this type of data structure because you would never need to have random access to the data. In actual practice, this is the basic type of storage used for the text in a word processor with one line of text per record. Actually, a program with any degree of sophistication would use a doubly linked list. This would be a list with two pointers per record, one pointing down to the next record, and the other pointing up to the record just prior to the one in question. Using this kind of a record structure would allow traversing the data in either direction.
The pointer in the new record is assigned the value "NULL", and the pointer "prior" is given the address of this new record because the next time we create a record, this one will be the prior one at that time. That may sound confusing but it really does make sense if you spend some time studying it.
When we have gone through the "for" loop 6 times, we will have a list of 7 structures including the one we generated prior to the loop. The list will have the following characteristics.
2. Each structure contains a pointer to the next structure.
3. The last structure has a pointer that points to NULL and can be used to detect the end
as shown below.
start->struct1 name breed age point->struct2 name breed age
point->struct3 name breed age point-> . . . . struct7 name breed age
point->NULL
It should be clear to you, if you understand the above structure, that it is not possible to simply jump into the middle of the structure and change a few values. The only way to get to the third structure is by starting at the beginning and working your way down through the structure one record at a time. Although this may seem like a large price to pay for the convenience of putting so much data outside of the program area, it is actually a very good way to store some kinds of data.
Aword processor would be a good application for this type of data structure because you would never need to have random access to the data. In actual practice, this is the basic type of storage used for the text in a word processor with one line of text per record. Actually, a program with any degree of sophistication would use a doubly linked list. This would be a list with two pointers per record, one pointing down to the next record, and the other pointing up to the record just prior to the one in question. Using this kind of a record structure would allow traversing the data in either direction.
