Download - Data types vbnet
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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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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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Basic data types
DateCovers 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
StringFor 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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Basic data types
BooleanFor 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
DecimalNumbers with decimal places, especially currency
Takes 16 bytes of storage
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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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Basic data typesSingle
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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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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Variables
Prefixessht = short dte = date
int = integer dec = decimal
sng = single bln = boolean
dbl = double str = string
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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 onceThis allows for easy program modification if value changesWe don’t use a Dim statement, but use
Const dblPi as double = 3.147
Const sngGravity as single = 9.8