Chapter 6_The Repetition Structure

Repetition structure (or loop)

Repeatedly processes instructions until condition is met

Looping condition

The requirement for repeating the instructions

Loop exit condition

The requirement for not repeating the instructions

Pretest loop

Evaluates condition prior to processing instructions

Posttest loop

Evaluates condition after processing instructions

Do…Loop statement

• Used to code both pretest loop and posttest loop

• Begins with Do clause, ends with Loop clause

• Instructions to be repeated are placed between Do and Loop clauses

• Use While or Until keyword before condition

• Condition must evaluate to Boolean True or False

While keyword,

Until keyword

Two variations of syntax

• One for pretest loop and one for posttest loop

While keyword

Indicates that instructions should be processed while condition is true

Until keyword

Indicates that instructions should be processed until condition becomes true

Pretest loop

Appears in Do clause

Posttest loop

Appears in Loop clause

Diamond

Represents loop condition in flowchart

endless loop

Infinite loop is also called _____.

Infinite loop

The condition to end the loop is never met

Counters and Accumulators

Used to calculate subtotals, totals, and averages

Counter

Numeric variable used for counting

Accumulator

Numeric variable for accumulating (adding together) something

Initializing

Assigning beginning value

Updating (incrementing)

• Adding a number to accumulator’s value

  • Number can be positive or negative

• Done within the loop body

Priming read

Use to prime (prepare or set up) loop by performing first action prior to entering loop

Update read

Allows user to update value that controls loop’s condition

Arithmetic Assignment Operators

• Used to abbreviate an assignment statement:

  • Containing an arithmetic operator

• Format

  variableName = variableName arithmeticOperator value

• Example

  • intAge = intAge + 1 can be abbreviated

    intAge += 1

For…Next statement

• Used to code a counter-controlled loop

• Processes instructions precise number of times

• Condition tested before processing (pretest loop)

• Must specify start value, end value, and step value

Start value and end value

provide looping range

Step value

increments or decrements counter

hexagon

For clause represented by a _____ in flowchart

For…Next

Comparing the For…Next and Do…Loop Statements

1. Either can be used to code counter-controlled loop

   • _____ is more convenient

2. Declaration, initialization, update, and comparison

   • Handled by the For clause

Do…Loop

Comparing the For…Next and Do…Loop Statements

1. Must declare and initialize the counter variables

2. And update the counter variable

3. Include the appropriate comparison in the Do clause

Initialized

Updated within loop

Counters and accumulators must be (2):

Nested repetition structure

Inner loop placed entirely within outer loop

Inner loop

is referred to as nested loop

Refresh method

• Ensures that computer processes any previous lines of code that affect interface appearance

• Syntax:

  • Me.Refresh()

    • Me refers to current form

Sleep method

• Delays program execution

• Syntax:

  • System.Threading.Thread.Sleep(milliseconds)

    • Millisecond: 1/1000 of second

List box

Displays list of choices from which user can select zero or more choices

SelectionMode property

Controls number of choices that can be selected

ListBox tool

Used to add list box to interface

3 to 8

List box can be made any size you want

Windows standard: _____ to _____ choices

Collection

Group of objects treated as one unit

Items collection

Refers to group of items in list box

Index

• Unique number that identifies each item in collection

• First item has _____ of zero

Items collection’s Add method or Items.Add method

• Used to add item to list box

• Usually entered in form’s Load event procedure

Sorted property

List box

• Determines order of displayed items

• If True,

  • newly added item is placed in its proper position

• If False

  • newly added item is placed at end of list

• Uses dictionary sort order

SelectedItem property

• Contains value of selected item

• Contains empty string when no item is selected

SelectedIndex property

• Contains value of selected item’s index

• Contains -1 when no item is selected

Default list box item

• Appears when application is first loaded

• Chosen by setting either SelectedItem or SelectedIndex property

SelectedValueChanged event,
SelectedIndexChanged event

• Occur when user or code statement selects item in list box

• Can use these events to process instructions when selection is made