Records in sequential files are stored one after another. The files can be read forward or backward but only one record at a time in the order in which they are written. New records can only be added to the end of the file. In some cases, existing records can be updated but their record length may not be changed. Existing records can not be deleted. If you need to delete records the normal technique is to read the old file as input and write the updated file as output. At the end of the process the two files can be renamed if desired.
ISAM files are actually two sequential files in one. There is an index file and a data file. The data file part is just like a regular sequential file although more sophisticated access methods provide for the ability to leave gaps for additional records to be inserted so not all new records would have to be added at the end of the file. The index file part contains only the primary keys in key sequence and the physical address of the associated data record. When you read an ISAM file with a key, the access method finds the matching key in the index and uses that to find the associated data record in the data file. The common searching method for indexes is a binary search. You should be able to find a formula that tells you how to calculate how many physical reads are required to find any index value. The technique starts by dividing the number of records in half and accessing the index record at the midpoint of the file. The search compares the index value with the one being sought and depending on whether it is higher or lower, halfs the appropriate section of the index again. The algorithm keeps dividing by 2 and comparing the search value to the index value until the record being sought is found. As you can see, even files containing millions of records require remarkably few physical reads to find any record. That is why indexes are so important to ISAM files and also for RDBMS'.
I assume this is a school assignment. Don't take the lazy route and give my answer unless you want to give me credit. Do some research of your own also.
