Question

Suppose a sequential file contains 50,000 records and 5 milliseconds is required to interrogate an entry. How long should we expect to wait when retrieving a record from the middle of the file?

Short answer

125 seconds

Step-by-step solution

Understand the Problem

We need to calculate the time it will take to retrieve a record located in the middle of a sequential file that contains 50,000 records.

Identify the Position of the Middle Record

In a file with 50,000 records, the middle record is approximately at the position 25,000 because the middle of a sequence of 50,000 records is halfway through.

Calculate the Retrieval Time for the Middle Record

As each entry requires 5 milliseconds to be interrogated, we can calculate the time to reach the middle record by multiplying 25,000 by 5 milliseconds: \(25,000 \times 5\, \text{ms} = 125,000\, \text{ms}.\)

Convert Milliseconds to a More Usable Time Unit

Convert 125,000 milliseconds into seconds by dividing by 1,000: \(125,000\, \text{ms} \div 1,000 = 125\, \text{seconds}.\)

Key concepts

Record Retrieval Time

When dealing with sequential file processing, understanding record retrieval time is crucial. This time refers to how long it takes to access a specific record in a file. Sequential files require traversing records one by one, making retrieval time dependent on the position of the desired record.

Consider the file from the exercise, containing 50,000 records, with each record interrogation taking 5 milliseconds. To retrieve a record from the middle of this file, every preceding record must be checked, leading to a linear search time. Such operations mean each preceding record adds to the total retrieval time.

The middle record in our example lies at the 25,000th position. Thus, retrieval entails multiplying the interrogation time by the number of records till the target record: \(25,000 \times 5\, \text{ms} = 125,000\, \text{ms}\). Converting this into seconds makes it easier to understand, resulting in a retrieval time of 125 seconds.

File Interrogation

File interrogation is the process of checking each record to determine if it is the one of interest. In sequential file processing, to find a specific record, each preceding record must be interrogated until the desired one is found. Each interrogation takes a fixed amount of time, in this case, 5 milliseconds per record.

In practice:

• The interrogation involves reading and verifying the data to ensure it matches the search criteria.
• Since every record shares the same access time, knowing the number of records helps estimate the total time consumed.
• Interrogation time becomes significant in larger files, affecting the overall efficiency of data retrieval.

Understanding file interrogation helps in efficiently calculating how long an entire retrieval process might take based on the number of records processed.

Position Calculation

Calculating the position of a record is a fundamental step in data retrieval operations. Knowing where a record is in a sequential file allows for the estimation of retrieval times. In our example, determining the middle record at position 25,000 was key to estimating the required retrieval time.

With a starting file of 50,000 records:

• The middle is calculated as \(\frac{50,000}{2} = 25,000\), which is straightforward for an evenly numbered sequence.
• In sequential setups, positions are crucial since access is linear. Every earlier record affects the position directly related to how long retrieval will take.
• Efficient position calculation contributes to effective data management and quicker access strategies in computing scenarios.

Position calculation simplifies target finding in extensive data sets, providing a practical approach for determining access times and streamlining processing operations.