Copyright © 2001 by Wiley. All rights reserved. Chapter 3: Variables, Assignment Statements, and Arithmetic Variables Assignment Statements Arithmetic

Introduction to Programming with Visual Basic 6.0 by McKeown and Piercy

Copyright © 2001 by Wiley. All rights reserved.

Chapter 3: Variables, Assignment Statements, and Arithmetic

Variables

Assignment Statements

Arithmetic Operators

Constants

Formatting Data

VB Errors

Chapter 3:Variables, Assignment

Statements, and Arithmetic


2Copyright © 2001 by Wiley. All rights reserved.


Variables



Constants

Formatting Data

VB Errors

Learning Objectives•Declare and use different types of variables in your project.•Use text boxes for event-driven input and output.•Use a four-step process to write code for event procedures•Write Visual Basic instructions to carry out arithmetic operations.•Describe the hierarchy of operations for arithmetic. •Understand how to store the result of arithmetic operations in variables using assignment statements. •Use comments to explain the purpose of program statements.•Discuss using Visual Basic functions to carry out commonly used operations.•Use command buttons to clear text boxes, print a form with the PrintForm method, and exit the project. •Describe the types of errors that commonly occur in a Visual Basic project and their causes.




Variables



Constants

Formatting Data

VB Errors

Variables• Variables are named locations in memory (memory cells) in

which we store data that will change at run time• Variable names in VB:

– Must begin with letter– can’t include period– can’t be over 255 characters– are not case-sensitive

• Example variable names:– VideoPrice for Video Rental Price– Taxes for taxes on this price– AmountDue for sum of price and taxes– YTDEarnings for year to date earnings– EmpLastName for employee’s last name




Variables



Constants

Formatting Data

VB Errors

Data Types• Two primary types of data: numeric and string• Numeric data can be used in arithmetic operations

– 3.154; 2300; -34; 0.0000354 are all numeric constants

• String data should not be used in arithmetic– “Dogs”; “123-45-6789”; “Dr. Brown” are all string

constants• Numeric data types can be subdivided into specific

types:- currency $55,567.78 - integer 255- single 567.78 - long (integer) 35,455- double 567.78129086 - Boolean True or False




Variables



Constants

Formatting Data

VB Errors

Declaring Variables

• Declare ALL variables with the DIM statement• General form:

– Dim Variable1 as type1, variable2 as type2, etc.

• For example,– Dim strMyName as String, sngMyValue as Single– Dim curTaxes as Currency, curPrice as Currency– Dim curAmountDue as Currency

• Two or more variables can be declared with same Dim statement but you must include the variable type

• If you fail to declare a variable after the Option Explicit statement has been entered, an error occurs at Run time

• Use variables rather than objects in processing since all textboxes are strings




Variables



Constants

Formatting Data

VB Errors

The Option Explicit Statement

• Begin ALL Forms with the Option Explicit command in the declarations procedure of the general object. This forces all variables to be declared

• To automatically include Option Explicit, go to the Tools|Options|Editor menu selection and check the box for “Require variable declaration”




Variables



Constants

Formatting Data

VB Errors

Event-driven Input

• In VB, we often use event-driven input where data is transferred from text boxes to variables by an event

• Use an assignment statement to do this:Control property or variable = value, variable, or

property

• Only variables or control properties can be on left of assignment statement




Variables



Constants

Formatting Data

VB Errors


• Must convert strings in text boxes to numeric with Val function, e.g.,– Price = Val(txtPrice.Text)

• Convert numeric variables to strings before assigning to text box, e.g.,– txtAmountDue.Text = Str(curAmountDue)

• Carry out calculations with assignment statements, e.g., – curTaxes = 0.07*curPrice– curAmountDue = curPrice + curTaxes




Variables



Constants

Formatting Data

VB Errors

Using Functions

• Sometimes we want to convert a string to a number or vice versa or to compute a single value

• To do this, we use a function that is nothing more than an operation that takes a multiple arguments and generates a single value

• variable = functionName(arg1, arg2, …)

• Example functions for converting data:– Val to convert a string to a number– Str to convert a number to a string– CCur to convert a string to a currency data type




Variables



Constants

Formatting Data

VB Errors

Using Assignment Statements for Calculations

• Sometimes, an expression will be on right of assignment statement

• An expression is a combination of one or more variables and/or constants with operators

• Operators are symbols used for carrying out processing




Variables



Constants

Formatting Data

VB Errors


() for grouping + for addition

^ for exponentiation - for subtraction

- for negation

* for multiplication

/ for division

\ for integer division

mod for modulus




Variables



Constants

Formatting Data

VB Errors

Hierarchy of Operations

• Operations within parentheses ( )• Exponentiation (^)• Negation (-)• Multiplication and division (*, /)• Integer division (\)• Modulo arithmetic (Mod)• Addition and subtraction (+, -)• String concatenation (&)




Variables



Constants

Formatting Data

VB Errors

Arithmetic Example• 3 * (curSalary - curTaxes)^2 - curBonus/curMonths

3 1 2 5 4

Order1 Subtract Taxes from Salary

2 Square the result

3 Multiply this result by 3

4 Divide Bonus by Months

5 Subtract result from first expression




Variables



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VB Errors

Symbolic Constants

• We can assign a name to a constant with the Const statement

const constant name as variable type = value

• ExampleConst sngRate As Single = 0.07




Variables



Constants

Formatting Data

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Comments

• To explain the purpose of a statement, a comment statement is added

• Any statement beginning with an apostrophe or REM is a comment

• Comments can be added to end of statements using apostrophe




Variables



Constants

Formatting Data

VB Errors

Formatting Data• To display information in a pleasant form, we can

use the Format function:variable or control = Format(variable, format

expression)

• Where the format expressions are in quotes and include;– Currency– Fixed– Standard– Percent– Scientific

• Example:txtTaxes.Text = Format(curTaxes, “currency”)




Variables



Constants

Formatting Data

VB Errors

Print Form, Clearing Entries and Setting Focus

• To print the form, use the PrintForm command

• To clear a text box, set it equal to the null string ”” (no space)

• To set the focus to a text box, use the Setfocus method

• For example, txtCustName.SetFocus sets focus to this textbox




Variables



Constants

Formatting Data

VB Errors

Using Other Arithmetic Functions• Other useful functions include

– Abs for absolute value Sqr for square root– FV for future value PV for present value– IRR for internal rate of return Pmt for payment– Ucase/Lcase to convert to upper/lower case– Len for length of a string– Date for the system date– DateValue for the date corresponding to string argument

• We will use Pmt to compute the monthly payment

– curMonPay = Pmt(sngRate, curNper, -curLoanAmt)– Pmt(.08/12,60,-10000) = $256.03




Variables



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Formatting Data

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Creating a Monthly Payment Calculator

• Assume you wanted to determine the monthly payment necessary to pay off a loan at a given interest rate in some number of months

• Use PMT function• PMT(rate, nper, pv) where

– rate = monthly interest rate– nper = number of months– pv = negative value of loan amount




Variables



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The Monthly Payment Form




Variables



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Compute Button Click event

Dim curAmount As Currency, intMonths As IntegerDim sngRate As Single, curPayment As CurrencycurAmount = CCur(txtAmount)intMonths = CInt(txtMonths.Text)sngRate = CSng(txtRate.Text)curPayment = Pmt(sngRate, intMonths, -curAmount)txtPayment.Text = Format(curPayment, “Currency”)txtAmount.Text = Format(curAmount, “Currency”)txtRate.Text = Format(sngRate, “Percent”)




Variables



Constants

Formatting Data

VB Errors

Visual Basic Errors• Syntax errors: caused by incorrect grammar, vocabulary, or

spelling. Also caused by using a keyword. Usually caught as you enter the statement. These are pointed out by VB and are usually easy to find and correct.

• Run time errors: errors not caught at entry but which involve an incorrect statement or bad data, e.g., dividing by zero. The presence of an error is detected by VB when you run the program, but you still have to find the source of the error. More difficult to correct than syntax errors.

• Logic errors: those that VB does not catch as being “wrong”, but which involve erroneous logic, say, only having a program include 11 months of data instead of 12. These are the hardest to find!

• Debugging is the art and science of finding errors. VB has debugging tools to be discussed later.

Documents

Copyright © 2001 by Wiley. All rights reserved. Chapter 3: Variables, Assignment Statements, and Arithmetic Variables Assignment Statements Arithmetic