COM148X1 Interactive Programming

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COM148X1Interactive Programming

• Lecture 3

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Topics Today

• String Functions
• Using Timer
• Looping

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String Functions
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String Class

• Visual Basic provides many useful string
  functions and they can be used through all String
  object
• Position of characters in string is starting from
  0
• All string functions will not alter the original
  string, another string variable is required to
  hold the result

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Commonly Use String Functions
Function Description Example
ToUpper() change the content to upper case s2 s.toUpper()
ToLower() change the content to lower case s2 s.ToLower()
Length() return the length of string len s.Length()
Substring(pos, len) return substring from pos with length len s2 s.substring(2, 5)
IndexOf( substring) return position of substring, return -1 if substring not found pos s.IndexOf(hello)
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Commonly Use String Functions (cont)
Function Description Example
Trim() remove leading and trailing space s2 s.toTrim()
Remove (pos, len) remove a number of characters from string starting from pos S2 s.Remove(3, 5)
Insert( pos, string) insert a string to pos s2 s.insert(3, Hello)
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Using Timer
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What is Timer

• Timer is an invisible stopwatchto do count down
  from present time
• Once timer finished count down, corresponding
  code set by programmer will be executed

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Locate Timer from Toolbox
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Timer Properties

• Enabled
• Count down will start if it is True
• Interval
• Number of milliseconds for count down, once
  finished count down, call-back function (written
  by programmer himself) will be executed

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Looping
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What is Looping

• Looping is a program control structure which
  allows computer executes part of program multiple
  times
• In Visual Basic, looping can be done by For Next
  loop or Do While loop

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For Next Loop

• For Next loop mainly used to do countable
  repetitions, for example, calculate the total
  income for a year (12 months)
• A counter variable (a variable that is used for
  count) is required for the loop to count the
  number of repetition

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For Next Loop 1
i v1
Action
yes
i i 1
i lt v2?
no
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For Next Loop 1 in VB

• For variable v1 To v2 actionNext variable
• Example
• For month 1 to 12 income InputBox(Income
  month) sum sum incomeNext month

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For Next Loop 2
i v1
Action
yes
i i c
i ? v2?
no
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For Next Loop 2 in VB

• For variable v1 To v2 Step c actionNext
  variable
• Example
• For id 1 to max_id step 2 print student name
  with ID id Next id

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For Next Loop 3
i v1
Action
yes
i i c
i ? v2?
no
exit before the loop complete
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For Next Loop 3 in VB

• For variable v1 To v2 Step c action part
  1 If (condition) Then Exit For End If action
  part 2Next variable
• Example
• For month 1 to 12 income InputBox(Income
  month) If (income lt 0) Then MsgBox(No more
  income, stop input) Exit For End if sum
  sum incomeNext month

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Do While Loop

• Unlike For Next loop, Do While loop mainly used
  to do uncountable repetitions, for example,
  supermarket calculator (different customers will
  buy different number of products)
• Do While loop can always be used to replace For
  Next loop but not the reverse in Visual Basic

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Do While Loop 1
no
Condition
yes
Action
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Do While Loop 1 in VB

• Do While condition actionLoop
• Example
• Do While price gt 0 sum sum price price
  InputBox(Enter price)Loop

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Do While Loop 2
Action
yes
Condition
no
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Do While Loop 2 in VB

• Do actionLoop While condition
• Example
• Do password InputBox(Password)Loop While
  password ltgt 21

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Exit Do While Loop Before Completion

• Similar to For Next loop, Do While loop can be
  exit before completion by using Exit Do statement

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Infinity Loop

• Loop that will never ends
• Actions that may cause infinity loop
• Modify counter variable during the execution of
  For Next loop
• Use wrong condition for Do While loop
• Forget to update variable which used in the
  condition of Do While loop
• Intentionally make it as infinity loop

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Nested Loop

• Loop exist inside another loop
• The loop inside will be executed multiple times
• Example
• For row 1 to 10 For column 1 to 5 name
  InputBox(Student name at row row , col
  column) do something with the
  name ... next columnnext row