COM148X1 Interactive 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 FunctionDescriptionExample 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’) FunctionDescriptionExample 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 v2 action Next variable Example For month = 1 to 12 income = InputBox(“Income “ & month) 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 action Next 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 1 If (condition) Then Exit For End If action part 2 Next variable Example For month = 1 to 12 income = InputBox(“Income “ & month) If (income < 0) Then MsgBox(“No more income, stop input”); Exit For End if sum = 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 condition action Loop Example Do While price > 0 sum = sum + price price = InputBox(“Enter price”) Loop

Do While Loop 2 Action Condition yes no

Do While Loop 2 in VB Do action 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 10 For column = 1 to 5 name = InputBox(“Student name at row = “ & row & “, col = “ & column) ‘ do something with the name ‘... next column next row