Transcript 1 Dimensional Arrays

Modular Programming
Splitting your program into functions
and procedures
Modular Programming
Well written code makes use of modular programming. This simply means
that programs are organised into modules or sub-programs.
Advantages

Each sub-program can be developed and tested as a standalone
program

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Bugs can be located within a specific sub-program
Maintenance can be carried out on each
Programs can be sub-divided and developed by many programmers
The two main types of modules are subroutines (procedures) and functions.
Procedures in VB6.0
A procedure is a subroutine that performs a specific task in a program.
Here’s how to create a procedure in VB6:
Private Sub InputNames()
Statements
End Sub
A procedure can be invoked by:
a) an event e.g. Private Sub cmdCalculate_Click()
b) being called when needed e.g. Call Validate
Functions in VB 6.0
A function is a subroutine that always returns a value e.g. a function could be
used to return the result of a calculation.
Here’s how to create a function in VB6:
Private Function CalcTax () As Single
Statements
End Function
If CalcTax()was a function then you could return a value to a label
e.g. lblAmount.Caption = CalcTax()
or you could return a value to an expression
e.g. Amount = Estimate * .2 + CalcTax() * .14
Note: The value
returned from the
function must always
be assigned to a
variable or a control!
Global Variables
Global variables are declared at the start of a program and can be used
anywhere in the program.
Pros and cons
 Can be used anywhere in the program
 Declared once at the start
 If value changes then changes for the rest of the program
 Can be changed by mistake
Local Variables
Local variables are declared within a procedure or function and can only be
used by that subroutine.
Pros
 Different variables can have the same name
 Save memory as created and destroyed during subroutine runtime
 Make subroutines ‘standalone’ so easy to reuse in other programs
 Reduce risk of accidental change of global value
Parameters
A parameter is simply the value of a variable that can be passed between
subroutines.
age = 25
variable
parameter
The two methods of parameter passing are:
i.
By reference (in and out)
IN
Subroutine
OUT
ii.
By value (in)
IN
Subroutine
Passing Parameters By Reference
Parameter passed is the actual variable itself. Any changes to the parameter
persist beyond the call i.e. change the original value.
Subroutine A
Subroutine B
age = 25
Private Sub check (ByRef age as integer)
age
25
42
age
25
age = 42
Note: An array should always
be passed by reference due to
memory overheads.
age
42
Passing Parameters By Value
A copy of the current value of parameter is passed in and any changes do not
affect the original value.
Subroutine A
Subroutine B
age = 25
Private Sub check (ByVal a)
age
25
a
25
a = 42
Note: In sub B the variable a
is an alias i.e. a copy of the
actual parameter.
a
42
alias
Advantages of Passing Values Using
Parameters
There are a number of advantages:
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Greater control of data flow
Reduced risk of accidental change
More efficient use of memory
Sub-programs are more ‘self-contained’
Improved reliability and robustness
Here’s how to pass parameters in VB6:
Call AddVat (cost, total)
Private Sub AddVat (ByVal c as currency, ByRef total as
currency)
statements
End Sub
1 Dimensional Arrays
When you need to store a lot of the
same kind of values
Why do we need Arrays?
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All of our programs up until now have used
variables that can only store one piece of
information.
If we wanted to create a program that got the
user to type in 10 scores we would need to
declare 10 variables, one for each score.
If we had to change the program to store 100
scores we’d be fed up typing the number of
variable declarations we would need.
Why do we need Arrays?
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We need one variable that has 10 spaces in it, one for each score
we need to store.
We call this type of variable an array and it’s made up of a variable
name and a subscript which tells the computer what item in the
array we want.
You need to be careful though because the first item of the array
has a subscript of 0 not 1.
Main Memory
Score(0)
Score(1)
Score(2)
Score(3)
Score(4)
Score(5)
Score(6)
Score(7)
Score(8)
Score(9)
5
2
10
7
7
4
3
0
8
5
Ten locations in memory are called score and the computer uses the subscript to
identify individual scores.
Declaring an Array
Just like simple variables, arrays need to be declared before they are
used. To declare an array of string values in VB6.0 you use the following
command.
Array Name
VB Data Type
Dim Name(9) as String
Tells the computer
you need a new
variable
This is the number of the last
item of the array. Because we
start from 0 this creates an array
with spaces for 10 pieces of
information
Exercise on Declaring Arrays
1.
The scores for 20 ice skaters, each score is a decimal number
between 0 and 10
Dim SkaterScore(19) as Single
2.
The names of 5 doctors
Dim DoctorName (4) as String
3.
The price of 8 items on a shopping receipt
Dim ReceiptItem(7) as Currency
4.
The total number of goals scored for each team in the premiership.
There are 12 teams
Dim PremiershipTeam(11) as Integer
5.
Whether a question in a 30 question paper was right or wrong,
Dim Answer(29) as Boolean
Assigning Values to an Array
Name(0) = “Mr Donaldson”
Name(1) = “Lynsey Clark”
Name(2) = “Graeme Craig”
…
…
Name(9) = “Stephen Kennedy”
The first item in the array
is the string “Mr
Donaldson”
The second item in the
array is the string “Lynsey
Clark”
The third item in the array
is the string “Graeme
Craig”
The tenth item in the array
is the string “Stephen
Kennedy”
Exercise on Assigning Values to an
Array
1.
The score 1.5 for the fifth skater in the SkatersScores array
SkaterScore(4) = 1.5
2.
The name “Dr Brown” for the 2nd doctor in the array you declared
to store doctors names in.
DoctorName(1) = “Dr Brown”
3.
The price 2.99 as the 5th item in the array you declared to store
items on a shopping receipt.
ReceiptItem(4) = 2.99
4.
The 23 goals scored by the team in 11th place in the array you
declared to store the total number of goals scored by each team in
the premiership.
PremiershipTeam(10) = 23
5.
Question 13 marked as wrong, (false), as the 13th item in the array
you declared to store whether 30 questions were right or wrong.
Answer(12) = false
Useful Functions for
Converting User Input
Handy ways to make sure your program
doesn’t crash
Making sure input is always
converted into a number
Function Used:- Val()
What it Does:Converts any expression into a number. An
expression that only contains non-numeric
characters such as A, *, £ etc or starts with
these characters is converted to a zero.
How to Use It:- Val(expression)
Code Example:- number = Val(txtNumber.text)
Changing the number of decimal
places
Function Used:- Round()
What it Does:Rounds an expression (that produces a
number) to the number of decimal places the
user has asked for.
How to Use It:- Round(expression, no of d.p)
Code Example:roundednumber = Round(DecimalNumber, 2)
Converting a number so you have just
the whole number part
Function Used:- Int()
What it Does:Returns the whole number part of a decimal
number and discards any digits after the
decimal place
How to Use It:- Int(expression)
Code Example:WholeNumber = Int(DecimalNumber)
Useful Functions and Keywords
for Formatting Output
Making the data you display neat and
tidy
Changing the Format of Data
Function Used:- Format()
What it Does:Converts any expression into a particular format that
can be displayed in a VB control like a text box or
picture box. Users can either type in a standard format
like “fixed” or “currency” or create their own custom
format
How to Use It:Format(expression, “format to be used”)
Code Example:picDisplay.print (Format(decimal, “fixed”)
Useful Keywords for Output
Keywords Used:- vbTab and vbNewline
What they Do:vbTab tells the computer to add a tab and vbNewline
tells the computer to start a new line
How to Use them:“piece of text” & vbTab & “2nd piece of text” & vbNewline
Code Example:picDisplay.Print(“Height:- “ & vbTab & height & vbNewline
Handling Strings
Slicing, dicing and rearranging strings of
characters
Working with Strings
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Often when we have programs that work with
text we need to change the text that users
have entered in some way.
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We can either
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split the text up into several strings and store them
in separate string variables, (creating substrings)
join two strings together, (concatenation).
Concatenation
Joining a piece of text (string literal) to the value stored
inside of a variable
A piece of text which is
inside quotation marks
Variable that
stores the cost
of diesel
Display = “The cost of diesel is £” & DieselCost
String Variable that
we are using to store
the concatenated
string
Concatenation
operator which joins
the variable and text
together
Finding the Length of a String
Every string has a length, which is the number of
characters contained in that string. We can use the Len()
function to find the length of a string
What is the length of the following string
“Why did the chicken cross the road?”
Function that returns the
length of the string
Length = Len(MyName)
Integer variable that holds
the length of the string
Name of the string variable
we want to find the length of
Creating Substrings
When we only want part of a string variable we need to
create a substring. For example we might want just the
title “Mr” from the name “Mr Donaldson”. We can use the
Mid() to return any part of another string for us
Number or Integer variable
that tells us the start of the
substring
Substring = Mid( StringToSplit, StartPosition, LengthOfSubstring)
String variable we want to
get the substring from
Number or Integer variable that tells us
how many characters we want in our
substring
Finding Substrings
Often we want the computer to find the start position of a
particular piece of text within a string so that we can use
Mid to extract this substring. To find a substring we use the
Instr() function to return a number that gives the location of
the string or 0 if it can’t find it.
Position in the string where we
want to start searching
Piece of text that we are
searching for, in this case a
space
StartPositionOfText = Instr( 1 , StringToSearch , “ ” )
Function that returns the start
position of a piece of text in a
string or 0 if its not there
String variable we want
to search through
Multiple Outcome Selection
Getting the computer to make a
decision between multiple
Multiple Outcome Selection
The CASE statement improves the IF..THEN..ELSE construct where more than
two conditions are possible.
‘Algorithm using nested IFs
‘Implement using CASE
If mark >= 70 Then
Select Case mark
grade = “A"
Case Is >= 70
Else
grade = “A"
If mark >= 60 Then
Case Is >= 60
grade = “B“
grade = “B"
Else
Case Is >= 50
If mark >= 50 Then
grade = “C"
grade = “C"
Case Else
Else
grade = “Fail"
grade = “Fail"
End Select
End If
End If
End If
CASE makes the code more readable so aids maintenance.