Index sequential files are files that are organised in a but which also have an index. A text book is a little like an index sequential file. The chapters of the book are organised in a order, one after the other, but there is also an index. The logical order of the chapters lets you work through each chapter in turn. The index lets you look up any particular topic. A file of employee records may be organised as an index sequential file. Sometimes, when the payslips are being produced, the file needs to be accessed sequentially during processing. Sometimes, when someone changes address, for example, an individual's file needs to be retrieved quickly.

An example:



Index sequential files can have many levels of . For a bank account number such as 45843500, there may be an index for the first two numbers. There will be one pointers in this index (because the first two digits in the bank account number could be 00, 01, 02 and so on up to 99). Each of the one hundred pointers points to a new area in memory. Each new area of memory will then have another index that has hundred pointers that can be used for the third and fourth digits, and so on. Any record can be quickly found by examining each pair of digits in a bank account number and using the indexes in .

The best way of describing an index sequential file in an exam is not to try and describe it using words but to draw a !