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Organisation and Structure of Data (Describe sequential, indexed…
Organisation and Structure of Data
Explain the purpose of files in data processing
Define a file in terms of records and fields
File
: a
set of related records
Record
: a
collection of related fields
(treated as one unit for processing.
Field
: a
single data item
(of a record).
Explain fixed and variable length fields and records and give examples of the appropriate use of each type
Definitions
Fixed Length Record
:
same number of bytes in each record
(and same number of fields).
Variable Length Record
:
different number of bytes in each record
(OR different number of fields).
Advantages and disadvantages of Fixed Length Records
Easier to program
- can calculate storage space required.
Wasted memory
- blank space within each field.
Faster to process
- computer knows where each field starts and ends.
Truncation
- may not be enough locations in a field, will truncate.
Advantages and disadvantages of Variable Length Records
No truncation
- fields can extend to any length.
Slower to process
- computer must find where field starts and ends.
No wasted memory
- no blank space within fields.
Harder to program
- difficult to calculate storage space required.
Design files and records appropriate for a particular application
Distinguish between master and transaction files
Master File:
a computer file containing relatively permanent information, usually updated periodically.
Master files are
sequential
, e.g. Customer details.
Transaction file:
a collection of records used in batch processing to update a master file.
Transaction files are
serial
, e.g. Customer meter readings
The process of
updating a master file...
https://drive.google.com/file/d/1CewfsC2xBdrP53I5YX71uowAvb17yTBv/view?usp=sharing
Explain the need for file security, including file backup, generations of files and transaction logs
File Security
Files need to be
protected
from things such as hardware/software failures, human error and malicious damage. They can be secured in several ways...
File backup
: a
redundant
(not in use)
copy
of files, usually
stored separately
from the
original
system. They can be used to
recover data
in the event of
catastrophic failure
of the original storage media.
Generations of Files
: storage of
several versions
of a
master
file. If one version is
corrupted
, the
previous version
(s) is still
available
.
Transaction log
: used with
on-line updating
-
stores all the update data
. It can be used in the case of
failure
, could
restore data
by being
combined
with the
previous
master/backup file, with
minimal data loss.
Describe the need for archiving files
Archiving is the process of storing data no longer in everyday use, but is kept for historical/security/legal reasons, and stored securely/off-site.
It
frees up resources
on the main computer system.
Describe sequential, indexed sequential and direct (random) file access
Serial
- new records
added to end
of field -
no set order
.
Slow
to
search
- no thought in storage.
Sequential
- stored
alphabetically
- can
cut
down time taken by using
binary search.
Direct Access File
- a
hashing algorithm
finds the
position
(address) on the disc from the
key field
. It uses an algorithm to
determine disc address.
https://drive.google.com/file/d/187FHSAznZz5hUeXZjYsM5xTmktsFXMYg/view?usp=sharing
Distinguish between the use of serial and sequential file access methods in computer applications
Indexed Sequential
To
add a record
: place in a block if possible; blocks are normally only partially filled to allow more entries later; if a block becomes full, an overflow area is used - access may become slow as more records are in overflow so re-organisation may become necessary.
To
delete a record:
record is marked as deleted but not physically removed.
Multi Level Index
: a main index contains the location of the next index. This process may extend to several levels and the last contains the physical address of the record.
Sequential
To
add a record
: make a new file (v2); copy all records before the position of the new record; add the new record to v2 file; write all the remaining records until the end of the v1.
To
delete a record
: make a new file (v2); copy all records before the position of the record to delete; do not copy the record to delete; write all the remaining records until the end of v1.
To
add multiple new records
: e.g. batch processing; sort before the update processes; this will avoid multiple updates.
Describe and design algorithms and programs for sequential file access and update
Explain and apply appropriate techniques for data validation and verification
Validation
: a
computer check to ensure that data is sensible and reasonable.
Verification
: a
human check ensuring that data has been copied correctly from one medium to another.
Double entry verification is requiring
user to enter
same data twice
for them to be
compared
and ensure they
match
. If they don't, the entry will be
rejected
.
Check Digit
- created by
performing calculations
on a number and then using the
result
of these calculations of the
'check digit'
in that number. It must always be a
positive
,
whole
number.
Validation techniques and invalid data
https://drive.google.com/file/d/1NY_tWI01kvcsvO3WdbtjwSJWtdx89NT7/view?usp=sharing
Explain and apply appropriate techniques for data validation and verification
Data
validation
- a
computer check to ensure that data is sensible and reasonable.
Range
Check
Presence
Check
Length
Check
Type
Check
Format
Check
Lookup
Check
Data
verification
- a
human check ensuring that data has been copied correctly from one medium to another.
Double Entry Verification is where the data is entered twice, the two sets of data are compared to check they match and if the data doesn't match, it is rejected.
A check digit is created by performing calculations on a number and then using the result of these calculations as the "check digit" in that number. It must always be a positive (or 0) whole number.
Design algorithms and programming routines that validate and verify data