1. Data Types & File Size Calculations
Data Structures
Data Types & File Size Calculations
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2. Learning Objectives Explain concepts of files, records, fields.
Explain the function of key fields.
Calculate an estimated file size given the number of records.
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3. Data Data stored in computers is normally connected in some way.
For example, some data about 20 students has a connection because it all refers to the same set of people.
Each person will have the same information stored about them, for instance their name, address, telephone number, exam grades…
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4. Fields & Records Each column is a field. Each row is a record. 45278
Account Number
Surname
Forename
Balance
45278
Smith
Sally
€10.00
1208
Jones
John
€20.00
3217
Cain
Shazad
€100.00
4310
White
Peter
€250.00
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5. Key Field
Some fields may contain the same items of data in more than one record.
e.g. there may be two people with the same name or balance.
It is important that the computer can identify individual records, and it can only do this if it can be sure that one of the fields will always contain different data in all the records.
The key field is unique and is used to identify the record.
In our example the key field would be the account number.
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6. Range / Fractional Real Precision Storage Requirements (bytes)
Data Types
Main Data Types
Data Types
Range / Fractional Real Precision
Storage Requirements (bytes)
Text (Characters) / String in VB
Any characters
1 per character
Char
Any character
1 byte
Integer (Numeric, Whole numbers, no fractions)
Byte 1
Integer
In Access stored by 2 bytes so +/- 32,768.
In VB stored by 4 bytes so approx. +/- 2 billion
2 - 4
Long (Integer)
In Access stored by 4 bytes so approx. +/- 2 billion.
In VB stored by 8 bytes so approx. +/– E+18.
4 – 8
Floating Point (Fractional Real Numbers)
Single 7 4
Double 15 8
Decimal 28 12
Boolean (Y/N True/False)
Often 1 byte is reserved
Date/Time

7. Summary of Data Types Storage Requirements
Storage Requirements (bytes)
Text
1 byte per character
Char
1 byte
Byte 0 – 255
Integer > 255
4 bytes
Decimal
8 bytes
Boolean
(True/False – Yes/No – 2 answers only)
Date / Time
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Note: Use this summary in exam questions. The previous slide is for general interest only.

8. All numbers/dates etc.. take up the same memory space
It does not matter how big or small a number is, it will always take up the specified number of bytes.
e.g. A variable declared as Byte:
Numbers 0, 1, 2 or 255 will take ALL up 1 byte each.
The size of the number does not matter.
e.g. A variable declared as Integer:
Numbers 0, 1, 2 or will ALL take up 4 bytes each.
Also note that you cannot pick and choose, e.g. a small number cannot be held as a byte and a big number held as an integer. The data type has to be chosen at the point of declaration and is chosen depending on how big the number might be, not what it actually is at any one time.
e.g. A variable declared as a decimal.
Numbers 3.1, 3.141, will ALL take up 8 bytes each.
The size of the number or length of the decimal part does not matter.

9. Text Fields must be “Fixed Lengths”
As text fields are 1 byte per character they are different lengths which makes it impossible to estimate their size.
So we must fix them to an appropriate maximum length.
I suggest you do this in 10’s:
e.g. 10 / 20 / 30 / 40 / 50 / … / 100 / …
Exams won’t require you do this but my quizzes will and it is more sensible anyway.
For example, the length of someone’s name should be fixed at 10 (or 20) characters long so, as each character is 1 byte, its size would be 10 bytes (or 20 bytes).
30 characters or more is too long and would be considered incorrect in an exam (and my quizzes).
Note that this means all names are this size, irrespective of its real length ; names are not 10 characters long would have spaces added to the end and names which are longer than this would be cut short.

10. 1. A library stores details of the books that are available.
Apart from title and author, state 3 other fields that it would be sensible for the library to store in this file, giving a reason why each of your chosen fields would be necessary.
(6)
One mark for each of three sensible fields with an extra mark for an explanation of the need for that field.
e.g.
ISBN/to identify book
Shelf number/ to allow for ease of search for book
Fiction or reference or children's (some form of category)/ to decide whereabouts in library it should go
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11. A library stores details of the books that are available.
State which field would be used as the key field of the record and explain why a key field is necessary.
(2)
Book number (ISBN)
because it is unique to that record and hence can be used as an identifier.
Name a suitable data type for this field.
String / Integer Why?
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12. Whether or not an order is outstanding.
2. A stock file in a warehouse has the following fields in each record. (i) State data types suitable for each of the fields.
Name of item.
Date of last delivery.
Price of item.
Whether or not an order is outstanding.
Number of that item left in stock.
Text / String
Date
Currency / Decimal
Boolean
Integer
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13. Whether or not an order is outstanding.
(ii) Given that there are approximately different items in the warehouse, estimate the size of the stock file. You should clearly show all the stages in the calculation.
Name of item.
Date of last delivery.
Price of item.
Whether or not an order is outstanding.
Number of that item left in stock.
10 bytes
8 bytes
1 bytes
4 bytes
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14. (ii) Given that there are approximately different items in the warehouse, estimate the size of the stock file. You should clearly show all the stages in the calculation. =
Name of item
Last delivery date
Price of item
Outstanding?
Number in stock
31 bytes
31 * =
bytes
10000 items = rows

15. (ii) Calculation Total 31 bytes 31 x 10000 = 310000 bytes Overhead:
10% of = * 0.1 = =
/ 1024 ~ 341 Kbytes
Notes:
You must write /1024 because computers do not work in base 10 as we do, but in base 2 – see Data Representation Presentation later. However, exams allow you to actually /1000 to make the calculation easier, as long you write /1024, then approximate (~) and actually /1000.
If you get answers in the millions you will need to convert it into Megabytes (Mb) which means / (~ bytes or 1 million bytes).
You could add 10% by directly
* 1.1 (as long as you allowed to use a calculator which only happens in paper 2, not paper 1).
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