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AS- Level WJEC CS SAQ > 1.6 Organisation and Structure of Data > Flashcards

1.6 Organisation and Structure of Data Flashcards

1
Q

What is the purpose of a file in data processing?

A

To store data which can be used by programs that process the data.

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2
Q

What is a file?

A

An organized collection of related records.

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3
Q

What is a record?

A

A collection of related fields.

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4
Q

What is a field?

A

A single data item.

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5
Q

Give 3 facts on a fixed length field (1).

A
  1. The number of characters to be stored is set in advance.
  2. Data entered can be shorter but not longer than field length.
  3. Same number of bytes in each record.
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6
Q

Give 3 facts on a fixed length field (2).

A
  1. Quicker to process as start/end locations are known.
  2. Wastes space as fields have blank space.
  3. Fixed length records will truncate long fields.
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7
Q

Give 2 examples on the use of a fixed length field.

A
  1. Postcode
  2. National insurance numbers
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8
Q

Give 4 facts on a variable length field.

A
  1. There is no maximum or a very high maximum field length.
    .
  2. Different number of bytes in each record.
  3. Saves storage space as no blank space.
  4. Avoids truncation as each field can extend to the number of characters needed.
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9
Q

Give 2 examples on the use of a variable length field.

A
  1. Address
  2. Personal statement

(Data that can be of any length).

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10
Q

State 2 advantages for a fixed length field.

A
  1. Access is fast as computer knows where each record starts.
  2. Easier to program as it is easier to calculate how much space is required.
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11
Q

State 2 disadvantages for a fixed length field.

A
  1. Records are usually larger and need more storage space.
  2. Slower to transfer (load or save) or transmit down a network.
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12
Q

State 3 advantages for a variable length field.

A
  1. The records will be smaller and will need less storage space.
  2. The records will load or save faster.
  3. The file will be faster to transfer or transmit down a network.
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13
Q

State 2 disadvantages for a variable length field.

A
  1. The computer will be unable to determine where each record starts so processing the records will be slower.
  2. Harder to calculate how much
    storage space is needed.
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14
Q

What is a transaction file? (2)

A
  1. It contains updates to be applied to a master file. eg. Recent purchases.
  2. It is a temporary file.
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15
Q

What is a master file? (2)

A
  1. The main file. eg. Bank balance.
  2. Semi-permanent, Most data is never changed but some of the fields may need to be updated occasionally.
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16
Q

What are the 4 access methods of a file?

A
  1. Sequential
  2. Indexed sequential
  3. Direct (aka random)
  4. Serial
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17
Q

What are sequential files?

A

Stored in order of a record key (numerical order) eg. Class register.

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18
Q

How is a new record added to a sequential file? (4)

A
  1. New copy of the file is made.
  2. Records before the new record are copied from old file to new.
  3. New record is added to new file.
  4. The rest of the records from the old file to the new.
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19
Q

How is a record deleted from a sequential file? (3)

A
  1. A new copy of the file is made.
  2. A file is copied to a new file one record
    at a time.
  3. The record to be deleted is not copied across.
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20
Q

State 2 advantages for sequential files.

A
  1. Easier to program as fewer overheads.
  2. Fast access to record.
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21
Q

What are indexed sequential files? (3)

A
  1. The record key and pointers are stored together in order of record key.
  2. The bulk of the record is stored anywhere on the disk.
  3. The pointer shows where the rest of the file is located.
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22
Q

What are the 2 parts a record in an indexed sequential file is split into?

A
  1. A record key and pointer
  2. The bulk of the record
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23
Q

What is an indexed sequential file similar to?

A

A table of contents in a book.

24
Q

How are new records added/ deleted to an indexed sequential file?

A

The record keys are added/ deleted as a normal sequential file.

25
Q

What is a multilevel index in an indexed sequential file?

A
  1. Main index which contain location of next index.
  2. May extend to many levels with the last index containing the physical address of the record.
26
Q

State 3 advantages for an indexed sequential file.

A
  1. Faster access than sequential because it can move directly to individual records.
  2. Avoids overheads of random access.
  3. If only sequential access is required it should be faster than random.
27
Q

What are direct access files?

A

Hashing algorithm is applied to the record key to produce a disk location.

28
Q

State 2 advantages for a random access file.

A
  1. Allows for very fast access irrespective of position in file.
  2. Suitable for large files which need this sort of access.
29
Q

When does a data collision occur and what is the solution?

A

When two data items are hashed to the same location, there needs to be an overflow area where the latest data is stored in linear structure.

30
Q

What is an overflow area?

A

Data is stored here when the disk block is full.

31
Q

What happens when an overflow area is full? (2)

A
  1. Access becomes slow.
  2. A new hashing algorithm is required and a larger file is needed.
32
Q

What are serial files? (2)

A
  1. Records stored in no particular order.
  2. New record are appended to the end of the file e.g. shopping list.
33
Q

Give 3 methods of file security.

A
  1. File Backup
  2. Generations of files
  3. Transaction logs
34
Q

Give an example of policy for backing up data (4).

A
  1. Copy data to remote or removable media.
  2. Repeat this process on a regular basis (hourly or daily).
  3. Store data off site, in a fireproof safe, or both.
  4. Lost data can be retrieved by copying a recent backup.
35
Q

How do file generations work for file backup? (4)

A
  1. The Grandfather (a) and transaction file (b) are the oldest files.
  2. Master file (a) and transaction file (b) were processed to produce master file (c) (the father level).
  3. Same process can be used to produce master file (e) (son level) using master file (c) and transaction file (d).
  4. If master files (e) and (c) becomes corrupted they can still be restored as long as you have the transactions files (b) and (d).
36
Q

What are transaction files? (2)

A
  1. A record of all the transactions that have taken place that day.
  2. In needed, the latest backup of the database can be updated using the transaction file.
37
Q

What is file archiving?

A

Copying/ storing data files no longer in use on a secure and safe medium.

38
Q

What is the organisation of an unsorted transaction file?

A

Serial

39
Q

What is the organisation of a master file?

A

Sequential

40
Q

State the difference between data validation and verification (2).

A
  1. Validation checks that data is in the required format. It is sensible.
  2. Verification makes sure data is inputted into the computer is accurate.
41
Q

State 3 data verification checks.

A
  1. Screen verification
  2. Double- keying
  3. Check digit
42
Q

Describe screen verification (2).

A
  1. A visual check by eye that no errors have been made.
  2. Printing out data that has been inputted, and sending it back to source of data to check
    e.g., printed proofs of books for proof- reading.
43
Q

Describe double- keying (2).

A
  1. Check to see if data has been keyed correctly.
  2. Data is inputted twice and the computer will only accept the data for processing if the two versions are identical e.g., new passwords.
44
Q

Describe check digit (3).

A
  1. On numerical data an extra digit is added to the data.
  2. This check digit is the result of a calculation on the number e.g., last digit of a bar code.
  3. When a bar code is re-entered the check digit verified.
45
Q

Give 7 methods of validation checks.

A
  1. Character
  2. Format
  3. Length
  4. Range
  5. Lookup
  6. Presence check
  7. Type check
46
Q

What is a character?

A

Only numeric characters in numbers.

47
Q

Give an example of a character.

A

Name “Paul23” is rejected.

48
Q

What is a format?

A

It checks that the data is in a particular format.

49
Q

Give an example of format.

A

Postcode e.g., CF83 8PU

50
Q

What is a length?

A

Data has the correct number of characters.

51
Q

Give an example of length.

A

Telephone numbers

52
Q

What is a range?

A

Allow a number between two values.

53
Q

Give an example of range.

A

Persons age

54
Q

What is a lookup?

A

User selects value from list rather than type in.

55
Q

What is a presence check? (2)

A
  1. Checks that something has been inputted.
  2. Reject empty field.
56
Q

What is a type check?

A

Checks that an appropriate data type was entered.

AS- Level WJEC CS SAQ (14 decks)

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