CHAPTER 3DATA TYPES, VARIABLES, AND EXPRESSIONS

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Terminology• Data Type – a category of data. A description of how the computer will

treat bits found in memory.• Variable – a named location in memory, treated as a particular data

type, whose contents can be changed.• Constant – a named location in memory, treated as a particular type,

whose contents cannot be changed.• Declaration – the act of creating a variable or constant and specifying

its type.• Literal – a hard-coded piece of data, part of the statement and not

based on a variable or constant declaration.• Operator – a symbol that describe how to manipulate data and

variables in memory• Expression – a combination of operators, variables, constants and/or

literals that produces a resulting piece of data• Assignment – copying the results of an expression into a variable.• Statement – a program instruction telling the CPU what to do.

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Numbers• Arithmetic Operations :

• +, -, /, *, \ (integer division), Mod (modulus, remainder), ^ (exponentiation)

• Numeric Data Types: Integer, Double

• Numeric Literals: (e.g. 10, 12.2, 0.395)• Numeric Variables

• Declaration, Assignment, Use in expressions• Numeric Expressions • Some Built-In Arithmetic Functions: Math.Sqrt, Int,

Math.Round

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Arithmetic Operations• Arithmetic operations in Visual Basic

+ addition- subtraction* multiplication/ division^ exponentiation

\ integer division (remainder is discarded)

Mod modulus (remainder of an integer division)

Operator Precedence in Numeric Expressions

• Exponentiation (^)• Unary identity and negation (+, –)• Multiplication and floating-point

division (*, /)• Integer division (\)• Modulus arithmetic (Mod)• Addition and subtraction (+, –)

first

lastParentheses can be used to override normal precedence(inner parentheses happen before outer parentheses)

Same-precedence operations occur in left-to-right order

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Numeric Expressions2 + 3

3 * (4 + 5)

2 ^ 3

13.2 + 4.5 / 3Text 49125 all 4 – separate w/comma, to 22333

All of these expressions involve numeric literals and arithmetic operators

Question: what will be the result of each of these expressions?

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.

23 \ 7 = 3 23 Mod 7 = 2 8 \ 2 = 4 8 Mod 2 = 0

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Numeric VariableA numeric variable is a named location in memory that will contain a number and can be modified throughout the program’s execution.

Example variable names:speed

distanceinterestRate

balance

Numeric Variable Declaration• Variable declaration (a statement beginning

with Dim):Dim speed As Double

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variable name data type

This creates a location in memory for containing a Double value. The Double data type refers to a number that can include a fractional part (i.e. places to the right of the decimal place.

Numeric Variable Declaration• You can declare multiple variables in the

same Dim statement

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The Integer data type refers to a whole number (no fractional part included)

Dim a, b As Double This creates two Double variables

Dim a As Double, b As IntegerThis creates one Double variable and one Integer variable

Numeric Variable Assignment• Assignment:

• speed = 50

• balance = balance + balance * interestRate

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variable name Numeric expression

Note: the = symbol is an assignment operator in this case. Sometimes it is used as a test for equality (a relational operator), for example if used in a test of an If-statement

In an assignment statement, the expression to the right of the = operator is fully evaluated first, then the resulting value is placed in the variable to the left of the = operator.

What is balance after stmt is run if balance = 100 and interestRate = 5% Text 2813 to 22333

Initialization• Numeric variables are automatically

initialized to 0:Dim varName As Double

• To specify a nonzero initial valueDim varName As Double = 50

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Initialization is a variable declaration combined with an assignment

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Incrementing• To add 1 to the numeric variable var

var = var + 1

• Or as a shortcutvar += 1

• Or as a generalizationvar += numeric expression

Other shortcuts: -=, *=, /=, etc.

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Some Built-in Arithmetic Functions

Functions return a value

Square root: Math.Sqrt(9) returns 3

Convert number to integer: Int(9.7) returns 9

Rounding: Math.Round(2.7) returns 3

Widening• Widening: assigning an Integer value to

a Double variable• Widening always works. (Every Integer

value is a Double value.)• No conversion function needed.

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Narrowing• Narrowing: assigning a Double value to an

Integer variable• Narrowing might not work. (Not every

Double value is an Integer value.)• Narrowing requires the Cint function.

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String LiteralA string literal is a sequence ofcharacters surrounded by quotation marks.

Examples:"hello"

"123-45-6789""#ab cde?"

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String VariableA string variable is a name to which astring value can be assigned.

Examples:countryssnword

firstName

String Variable (continued)• Declaration:

Dim firstName As String

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variable name data type

• Assignment:firstName = "Fred"

Remember – in general an assignment statement has a variable name to the left of = and an expression to the right. The data type of the expression should be consistent with the data type of the variable. For example, you should not assign a String expression into a Double variable.

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“

The string "", which has no characters, is called the empty string or the zero-length string.

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Concatenation Combining two strings to make a new string

quote1 = "We'll always "quote2 = "have Paris."quote3 = quote1 & quote2 & " - Humphrey Bogart"

The variable called quote3 will contain:We'll always have Paris. - Humphrey Bogart

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Concatenation can be done using either the ampersand symbol (&) or the plus symbol (+)

Appending• To append str to the string variable var

var = var & str

• Or as a shortcutvar &= str

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Appending ExampleDim var As String = "Good"var &= "bye"

Resulting value in var: Goodbye

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Strings• String Properties and Methods:

Length ToUpperTrim ToLowerIndexOf Substring

String Properties and Methods "Visual".Length is 6.

"Visual".ToUpper is VISUAL.

"123 Hike".Length is 8.

"123 Hike".ToLower is 123 hike.

"a" & " bcd ".Trim & "efg" is abcdefg.

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Properties are data items associated with a class of objects. Methods are functions or subroutines associated with a class of objects. These will be discussed in detail in future lectures.

Positions in a String Positions of characters in a string are

numbered 0, 1, 2, ….Consider the string “Visual Basic”.Position 0: VPosition 1: iPosition 7: BSubstring “al” begins at position 4

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Substring Method Let str be a string.str.Substring(m, n) is the substring of length

n, beginning at position m in str.

“Visual Basic”.Substring(2, 3) is “sua”“Visual Basic”.Substring(0, 1) is “V”

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IndexOf Method Let str1 and str2 be strings.str1.IndexOf(str2)

is the position of the first occurrence of str2 in str1.(Note: Has value -1 if str2 is not a substring of str1.)

"Visual Basic".IndexOf("is") is 1."Visual Basic".IndexOf("si") is 9."Visual Basic".IndexOf("ab") is -1.

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Dates• Date literal: #7/4/1776#

• Declarations: Dim indDay As Date Dim d As Date = CDate(txtBox.Text) Dim indDay As Date = #7/4/1776#

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The CDate function converts a string to a date.

Date literals are enclosed in pound signs #.

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.

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Option Strict (continued)• Select Options from the Tools menu• In left pane, expand Projects and Solution• Select VB Defaults• Set Option Strict to On

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Option Strict (continued)

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Auto Correction

With Option Strict OnDim dblVar As Double, intVar As IntegerDim strVar As String

Not Valid: Replace with:intVar = dblVar intVar = CInt(dblVar)dblVar = strVar dblVar = CDbl(strVar)strVar = intVar strVar = CStr(intVar)

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Data ConversionBecause the contents of a text box is always a string, sometimes you must convert the input or output.

dblVar = CDbl(txtBox.Text)

txtBox.Text = CStr(numVar)

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converts a String to a Double

converts a number to a string

Named Constants• Declared with Const CONSTANT_NAME As DataType = value

• Value cannot be changed.

Examples: Const MIN_VOTING_AGE As Integer = 18 Const INTEREST_RATE As Double = 0.035 Const TITLE As String = "Visual Basic"

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Three Types of Errors• Syntax error• Runtime error• Logic error

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Some Types of Syntax Errors• Misspellings lstBox.Itms.Add(3)• Omissions lstBox.Items.Add(2 + )• Incorrect punctuation Dim m; n As Integer

Syntax Error List Window Dim m; n As Double lstResults.Items.Add(5 lstResults.Items.Add(a)

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A Type of Runtime ErrorOverflow error – this is an Exception, and will

cause the program to abort unless caught.

Dim numVar As Integer = 1000000numVar = numVar * numVar

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This is because a variable of Integer data type can only be assigned whole number values between about -2 billion and 2 billion. If you want larger numbers, you can use the Long data type.

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A Logical ErrorDim average As DoubleDim m As Double = 5Dim n As Double = 10average = m + n / 2

Value of average will be 10. Should be 7.5.This is because division takes place before addition. Using parentheses to override normal operator precedence will make this correct:

average = (m + n) / 2

Code comments• Commented code begins with ‘ and is green

• ‘determine if user has more input• Required in all remaining programs:

• 'Program name:• 'Student name:• 'My submission of this program indicates that I

have neither received nor given substantial assistance in writing this program.

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