Transcript Variables
Variables "There are 10 kinds of people in the world today – those who understand binary and those who don't!” Unknown What is a Variable (Object)? Inside your computer are various bits of computer technology CD-ROM Hard Disk Drives RAM (Random Access Memory) Variables allow us to control the memory, the RAM A Simple Calculation Consider 3 = 1 + 2 A computer needs to set aside three areas of RAM One for the result Two for the other two numbers It will also perform the calculation in binary for example… 00000011=00000001 + 00000010 We as programmers really don’t want to get involved in thinking about memory and binary so we use variable objects to do the dirty work for us. Examining the Swap Offer Functionality Once we have signed up and logged onto the system we see the following options… We shall examine the code for “Submit Offer”. To do this we shall add a break point to the code. (F9) Declaring Variables Notice the following lines of code… These lines of code “declare” variables. “Dim” tells the language to set aside sections of RAM with the names we specify Dim What this is doing is telling the computer to allocate one or more addresses in RAM, give it a name and limit the type of data it can hold. We are creating a variable object so that we can control some of the RAM. e.g. Dim ErrMsg as String Declares a variable called ErrMsg with the String data type. (Dim may be thought of as Declared In Memory even though it really means Dimension.) So What is RAM? RAM may be thought of as a series of boxes that allow us to store data in. With these declarations Dim ErrMsg As String Dim OfferTitle As String Dim Description As String Dim SwapNo As Integer Dim Userno As Integer Dim EMail As String Dim UserName As String We have created seven boxes in RAM for storing data Data Types The data type is the name of the type of data that the variable is able to store The Integer data type only allows us to store whole numbers For example we could store in the variable SwapNo the following values… 0, 1 200, 50 i.e. all numeric whole numbers But not the following… “Kilroy Wos Ere”, 3.7 because… “Kilroy Wos Ere” 3.7 text not numbers not a whole number (This would work but it would be rounded!) Things to try What happens if… You delete the line Dim Description As String and try to run the program? (Notice the error messages and the tools to help you locate problems, use undo to fix the error.) You rename the variable SwapNo to SwapNumber and run the program? (As before notice the error messages and then fix the error.) You change the data type of Description from String to Integer? (You will need to use the program to see the problem.) Assignment What does the word assignment mean? To assign something to a variable we use the assignment operator… = So to assign the number 1 to the FirstNumber variable we would use the following line of code… FirstNumber = 1 The Assignment Operator Using the debugger (F9, F10) we shall examine how the following highlighted line of code works. Assigning Literal Constants There are times when we simply want to give a variable a value, often to initialise it. A literal constant is a set value specified by you the programmer. Take a look at the following… Dim DaysLeft As Integer DaysLeft = 365 This could also be written like so… Dim DaysLeft As Integer = 365 Performing Simple Calculations Take a look at the following code... Dim FirstNumber As Integer Dim SecondNumber As Integer Dim Result As Integer FirstNumber = 20 SecondNumber = 30 Result = FirstNumber + SecondNumber What is happening here and what will be the value of Result? Finding the Definition of a Variable If you are looking at a section of code and you see a variable but cannot find where it is declared right click on it like so... Data Type Conversion Errors The process of assigning data unsuitable for the specified data type is called a data type conversion error e.g. Dim MyAge as Integer MyAge = “Fred Bloggs” Overflow Errors Different data types allow different ranges of values to be stored in them For example the largest number a variable with the Integer data type may store is 2147483647. Attempting to store a number larger than this will result in an overflow error. e.g. Dim MyAge As Integer MyAge = 2147483648 Rules for Naming your Variables Make your Variable names Meaningful Don’t use A B & C for your variable names Not all characters may be used in variable names Spaces are not allowed My Variable – bad MyVariable – good Variables may contain a number but not start with one Use Camel Case when naming your variables InputVariable - good inputvariable - bad Summary A variable allows us to control a section of RAM which has been given a name by you the programmer A variable has a data type which controls what sort of data it may store. (Any attempts to assign the wrong type of data to a variable will produce an error!) A variable must be declared before you may use it. The key word Dim is used in VB to do this A variable has a value this value is the data stored at whatever location in RAM is used The value of a variable is assigned using the assignment operator “=”. This may be a discrete value e.g. 1 Value=1 or the result of a calculation e.g.… Result = FirstNumber + SecondNumber Questions Write down variable declarations that would store the following values: For example, an address: Answer 1. A person’s name 2. A person’s age 3. A telephone number 4. A date of birth 5. The price of a kettle : Dim AnAddress as String Questions 6. Complete the following code that assigns the value 20 to a variable called ClassSize: Dim ClassSize as Integer ClassSize 7. Complete the following code that assigns the name “Fred Bloggs” to a variable called StudentName: Dim StudentName as String StudentName Questions 8. A form has a text box called txtUserName. a. Write a suitable variable declaration for a variable to store this data: b. Write a suitable assignment statement that reads in this value from the form: 9. Write an assignment statement that assigns the result of 2 + 2 to variable called Result: 10. Write an assignment statement that assigns the result of 4.5 x 2 to a variable called Result: