Visual Basic
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Visual Basic

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Visual Basic Visual Basic Presentation Transcript

  • It is easier to read, understand and manage things if they are broken down into sections. Books are divided into chapters, essays are written in paragraphs, school subjects are split into units, school years are split into register classes. All of these things have been broken down into manageable ‘chunks’. Each ‘chunk’ has something in common. The chapter in a book is one part of the story. The unit at school is all on one topic. One register class all have the same guidance teacher, may all take German or all play in the school wind band. There is usually a list at the start of things to show what each of the ‘chunks’ are. The contents page at the beginning of a book, a course description of a subject at school or a list of register classes. Procedures & Parameters Sub-programs make your program more readable , maintainable, modular and robust . The same applies to programming. It is much easier to read, understand and manage a program if it is broken down into sections. Each section should perform a main step from your design. These sections are often known as procedures . In Visual Basic a procedure is called a sub-program When you write out the pseudocode for your program design then the main steps listed will each become sub-programs. At the beginning of your program, after you have listed the variables you will use, you will have to list the sub-programs you will use as well.
  • For Example… This would be written in code as: It’s worth remembering that the order you list the sub-programs in is the order they will run in. Private Sub cmdCalculate_click() DIM number1 as integer DIM number2 as integer DIM answer as integer Call get_numbers Call calculate Call display_result END Sub Variable statements Sub-program statements Pseudocode Sub-program name Main Steps 1. Get two numbers get_numbers 2. Calculate the sum calculate 3. Display the answer in message box Display_result
  • Private Sub cmdCalculate_click() DIM number1 as integer DIM number2 as integer DIM answer as integer Call get_numbers( ) Call calculate( ) Call Display_result( ) END Sub Sub get_numbers ( ) Number1 = InputBox(“Enter the first number”) Number2 = InputBox(“Enter the second number”) End Sub Sub calculate ( ) Answer = number1+number2 End Sub Sub Display_result( ) txtAnswer.text = answer End Sub So a whole program written with sub-programs would look something like this: Variable statements Sub-program get_numbers Sub-program calculate Sub-program display_result Sub-program statements
  • Private Sub cmdCalculate_click( ) DIM number1 as integer DIM number2 as integer DIM answer as integer Sub Display_result( ) txtAnswer.text = answer End Sub Sub calculate ( ) Answer = number1+number2 End Sub Sub get_numbers ( ) Number1 = InputBox(“Enter the first number”) Number2 = InputBox(“Enter the second number”) End Sub END Sub Call Display_result( ) Call calculate( ) Call get_numbers( ) Running order of a program which uses sub-programs
  • There are two different ways you can use variables within your program. Global Variables and Local Variables Variables Global Variables Global variables are variables whose value is available throughout your program for use by different sub programs. Global variables are declared right at the beginning of your program, before you call any sub programs. It is NOT good programming practise to make all your variables global. Global variables are more likely to be altered by mistake and produce errors in your code. Where possible, Local variables should be used. It will always be necessary to have some global variables as you will normally want to move some data between different sub programs, for example a value taken in in one sub program might be used in a calculation in another and displayed in a third.
  • Local Variables Local variables are variables which are used in only one sub program. They are not declared at the beginning of your code – instead the dimensioning takes place within the sub program. The value stored in a local variable can only be changed, used and viewed in the sub routine it was declared in. You should try to use Local variables whenever possible as it reduces the chances of values being changed by accident elsewhere in your code and makes the program easier to maintain and update. It is also considered good programming practise to use Local variables where possible.
  • Global Variable number Dimensioned at the start of the program Used in different sub programs – the value from one is passed into and used in the other Local variables counter and answer dimensioned within the sub program and used only in that sub program Local and Global variables
  • Sub-Programs & Parameters You can’t use sub-programs in Visual Basic without also using parameters . Parameters handle the flow of data within your program for all global variables. A parameter can be either a variable or a value . Parameters can be passed into and out of a sub-program. Parameters improve the reliability and robustness of your program You must list all the parameters you intend to use in each sub- program. This list must be included as part of the CALL statement and as part of the actual sub-program.
  • There are two different ways to pass parameters between sub-programs. You can pass parameters by VALUE or you can pass parameters by REFERENCE Passing Parameters Passing Parameters by Value Parameter Passing by VALUE is when a value is passed into a subprogram for use by the subprogram, but the changed value is not passed out. Parameters passed by Value are indicated by putting brackets around the parameter name. Passing Parameters by Reference Parameter Passing by REFERENCE is when a value is passed into a subprogram for use by the subprogram, and then the changed value is passed out to be used by other sub-programs. Parameters are passed by Reference by default and are simply listed in the parameter list.
  • Private Sub cmdBegin_Click() Dim number1 As Integer Dim number2 As Integer Dim answer As Integer Call get_numbers (number1, number2) Call calculate ((number1), (number2), answer) Call display_answer ((answer)) End Sub Sub get_numbers (number1, number2) number1 = InputBox("Enter number 1") number2 = InputBox("Enter number 2") End Sub Sub calculate (number1, number2, answer) answer = number1 + number2 End Sub Sub display_answer (answer) txtAnswer.Text = answer End Sub Example Comments The parameters are shown in blue number1 and number2 are changed in the get_numbers sub-program and the changed data needs to be used elsewhere so are passed by reference . The same parameters are listed here in the exact same order they are listed in the main program. number1 and number2 are simply passed into the calculate sub-program but are not changed and so are passed by value , answer is changed and needs to be passed by reference . answer is passed into the display_answer sub-program but is not changed and so is passed by value .
  • Sub-programs and parameters make designing your program even more crucial. You need to indicate in your design where and how parameters will be used. This could be by adding data flows to structure diagrams Or by indicating data flow in pseudocode. number1 number2 number1 number2 answer answer 1. Set up variables 2. Get numbers OUT: number1, number2 3. Calculate answer IN: number1, number2. OUT: answer 4. Display answer IN: answer 5. End Design Calculate Display answer Get Numbers Main
  • Try this program out in Visual Basic.
  • IF register_class = “4A” OR register_teacher = “4I” OR register_teacher = “5A” THEN guidance_teacher = “Ms Graham” Else IF register_teacher = “4B” OR register_teacher = “4G” OR register_teacher = “5C” THEN guidance_teacher = “Miss Stott” Else IF register_teacher = “4C” OR register_teacher = “4K” OR register_teacher = “5E” THEN guidance teacher = “Mrs Skellern” … (and so on) END IF IF register_class = “4A” THEN guidance_teacher = “Ms Graham” Else IF register_teacher = “4B” THEN guidance_teacher = “Miss Stott” Else IF register_teacher = “4C” THEN guidance teacher = “Mrs Skellern” … (and so on) END IF Using the CASE statement A CASE statement is a more efficient way of performing a multiple choice in programming. You could use the IF… THEN… ELSE IF… ELSE IF… ELSE IF … but it gets quite messy and you have to repeat the condition of the choice each time. For Example: The IF method gets even more messy if you have multiple possibilities giving the same result. For Example:
  • SELECT CASE register_ class CASE IS “4A”,”4I”,”5A” guidance_teacher = “Ms Graham” CASE IS “4B”, ”4G”,”5C” guidance_teacher = “Miss Stott” CASE IS “4C”, “4K”, “5E” guidance_teacher = “Mrs Skellern” END SELECT The CASE statement will perform this same task much more neatly and efficiently. SELECT CASE register_ class Case Is “4A”,”4I”,”5A” guidance_teacher = “Ms Graham” Case Is “4B”, ”4G”,”5C” guidance_teacher = “Miss Stott” Case Is “4C”, “4K”, “5E” guidance_teacher = “Mrs Skellern” End Select This is the variable you are checking This is the first condition (or conditions) to check if it meets. The comma means OR. This is what happens if the condition is met. There can be more than one line of code here.
  • The CASE statement treats number ranges differently from other Visual Basic structures. The default for a CASE condition is CASE IS. So CASE IS can be exactly something or more or less than one number. You cannot have a number range – CASE IS >20 AND <40 . If you need a number range you need to use CASE…..TO – CASE 20 TO 40
  • When using random numbers you need to use the RND command. Randomize is the word used to mix up a list of numbers each time a program is run. It is a pre-defined function . The value of the random number is between 0 and 1 (a real number or a fraction). If we want a bigger number we have to multiply the random number by the biggest amount we want. To see how random numbers work, look at the following program: Create random number Display random number Output Random Numbers
    • As you can see, the RND function produces random numbers between 0 and 1
    • To produce whole random numbers between 1 and 10 we would have to do three things to the line number = RND.
    • We would need to multiply by 10 to get a random fraction between 0 and 10
    • We would need to chop off the fraction part using the function INT (changes a number to the nearest whole number)
    • Lastly, we need to add 1 because the highest number would always be 9 as it would be rounded down by the INT function.
    • So , Number = RND will become
    • Number = INT (RND * 10) + 1
    Output Inserting the word RANDOMIZE gives truly random results
  • Validation and Conditional Loops You need to remember to validate the input to any program. This will greatly reduce run-time errors caused by unexpected data and will also assist the users to enter the right data and be clear what they are doing. It will also help the testing phase. A simple validation could be something like entering a price which is less than £10. Private Sub cmdBegin_Click() DIM cost as Integer Cost = INPUTBOX(“Please enter the cost”) txtCost.text = cost END SUB Private Sub cmdBegin_Click() DIM cost as Integer DO Cost = INPUTBOX (“Please enter the cost”) LOOP UNTIL cost < 10 txtCost.text = cost END SUB No validation – you can enter what you like Simple validation Keeps asking until you enter a valid number – no error message If you wanted to make sure the cost of an item was less that £10 you would need to keep asking the user to enter the cost UNTIL it was less than £10. They might enter it correctly the first time, in which case they don’t need to be asked again or they might get it wrong one hundred times in which case the program will loop until they enter a cost that is less than £10. The problem here is that there is no message to tell the user why they keep getting asked for the same thing. You can add a condition which will check if the input is ok and print a helpful error message if it is not. This program will ask the user to enter the cost of an item and will display it in a text box. However, there is no way to check whether the cost is less than £10. To check this (this is called VALIDATION) you will need to use a conditional loop. A conditional loop is where some lines of code in your program will repeat until a certain condition is met.
  • Private Sub cmdBegin_Click() DIM cost as Integer DO Cost = INPUTBOX (“Please enter the cost”) IF cost >=10 THEN MSGBOX(“The cost must be less than £10”) END IF LOOP UNTIL cost < 10 txtCost.text = cost END SUB Private Sub cmdBegin_Click() DIM cost as Integer DO Cost = INPUTBOX (“Please enter the cost”) IF cost >=10 THEN MSGBOX(“The cost must be less than £10”) END IF LOOP UNTIL cost > 0 AND cost < 10 txtCost.text = cost END SUB This is better, every time an invalid cost is entered a message is displayed stating why and asking for the data again. This is correct – as it is a cost it also checks the number is greater than 0. The error message is still displayed and there is no chance the data entered will be inappropriate. It is much more difficult to validate text input, you can help eliminate problems by converting all text input to either upper or lower case, as Visual Basic considers them as different in a comparison - “john” would be considered as different from “John”. It is not always appropriate to convert all text to upper or lowercase. You can also count the number of characters entered – this can help with things like postcodes which are a fixed length.
  • So far all the variables you have used have been single item variables. As you write more complex programs you will find you need many similar variables in one program. If you were going to read in 10 pupils marks you could have 10 individual variables. Dim mark1 as integer Dim mark2 as integer Dim mark3 as integer Dim mark4 as integer Dim mark5 as integer Dim mark6 as integer Dim mark7 as integer Dim mark8 as integer Dim mark9 as integer Dim mark10 as integer This would also require 10 input statements Mark1 = inputbox(“Please enter Mark 1”) Mark2 = inputbox(“Please enter Mark 2”) Mark3 = inputbox(“Please enter Mark 3”) Mark4 = inputbox(“Please enter Mark 4”) Mark5 = inputbox(“Please enter Mark 5”) Mark6 = inputbox(“Please enter Mark 6”) Mark7 = inputbox(“Please enter Mark 7”) Mark8 = inputbox(“Please enter Mark 8”) Mark9 = inputbox(“Please enter Mark 9”) Mark10 = inputbox(“Please enter Mark 10”) This would require 10 variables txtMark1.text = mark1 txtMark2.text = mark2 txtMark3.text = mark3 txtMark4.text = mark4 txtMark5.text = mark5 txtMark6.text = mark6 txtMark7.text = mark7 txtMark8.text = mark8 txtMark9.text = mark9 txtMark10.text = mark10 And 10 output statements This is obviously an awful lot of repetitive code. There is a much better way of storing and using data which is related. This is using an array. Using Arrays
  • An array stores a number of variables in a special data structure. If you wanted to store a large amount of one type of data then you could use an array . An array to holds them all together and treats them as one thing . The program can then refer to the whole array or any single element of the array. You would declare an array like this: Array name The number of elements in the array Array data type DIM mark(10) as Integer The memory sets aside an area divided into elements Mark 1 2 3 4 5 6 7 8 9 10 So this element is referred to as Mark(6) and this one is Mark (8) An input statement would look like this: Mark(3) = Inputbox(“Please enter mark 3) The data entered would be placed in the third element of the array.
  • To eliminate the need for multiple INPUT and OUTPUT statements you would use a FOR loop for array operations. FOR counter = 1 TO 10 Mark( counter ) = Inputbox(“Please enter the mark “) NEXT counter FOR giraffe = 1 TO 10 lstResults.additem Mark( giraffe ) NEXT counter Whatever variable is used in the FOR loop is also used for the element position in the Array. As the loop goes round, the loop variable (counter or giraffe above) is incremented (goes up by one) so first time round counter = 1 and the input goes in Mark(1), next time round counter = 2 so the input goes in Mark(2) and so on. Instead of 10 input statements or 10 output statements, those three lines of code will input or output all 10 data items.
  • You can also use FOR loops and arrays to take in more than one set of related data. For example if you wanted to read in 6 student names and their marks. You need to keep the names and marks separate because you might want to use the numeric data for comparisons or calculations. You will need to declare 2 arrays one for names and one for marks. You will only need one FOR loop. Name Mark Jim 37 Bob 26 Sue 52 Jan 86 John 17 Peter 79 37 26 52 86 17 79 Jim Bob Sue Jan John Peter Name Mark (1) (2) (3) (4) (5) (6) (1) (2) (3) (4) (5) (6) So this data Would be stored like this This means that the name and mark are stored in different arrays but in the same position, so could be accessed at the same time using one FOR loop
  • DIM name(6) as String DIM mark(6) as Integer DIM pupil as Integer DIM result as Integer FOR pupil = 1 TO 6 name( pupil ) = Inputbox(“Please enter the name”) mark( pupil ) = Inputbox (“please enter the mark”) NEXT pupil FOR result = 1 TO 6 lstClassResults.additem name( result ) & “ “ & mark( result ) NEXT result This code uses one FOR loop to read in both the name and the mark. A different FOR loop is used to display the data in a list box.
  • String Operations Visual Basic considers upper and lower case text to be different, so if you had a quiz program where the answer should be true and the user entered TRUE, when you perform the comparison to see if they got the answer right the program would say they got it wrong. You can convert string data to upper and lower case to help prevent problems like this. To convert test to upper case or lower case you would use the UCase or LCase functions. lowercase_string = LCase(string_variable) uppercase_string = UCase(string_variable) Where string_variable is the original data and lowercase_string and uppercase_string is where the converted data is stored. You can use the Len function to find out how many characters are in a string variable. It counts spaces as well as characters. You will need to store the length as a numeric variable. Numeric_variable = Len(string_variable) If string_variable = “John Smith” then numeric_variable = 10 If string_variable = “Hello” then numeric_variable = 5
  • One of the operations you can do with string variables is string concatenation – this is adding the two strings together. It is as simple as this: Newstring = string1+string2 DIM initial as String DIM surname as String DIM userID as String initial = Inputbox(“Please enter your initial”) surname = Inputbox(“Please enter your surname”) userID = surname+initial INPUT : initial = V surname = Mackenzie OUTPUT : MackenzieV String Operations - Concatenation
  • String Operations – Sub String A Sub string operation is where you separate part of the string value. You can take the first character, the middle three characters, the last two, the first seven. Basically you can separate whatever number of characters you want from wherever you want in the string. You need to use the Mid$ function for this: String_output = Mid$(string_input, start position, number or characters) String_out = Mid$(string_input, 2, 3 ) So if string_input = “Greetings” Then String_out = “ree” If string_input = “Computing is Fun” Then String_out = “omp” Character to start at How many characters to extract
  •