Chapter 3 Variables Input and Output 3 1

3. 1 Numbers • • • Arithmetic Operations Variables Augmented Assignments Mathematical Functions The Integer Data Type Two Other Integer Operators © 2017 Pearson Education, Inc. Hoboken, NJ. All rights reserved. 2

Numbers (continued) • • • The Decimal Data Type Multiple Declarations Parentheses, Order of Precedence Three Kinds of Errors The Error List Window © 2017 Pearson Education, Inc. Hoboken, NJ. All rights reserved. 3

Arithmetic Operations • Numbers are called numeric literals • Five arithmetic operations in Visual Basic + addition - subtraction * multiplication / division ^ exponentiation © 2017 Pearson Education, Inc. Hoboken, NJ. All rights reserved. 4

Displaying Numbers Let n be a number or a numeric expression. The statement lst. Box. Items. Add(n) displays the value of n in the list box. © 2017 Pearson Education, Inc. Hoboken, NJ. All rights reserved. 5

Example 1: Form © 2017 Pearson Education, Inc. Hoboken, NJ. All rights reserved. 6

Example 1: Code and Output Private Sub btn. Compute_Click (. . . ) Handles btn. Compute. Click lst. Results. Items. Add(5) lst. Results. Items. Add(2 * 3) lst. Results. Items. Add((2 ^ 3) – 1) End Sub Output in list box 5 6 7 © 2017 Pearson Education, Inc. Hoboken, NJ. All rights reserved. 7

Numeric Variable A numeric variable is a name to which a number can be

Variables • Declaration: Dim speed As Double variable name data type • Assignment: speed

Initialization • Numeric variables are automatically initialized to 0. Dim var. Name As Double • To specify a nonzero initial value Dim var. Name As Double = 50 © 2017 Pearson Education, Inc. Hoboken, NJ. All rights reserved. 10

Numeric Expressions Numeric variables can be used in numeric expressions. Dim balance As Double = 1000 lst. Box. Items. Add(1. 05 * balance) Output: 1050 © 2017 Pearson Education, Inc. Hoboken, NJ. All rights reserved. 11

Assignment Statement Dim num. Var 1 As Double = 5 Dim num. Var 2 As Double = 4 num. Var 1 = 3 * num. Var 2 lst. Box. Items. Add(num. Var 1) Output: 12 © 2017 Pearson Education, Inc. Hoboken, NJ. All rights reserved. 12

Augmented Assignments • To add 1 to the numeric variable var = var + 1 • Or as a shortcut var += 1 • A generalization var += numeric expression © 2017 Pearson Education, Inc. Hoboken, NJ. All rights reserved. 13

Built-in Functions return a value Math. Sqrt(9) returns 3 Int(9. 7) returns 9 Math.

Integer Data Type • Variables of type Double can be assigned both whole numbers and numbers with decimals. • The statement Dim var. Name As Integer declares a numeric variable that can only be assigned whole number values between about -2 billion and 2 billion. © 2017 Pearson Education, Inc. Hoboken, NJ. All rights reserved. 15

Two Integer-Valued Operators • Integer division (denoted by ) is similar to ordinary long division except that the remainder is discarded. • The Mod operator returns only the integer remainder after long division. 23 7 = 3 23 Mod 7 = 2 82=4 8 Mod 2 = 0 © 2017 Pearson Education, Inc. Hoboken, NJ. All rights reserved. 16

Decimal Data Type • The statement Dim var. Name As Decimal declares a numeric variable that is not subject to rounding errors. • Variables of type Decimal are used in this book for all financial calculations. © 2017 Pearson Education, Inc. Hoboken, NJ. All rights reserved. 17

Multiple Declarations Dim a, b As Double Two other types of multiple-declaration statements are Dim a As Double, b As Integer, c As Decimal Dim a As Double = 2, b As Integer = 5, c As Decimal = 1. 05 D © 2017 Pearson Education, Inc. Hoboken, NJ. All rights reserved. 18

Parentheses • Parentheses should be used liberally in numeric expressions. • In the absence of parentheses, the operations are carried out in the following order: ^, * and /, , Mod, + and -. © 2017 Pearson Education, Inc. Hoboken, NJ. All rights reserved. 19

Three Kinds of Errors • Syntax error • Runtime error • Logic error ©

Some Types of Syntax Errors • Misspellings lst. Box. Itms. Add(3) • Omissions lst. Box. Items. Add(2 + ) • Incorrect punctuation Dim m; n As Integer © 2017 Pearson Education, Inc. Hoboken, NJ. All rights reserved. 21

A Type of Runtime Error Overflow error Dim num. Var As Integer = 1000000

A Logical Error Dim average As Double Dim m As Double = 5 Dim n As Double = 10 average = m + n / 2 Value of average will be 10. Should be 7. 5. © 2017 Pearson Education, Inc. Hoboken, NJ. All rights reserved. 23

Error List Window Dim m; n As Double lst. Results. Items. Add(5 lst. Results.

3. 2 Strings • • Variables and Strings Using Text Boxes for Input and Output Option Explicit and Option Strict Concatenation String Properties and Methods Indices and Substrings The Empty String © 2017 Pearson Education, Inc. Hoboken, NJ. All rights reserved. 25

Strings (continued) • • Initial Value of a String Variable Widening and Narrowing Data types of Literals and Expressions Internal Documentation Line Continuation Scope of a Variable Auto Correction © 2017 Pearson Education, Inc. Hoboken, NJ. All rights reserved. 26

String Literal A string literal is a sequence of characters surrounded by quotation marks. Examples: "hello" "123 -45 -6789" "#ab cde? " © 2017 Pearson Education, Inc. Hoboken, NJ. All rights reserved. 27

String Variable A string variable is a name to which a string value can

String Variable (continued) • Declaration: Dim first. Name As String variable name data type

String Variable (continued) You can declare a string variable and assign it a value at the same time. Dim first. Name As String = "Fred" © 2017 Pearson Education, Inc. Hoboken, NJ. All rights reserved. 30

Add Method Let str be a string literal or variable. Then, lst. Box. Items.

String Variable You can assign the value of one string variable to another string variable. Dim str. Var 1 As String = "Hello" Dim str. Var 2 As String = "Goodbye" str. Var 2 = str. Var 1 lst. Output. Items. Add(str. Var 2) Output: Hello © 2017 Pearson Education, Inc. Hoboken, NJ. All rights reserved. 32

Variables and Strings Private Sub btn. Display_Click(. . . ) Handles btn. Display. Click Dim president As String president = "George Washington" lst. Output. Items. Add("president") lst. Output. Items. Add(president) End Sub Output: president George Washington © 2017 Pearson Education, Inc. Hoboken, NJ. All rights reserved. 33

Using Text Boxes for Input and Output • The contents of a text box is always a string. • Input example: str. Var = txt. Box. Text • Output example: txt. Box. Text = str. Var © 2017 Pearson Education, Inc. Hoboken, NJ. All rights reserved. 34

Data Conversion Because the contents of a text box is always a string, sometimes you must convert the input or output. dbl. Var = CDbl(txt. Box. Text) converts a String to a Double txt. Box. Text = CStr(num. Var) converts a number to a string © 2017 Pearson Education, Inc. Hoboken, NJ. All rights reserved. 35

Option Strict • Visual Basic allows numeric variables to be assigned strings and vice versa, a poor programming practice. • To prevent such assignments, set Option Strict to On in the Options dialog box. © 2017 Pearson Education, Inc. Hoboken, NJ. All rights reserved. 36

Option Strict (continued) • • Select Options from the Tools menu In left pane, expand Projects and Solution Select VB Defaults Set Option Strict to On © 2017 Pearson Education, Inc. Hoboken, NJ. All rights reserved. 37

Option Strict (continued) © 2017 Pearson Education, Inc. Hoboken, NJ. All rights reserved. 38

With Option Strict On Dim dbl. Var As Double, int. Var As Integer Dim str. Var As String Not Valid: Replace with: int. Var dec. Var dbl. Var str. Var = = dbl. Var str. Var int. Var = = CInt(dbl. Var) CDec(str. Var) CDbl(str. Var) CStr(int. Var) © 2017 Pearson Education, Inc. Hoboken, NJ. All rights reserved. 39

Concatenation Combining two strings to make a new string quote 1 = "We'll always " quote 2 = "have Paris. " quote = quote 1 & quote 2 txt. Output. Text = quote & " - Humphrey Bogart" Output: We'll always have Paris. - Humphrey Bogart © 2017 Pearson Education, Inc. Hoboken, NJ. All rights reserved. 40

Appending • To append str to the string variable var = var & str

Appending Example Dim var As String = "Good" var &= "bye" txt. Box. Text

Comment on Example 4 Consider txt. Output. Text = num. Of. Keys & " keys" The ampersand automatically converts num. Of. Keys into a string before concatenating. We do not have to convert num. Of. Keys with CStr. © 2017 Pearson Education, Inc. Hoboken, NJ. All rights reserved. 43

String Properties and Methods "Visual". Length is 6. "Visual". To. Upper is VISUAL. "123 Hike". Length is 8. "123 Hike". To. Lower is 123 hike. "a" & " bcd ". Trim & "efg" is abcdefg. © 2017 Pearson Education, Inc. Hoboken, NJ. All rights reserved. 44

Positions in a String Positions of characters in a string are numbered 0, 1, 2, …. Consider the string “Visual Basic”. Position 0: V Position 1: i Position 7: B Substring “al” begins at position 4 © 2017 Pearson Education, Inc. Hoboken, NJ. All rights reserved. 45

Substring Method Let str be a string. is the substring of length n, beginning at position m in str. Substring(m, n) “Visual Basic”. Substring(2, 3) is “sua” “Visual Basic”. Substring(0, 1) is “V” © 2017 Pearson Education, Inc. Hoboken, NJ. All rights reserved. 46

Index. Of Method Let str 1 and str 2 be strings. str 1. Index. Of(str 2) is the position of the first occurrence of str 2 in str 1. (Note: Has value -1 if str 2 is not a substring of str 1. ) "Visual Basic". Index. Of("is") is 1. "Visual Basic". Index. Of("si") is 9. "Visual Basic". Index. Of("ab") is -1. © 2017 Pearson Education, Inc. Hoboken, NJ. All rights reserved. 47

The Empty String • The string "", which has no characters, is called the empty string or the zero-length string. • The statement lst. Box. Items. Add("") skips a line in the list box. • The contents of a text box can be cleared with either the statement txt. Box. Clear() or the statement txt. Box. Text = "" © 2017 Pearson Education, Inc. Hoboken, NJ. All rights reserved. 48

Initial Value of a String Variable • By default the initial value is the keyword Nothing • Strings can be given a different initial value as follows: Dim name As String = "Fred" © 2017 Pearson Education, Inc. Hoboken, NJ. All rights reserved. 49

Widening • Widening: assigning an Integer value to a Decimal or Double variable • Widening always works without any loss of data. (Every Integer value is a Decimal value and a Double value. ) • No conversion function is needed. © 2017 Pearson Education, Inc. Hoboken, NJ. All rights reserved. 50

Narrowing • Narrowing: assigning a Double value to an Integer or Decimal variable • Narrowing might not work. (Not every Double value is an Integer or Decimal value. ) • Narrowing requires the CInt or CDec function. © 2017 Pearson Education, Inc. Hoboken, NJ. All rights reserved. 51

Comments Private Sub btn. Compute_Click(. . . ) Handles btn. Compute. Click 'Calculate the balance in an account Dim rate As Decimal 'Annual rate of interest Dim cur. Balance As Decimal 'Current balance © 2017 Pearson Education, Inc. Hoboken, NJ. All rights reserved. 52

Internal Documentation 1. Other people can easily understand the program. 2. You can understand the program when you read it later. 3. Long programs are easier to read because the purposes of individual pieces can be determined at a glance. © 2017 Pearson Education, Inc. Hoboken, NJ. All rights reserved. 53

Line Continuation A long line of code can be continued on another line by using an underscore (_) preceded by a space msg = "I'm going to make " & _ "him an offer he can't refuse. " © 2017 Pearson Education, Inc. Hoboken, NJ. All rights reserved. 54

Implicit Line Continuation The line continuation character can be omitted after a comma, ampersand, or arithmetic operator. msg = "I'm going to make " & "him an offer he can't refuse. " average = sum. Of. Numbers / number. Of. Numbers © 2017 Pearson Education, Inc. Hoboken, NJ. All rights reserved. 55

Scope • The scope of a variable is the portion of the program that can refer to it. • Variables declared inside an event procedure are said to be local to the procedure and to have procedure scope. They are only available to the event procedure in which they are declared. © 2017 Pearson Education, Inc. Hoboken, NJ. All rights reserved. 56

Scope (continued) • Variables declared outside an event procedure are said to have class scope and are available to every event procedure. • Usually declared after Public Class form. Name (In Declarations section of Code Editor. ) © 2017 Pearson Education, Inc. Hoboken, NJ. All rights reserved. 57

Auto Correction Description-of-Error box © 2017 Pearson Education, Inc. Hoboken, NJ. All rights reserved.

Auto Correction Suggested-Change box © 2017 PEARSON EDUCATION, INC. HOBOKEN, NJ. ALL 59

Automatic Colorization Comments – green String literals – maroon Keywords – blue Class Name – turqoise Note: Examples of keywords are Handles, Sub, and End. Examples of class names are Form 1, Math, and Message. Box. © 2017 Pearson Education, Inc. Hoboken, NJ. All rights reserved. 60

3. 3 Input and Output • • Formatting Numeric Output Dates as Input and Output Using a Masked Text Box for Input Getting Input from an Input Dialog Box Using a Message Dialog Box for Output Named Constants Formatting Output with Zones (Optional) © 2017 Pearson Education, Inc. Hoboken, NJ. All rights reserved. 61

Formatting Numeric Output with the To. String Method String Value (12345. 628). To. String(“N 1”) 12, 345. 6 (12345. 628). To. String(“C 2”) $12, 345. 63 (0. 183). To. String(“P 0”) 18 % © 2017 Pearson Education, Inc. Hoboken, NJ. All rights reserved. 62

Dates as Input and Output • Date literal: #7/4/1776# • Declarations: Dim ind. Day As Date Dim d As Date = CDate(txt. Box. Text) Dim ind. Day As Date = #7/4/1776# © 2017 Pearson Education, Inc. Hoboken, NJ. All rights reserved. 63

Masked Text Box Control Similar to an ordinary text box, but has a Mask property that restricts what can be typed into the masked text box. Tasks button © 2017 Pearson Education, Inc. Hoboken, NJ. All rights reserved. 64

Masked Text Box Control Click on the Tasks button to reveal the Set Mask property. Click Set Mask to invoke the Input Mask dialog box. © 2017 Pearson Education, Inc. Hoboken, NJ. All rights reserved. 65

Input Mask Dialog Box © 2017 Pearson Education, Inc. Hoboken, NJ. All rights reserved.

Mask A Mask setting is a sequence of characters, with 0, L, and & having special meanings. • 0 Placeholder for a digit. • L Placeholder for a letter. • & Placeholder for a character © 2017 Pearson Education, Inc. Hoboken, NJ. All rights reserved. 67

Sample Masks • State abbreviation: LL • Phone number: 000 -0000 • Social Security

Getting Input from an Input Dialog Box string. Var = Input. Box(prompt, title) full. Name = Input. Box("Enter your full name. ", "Name") title prompt © 2017 Pearson Education, Inc. Hoboken, NJ. All rights reserved. 69

Using a Message Dialog Box for Output Message. Box. Show(prompt, title) Message. Box. Show("Nice try, but no cigar. ", "Consolation") title prompt © 2017 Pearson Education, Inc. Hoboken, NJ. All rights reserved. 70

Named Constants • Declared with Const CONSTANT_NAME As Data. Type = value • Value cannot be changed by other statements. Examples: Const MIN_VOTING_AGE As Integer = 18 Const INTEREST_RATE As Decimal = 0. 035 D Const TITLE As String = "Visual Basic" © 2017 Pearson Education, Inc. Hoboken, NJ. All rights reserved. 71