national diploma unit 4 introduction to software development data types, variables and constants
TRANSCRIPT
National Diploma Unit 4Introduction to Software Development
Data types, variables and constants
Basic ideas
Computers are very good at doing sums….but only if we tell them what to do!Programs are best if they are kept general in natureE.g. a=a+b is better than a=2+1This allows us to decide the values of a and b within certain limits
Data types
Depending on the limits we want and the type of data, we can choose to use a particular data typeIf none of the basic ones fit…you can even create your own (but you need to tell VB everything about it first)
Basic data types
Integer A whole number in the range –32,768
to 32,767 Needs 2 bytes of storage space
Long A whole number in the range –
2,147,483,648 to 2,147,483,647 Needs 4 bytes of storage
Basic data types
Single Floating point numbers in the range
-3.402823E38 to –1.401298E-45 for negative values and 1.401298E-45 to 3.4o2823E38 for positive
Takes 4 bytes of storage This should be enough for anyone
but…………
Basic data types
Double Make yours a double if…… A single doesn’t 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 is more economical on storage
Basic data types
Currency Especially for financial data Uses 8 bytes of storage (Remember that a single only uses 4) Uses 4 decimal points for accuracy and
to ensure there are no rounding errors Range is –922,337,203,685,477.5808
to 922,337,203,685,477.5807
Basic data types
Decimal Any number scaled to a power of 10 Can be a whole number or just a
fractional part Range for whole numbers
79,228,162,514,264,337,593,543,950,335 + or –
For fractional numbers: 7.9228162514264337593543950335 + or –
Basic data types
Byte Used to store a number or character Number range =0-255 Character = 1 ASCII Stored in 1 byte so very economical
Basic data types
Boolean For true or false values Can be used for numbers
0=false Any other value = true
Uses 2 bytes of storage
Basic data types
Date Anything from 1st Jan 100 to 31st Dec
9999 Covers most of the useful range!! Time information is also stored as
hours, minutes and seconds Uses 8 bytes of storage
Basic data types
String For text and numbers that you don’t
want to do any maths on e.g. a NI number or telephone number
You can fix the length or let it be variable length
Fixing the length also fixes the amount of storage space
Basic data types
Variant When VB doesn’t know what data
type it is dealing with Uses 16 bytes for numeric data and
22 bytes overhead + the string itself for a string variable
Good programmers leave this one well alone It is wasteful of storage It doesn’t help with documentation at all
Basic data types
Object For when new objects are declared in
an object oriented program Uses 4 bytes of storage All new objects must belong to a
class which defines what characteristics they must have
The class is like a biscuit cutter and objects are the biscuits
Variables
Programs can’t do anything until you have reserved the space in memory that will hold valuesDim intNumber as integer This is a typical declaration statement It reserves a 2 byte slot of memory to
hold the value of intNumber
intNumber=3 Assigns the value of 3
Variables
Naming conventions We will use Hungarian notation which is
accepted a 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
Variables
Prefixes int = integer lng=long sng=single dbl=double byt=byte cur=currency dtm=date bln=boolean str=string vnt=variant obj=object
Variables
Variables also have scope and lifetimeDim intNumber as integerIn this statement, Dim is a local declaration if it is associated with one control or sub procedureIf it is in the general declarations area of the form, it has form-wide scope but is still private
Variables
Public intNumber as integerThis in the general declarations area of a form makes a variable publicly accessibleThis might be useful but don’t declare all variables public – much better to restrict their scope and keep them local to avoid confusion
Variables
Static intNumber as integerUsually variables reach the end of their lifetime at the end of a sub procedureThe memory they were using can then be “recycled”Static allows a variable to retain its value for the whole of the programThis allows its value to be used by other bits of the program (normally it can’t)
Constants
These are special items of data whose value will not be allowed to change at run timeExamples include pi, the speed of light, acceleration due to gravity and possibly VAT etcConst dblPi as doubleConst sngGravity as single ….etc
Name the variable
Write down some suitable identifiers for these variables: VAT a telephone number Today’s date Whether or not the sun is shining Your name The price of a can of coke
Name the variable
Remember to use meaningful names (no spaces or dots allowed)Use Hungarian notation to show the data type
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