com148x1 interactive programming lecture 3. topics today string functions using timer looping
TRANSCRIPT
COM148X1Interactive Programming
Lecture 3
Topics Today
String Functions Using Timer Looping
String Functions
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
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() 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”)
Commonly Use String Functions (cont’)
Function Description Example
Trim() remove leading and trailing space s2 = s.Trim()
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”)
Using Timer
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
Locate Timer from Toolbox
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
Looping
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
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
For Next Loop 1
i = v1
Action
i = i + 1 i v2?
yes
no
For Next Loop 1 in VB
For variable = v1 To v2action
Next variable Example
For month = 1 to 12income = InputBox(“Income “ & mo
nth)sum = sum + income
Next month
For Next Loop 2
i = v1
Action
i = i + c i v2?
yes
no
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 odd id‘…
Next id
For Next Loop 3
i = v1
Action
i = i + c i v2?
yes
no
exit before the loop complete
For Next Loop 3 in VB For variable = v1 To v2 Step c
action part 1If (condition) Then
Exit ForEnd Ifaction part 2
Next variable Example
For month = 1 to 12income = InputBox(“Income “ & month)If (income < 0) Then
MsgBox(“No more income, stop input”);
Exit ForEnd ifsum = sum + income
Next month
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
Do While Loop 1
Action
Condition
yes
no
Do While Loop 1 in VB
Do While conditionaction
Loop Example
Do While price > 0sum = sum + priceprice = InputBox(“Enter price”)
Loop
Do While Loop 2
Action
Condition yes
no
Do While Loop 2 in VB
Doaction
Loop While condition Example
Do password = InputBox(“Password”)
Loop While password <> “##21#”
Exit Do While Loop Before Completion
Similar to For Next loop, Do While loop can be exit before completion by using Exit Do statement
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
Nested Loop
Loop exist inside another loop The loop inside will be executed multiple times Example
For row = 1 to 10For column = 1 to 5
name = InputBox(“Student name at row = “ & row & “, col = “ & column)
‘ do something with the name‘...
next columnnext row