Answers

A3.8 Fields, records and file organisation

Data structures · A level · AQA 7517 4.2.1.3, Eduqas A500QS 2.4 · about 25 min

What this lesson is about

Text and binary files, fixed-length records, and serial, sequential, indexed sequential and direct access files.

• Fields, records and files
• Text files and binary files
• Serial files
• Sequential files
• Indexed sequential files
• Direct access files

Questions 6 marks in all

1. [1 mark]What is the difference between a serial file and a sequential file?

   1. AA sequential file keeps its records in key order; a serial file keeps them in the order they were added
   2. BA serial file keeps its records in key order; a sequential file keeps them in the order they were added
   3. CA serial file has an index; a sequential file does not
   4. DA sequential file uses a hashing function to place records
   Answer: A. Both are read from the start, but only a sequential file is ordered by its key field.

2. [1 mark]Which file organisations let a program go straight to, or near, a record without reading from the start of the file?

   Tick every answer that is true.

   1. AIndexed sequential
   2. BDirect access
   3. CSerial
   4. DSequential
   Answer: A, B. An index or a hashing function gives the record's location. Serial and sequential files are read from the start.

3. [1 mark]A file holds fixed-length records of 32 bytes each, starting at byte 0. At which byte does record number 5 start, counting records from 0?

   Answer: 160. Record n starts at n x record length = 5 x 32 = 160.

4. [1 mark]An airline's booking system looks up and changes one seat record at a time as customers call. Which file organisation suits it best?

   1. ADirect access
   2. BSerial
   3. CSequential
   4. DA text file read line by line
   Answer: A. Direct access finds any single record with one calculation, which is what one-at-a-time look-ups need.

5. [1 mark]Put the steps for updating a sequential master file from a transaction file in order.

   Number the lines 1 to 5 to put them in the right order.

   1. When the keys match, apply the transaction and write the updated record
   2. Sort the transaction file into the same key order as the master file
   3. When one file runs out, copy the rest of the other file
   4. Write whichever current record has the smaller key to the new master file, and read the next from that file
   5. Read the first record from each file
   Answer:

   Sort the transaction file into the same key order as the master file
   Read the first record from each file
   Write whichever current record has the smaller key to the new master file, and read the next from that file
   When the keys match, apply the transaction and write the updated record
   When one file runs out, copy the rest of the other file

   Both files must be in key order so a single merge pass can compare them record by record.

6. [1 mark]What is an advantage of a binary file over a text file for storing records?

   1. AValues are stored in their internal form, so records are smaller and need no conversion from characters
   2. BA person can read and edit it in any text editor
   3. CIt does not need the program to know the record layout
   4. DIt can only hold strings
   Answer: A. A number takes a fixed few bytes instead of one character per digit. The price is that people cannot read it and the program must know the layout.

The task: update the master file

# the two lines every program starts with: the commands, then the robot
from bugbot import *
connect()

with open("master.txt") as f:
    master = f.read().splitlines()
with open("results.txt") as f:
    results = f.read().splitlines()

# the two lines every program starts with: the commands, then the robot
from bugbot import *
connect()

with open("master.txt") as f:
    master = f.read().splitlines()
with open("results.txt") as f:
    results = f.read().splitlines()

out = open("new_master.txt", "w")
out.write(master[0] + "\n")
m, t = 1, 1
improved, added = 0, 0
while m < len(master) or t < len(results):
    if t >= len(results):
        out.write(master[m] + "\n")
        m = m + 1
        continue
    if m >= len(master):
        out.write(results[t] + "\n")
        added = added + 1
        t = t + 1
        continue
    mid, mname, mbest = master[m].split(",")
    tid, tname, tcm = results[t].split(",")
    if int(mid) < int(tid):
        out.write(master[m] + "\n")
        m = m + 1
    elif int(tid) < int(mid):
        out.write(results[t] + "\n")
        added = added + 1
        t = t + 1
    else:
        if float(tcm) > float(mbest):
            out.write(f"{mid},{mname},{tcm}\n")
            improved = improved + 1
        else:
            out.write(master[m] + "\n")
        m = m + 1
        t = t + 1
out.close()
print(f"{improved} records improved, {added} added")