If you think of the rows (or columns) of a two-dimensional array as records, then what has been demonstrated amounts to five basic ways to store records so that selection and traversal are possible. The methods for storing a collection of records are as follows:
1. Fixed-length records of the same type are stored sequentially in a single array and accessed by calculating record and member offsets.
2. In the C structure, each array entry is a record, perhaps including unions.
3. Fixed-length records of the same type are stored in multiarrays, each of appropriate type to match a member type.
4. Use relative pointer arrays to records stored in an array.
5. Use pointer variable arrays to records stored in the dynamic memory.
The first approach is enlightening because it makes explicit the offset calculations that are usually provided invisibly by the compiler. A severe disadvantage of this approach is that the representation of different type records causes difficulties?/FONT>awkward in some languages, and impossible in others.
The second approach is the elegant form of this representation. It is possible in a language that provides the record data structure in which calculations are provided by the compiler. The disadvantage of the first approach then disappears.
The method of multiple arrays is applicable to all languages that provide arrays, whether or not they have strong typing or the record data structure. This method, and the first, sacrifice the ability to refer to an entire record by a single term, as provided by the newer C compilers.
The two pointer-oriented methods work for collections of variable-length records but are convenient only when the number of distinct lengths is small. (The more general case is treated in the next chapter.) Relative pointers are easy to use but have the drawback that embedding records in an array raises the problems of data types clashing.
Pointers to dynamic memory are more difficult and dangerous to use, for reasons mentioned later, but they provide extra flexibility. In any event, storing records in an array or in dynamic memory eliminates the need to declare a variable name for them at the outset of a program; records can thus be created dynamically during program execution.
The array is limited in the number of records it can store by its fixed length, which must be declared prior to program execution. The capacity of dynamic memory can be much larger and need not be declared a priori, but it is determined by the computer system. As will become more apparent, dynamic memory allows more efficient use of available storage.