Types of Files
There are two main ways a file can be organized:
- Sequential Access — The data are placed in the file in a sequence like beads on a string. Data are processed in sequence, one after another. To reach a particular item of data, all the data that proceeds it first must be read.
- Random Access — The data are placed into the file by going directly to the location in the file assigned to each data item. Data are processed in any order. A particular item of data can be reached by going directly to it, without looking at any other data.
A sequential file works like a reel of tape. (In fact, sequential files are often stored on reels of magnetic tape.) Data in a sequential file are processed in order, starting with the first item, the processing the second, then the third and so on. To reach data in the middle of the file you must go through all the data that preceeds it.
A random access file works like a sampler box of chocolate. You can access a particular item by going directly to it. Data in a random acess file may be accessed in any order. To read data in the middle of the file you can go directly to it. Random access files are sometimes called direct access files.
You might think that all files should be random access. But random access files are much more difficult to implement and to process. They are expensive in computer time and resources. However, they are convenient for the users of a program. For example, a corporation can easily answer telephone queries from a customer if it can go directly to the customer's records.
