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Variables and Datatypes

Software Design & Development

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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

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Name the variable

Write down some suitable identifiers for these variables:

VATYour telephone numberToday’s dateWhether or not the sun is shiningYour nameThe price of a can of coke

Remember to use naming conventions and correct data types!