1. Variables and Datatypes
Software Design & Development
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2. Data Types
A program can’t do anything until you have reserved a
space in memory for the values used in it
This is called declaration of variables
When you declare a variable you must:
Give it a sensible name
Give it a data type depending on what kind of
value it will hold
Ask yourself will it hold just letters or whole
numbers or numbers with a decimal point or
dates…..etc
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3. Data types
Depending on the type of data used by our program,
we need to choose to use a particular data type
Some programs only use whole numbers – we can’t
have ½ a person!
Other programs only deal with numbers up to a few
thousand – if we count the students in the College
Other programs deal with huge numbers such as
distances in space
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4. Basic data types
Date
Covers most of the useful range!!
Anything from 1st Jan 100 to 31st Dec 9999
Time information is also stored as hours, minutes and
seconds
Uses 8 bytes of storage
String
For text e.g. names, addresses, postcodes
For numbers that you don’t want to do any maths on e.g. A
telephone number
You can fix the length or let it be variable
Fixing the length also fixes the amount of storage space
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5. Basic data types
Boolean
For true or false values e.g. Yes / No answers
Can be used for numbers
 0 = false
 Any other value = true
Uses 2 bytes of storage
Decimal
Numbers with decimal places, especially currency
Takes 16 bytes of storage
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6. Basic data types
Short
whole numbers in the range –32,768 to 32,767
Examples: -32 or 10,000
needs 2 bytes of storage space
Integer
A whole number in the range –2 billion to + 2 billion
Examples: 20,000,000 or -400,000
needs 4 bytes of storage
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7. Basic data types
Single
Numbers with decimal places and fractions
in the range -3.402823E38 to –1.401298E-45 for negative
values and 1.401298E-45 to 3.4o2823E38 for positive
Examples: 3·142, -700·75
Takes 4 bytes of storage
Double
For incredibly large or small values use a double, as a single
wont provide the range or accuracy that you need
Takes 8 bytes of storage
Bear this last fact in mind, if you don’t need this level of
accuracy, declare a single as it uses half the storage space
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8. Variables - Naming conventions
Hungarian notation is used in naming variables
This is accepted as good programming practice
Prefix the variable name with a 3 letter identifier to
indicate the data type
The variable name should be meaningful - good
programmers can read the code of a program and
know what it does
You will need to use these conventions
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9. Variables
Prefixes
sht = short dte = date
int = integer dec = decimal
sng = single bln = boolean
dbl = double str = string
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10. Constants
Use a Constant where a value is needed but does not
change while program is running
e.g. VAT, Annual Leave, Pi, Gravity
Constants are always global as they are only set up
once
This allows for easy program modification if value
changes
We don’t use a Dim statement, but use
Const dblPi as double = 3.147
Const sngGravity as single = 9.8
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11. Name the variable
Write down some suitable identifiers for these
variables:
VAT
Your telephone number
Today’s date
Whether or not the sun is shining
Your name
The price of a can of coke
Remember to use naming conventions and
correct data types!
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