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CS 106, Winter 2009Class 6, Section 4

Slides by: Dr. Cynthia A. Brown, cbrown@cs.pdx.eduInstructor section 4: Dr. Herbert G. Mayer, herb@cs.pdx.edu

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Numeric Data Types

• Double– Covers a huge range of values– Used for almost all computations– Can lose some accuracy in long computations (not

a concern for the programs we do in this class)• Integer– Used for counting– Always precise– Smaller range of values than Double

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Types of Results

• Adding two integers gives an integer, adding two doubles gives a double

• Dividing two integers using / gives a double!• Which is why we have \ and mod: 14 \ 4 = 3, and 14 mod 4 = 2• The exponential function always gives a double as

a result• VB will change integers into doubles in mixed

computations, but not the reverse

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String Data Type

• Besides numbers, strings are the other kind of data we will usually be working with

• Strings are strings of characters, so let’s look at characters first

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Characters• 256 characters used to be enough for upper and

lower case letters, numbers, punctuation, and a few oddities

• The 8-bit byte was the standard character size, in the ASCII code

• Now we want Chinese, Arabic, etc so the new system has 16 bits, allowing for 216 = 65,536 characters. The representation system is called Unicode.

• It’s very important to have standard representations for characters!

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Strings

• A string is a sequence of characters, typically stored in several words as a length and a list of characters (details vary)

• Strings are declared as followsDim professor as Stringprofessor = “Cindy Brown”

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String Operations: Concatenation

DimaString, bString, cStringas StringaString = “Hello”bString = “Dolly”cString = aString&“ “&bString‘insert a space

The value of cString is now “Hello Dolly”• You can also use aString&= “ “ &bString

(this changes aString to “Hello Dolly”)

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More String Operations

• There are plenty of useful string operations– str.Length‘length of the string– str.ToUpper‘convert to upper case– str.ToLower‘convert to lower case– str.Trim‘remove leading and trailing blanks– str.Substring(m,n) ‘substring length n starting at m– str.IndexOf(str2) ‘start of str2 in str

• Here str is the name of a String variable. See page 82 for examples

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Examples

We’ll pause here and run a few examples

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Other Data Types

• Boolean: named after logician George Boole• Can take only one of two values: True and

FalseDimvarNameas Boolean

• There are several other data types, which we will introduce if we need them

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Strict and Explicit

• You should turn on the options for Strict and Explicit in the VB system (see book pg 78)

• This will make the system enforce declarations for variables, and will make you explicitly convert values to different types when needed

• This helps by catching errors and is a very good programming practice

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Data Types

• We have learned about several data types• Double: for general computation• Integer: for counting• String: for working with text• Boolean: True and False

• Variables must be declared as one of these data types (or some other valid type), using a Dim statement

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Casting

• Converting information from one data type to another is called casting. (CInt, CStr)

• Casting is most often needed when printing numeric values in text boxes or list boxes, or reading them from text boxes

DimnumVaras DoublenumVar = 124txtBox.Text = CStr(numVar + 2) ‘convert to string

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Scope of Variables

• Variables can be declared within a sub-procedure or function, or at the level of the class

• A variable that is declared at the level of the class (also called a global variable) can be seen by all the sub-procedures and functions, and is in existence as long as the program is running

• A variable declared within a sub-procedure or function (also called a local variable) can only be seen there, and exists only as long as the sub-procedure or function is running

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Global vs LocalPublic Class formDemoDimvarAas Double‘global variable

Private Sub butRed_Click(…) HandlesbutRed.ClickDimvarB as String‘local variable<some code>End Sub

End Class

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Tricky CasePublic Class formDemoDimvarAas Double = 1‘global variable

Private Sub butRed_Click(…) HandlesbutRed.ClickDimvarA as Double = 1‘local variablevarA += 1 ‘local varA now = 2, global varA still = 1End Sub

Private Sub butBlue_Click(…) HandlesbutBlue.ClickDimvarB as Double‘local variablevarB = varA‘local varB now = 1, using global varAEnd SubEnd Class

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Types of Errors

• As you start to write code you will encounter errors

• This is a natural part of programming. Very few people can write a perfect program the first time

• There are three major types of errors– Syntax errors– Runtime errors– Logic errors

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Syntax Errors

• A syntax error is a mistake in the form of the program: writing something that is not a legal VB statement

• The editor will catch syntax errors and try to figure out what the problem is

• You cannot run your program if it has any syntax errors in it

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Runtime Errors

• With a runtime error the program will start to run, but then something happens that the system can’t handle, and you get a runtime error message

• Typical causes include dividing by zero (dividing by a variable whose value has become zero) or having a number become too large to fit in its data type

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Logic Errors

• With a logic error, the program runs fine but produces a wrong answer (an answer you didn’t expect) or behaves in a way you didn’t want it to

• Reading over the code may show you the source of the error

• As with runtime errors, using the debugger is often the quickest way to identify where the error takes place

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The Debugger

• Pages 617-618 have a quick overview of how to use the debugger

• Appendix D has a nice, thorough discussion with an example. I strongly suggest you work through that example and read the rest of the appendix

• This will save you a whole lot of time in the long run

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Formatting Output

• Output can appear in a text box, list box, or file

• Formatting lets us present neatly arranged columns, control the number of decimal places shown, etc.

• Output is almost always a string. There are cases where people make files of other data types, but we will not encounter those

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Formatting Sampler

• FormatNumber(123.628, 1): 123.6• FormatCurrency(123.628,2): $123.63• FormatCurrency(-1000): ($1000)• FormatPercent(0.185,2): 18.50%

• See the book for formatting with zones: how to make nice neat columns (pg 97)

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Postponed Topic from Chapter 3

• Reading data from files and writing to files is very important

• It’s the only way to have a permanent record of what happened in your program

• We’ll skip it for now, come back if we have time

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Break

10 min

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New Assignment

• This will be the first programming assignment• It’s due ---• Let’s take a look….

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Class example

Create an order process for an ice cream store. The user should be able to choose the number of scoops to buy. Each scoop costs $.75. After the user chooses the number of scoops, the program computes the cost, adding a 15% tip for the server.

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Interface Objects

• Place to enter the number of scoops (txtScoops)

• Place to display the price (txtPrice)• A Buy button (butBuy) [I thought of this while

doing the use case…how will the program know the user is finished entering the number?]

• Information for the user (labels)

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Use Case 1

• The user enters a number of scoops in the Scoops window

• The user pushes the Buy button• The program figures out how much to charge

by taking the number of scoops times .75 and adding the 15% charge

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Use case 2

• The user has not entered anything in the Scoops window

• The user pushes the Buy buttonExperiment shows the program blows up with a runtime error. Once we have conditionals we can test for this (Chapter 4)Experiment shows a non-number causes a blowup too

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Objects and Events

• Scoops text box– User enters a value

• Buy button– User pushes the button

• Price window– Program writes in the window

• The only really non-trivial one is pushing the Buy button

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FlowchartPush Buy button

Input a number?yes no

Handle bad input

end

Compute costAdd tip

end

Print result

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Variables and Constants

• What does the program need to remember?– How much a scoop costs– What percent the tip should be– How many scoops the user is buying

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Let’s look at the code…