[VB Super Simple Intro] 9. Program Execution Order-1

Preface

The author has been quite busy lately, so this tutorial series has been updating rather slowly. I saw some readers urging me to update in the WeChat official account backend, so I squeezed out some time to write a post~ Some of you worry that I won't have time to keep updating. Let me reiterate: as long as people are reading, I'll make time to keep updating, so please rest assured.

About Program Execution Order

When we write programs, we write them line by line, and when the program runs, it also runs line by line. However, real-world requirements are very complex. If we could only run line by line, our programs couldn't be called "programs"—they'd only be called "batch processing."

In VB, we have three ways to control the program's execution order: judgment, loops, and jumps. Below we'll learn each of them in turn.

Judgment

Judgment, as the name suggests, is judgment—for example, judging which of two numbers is larger, judging whether a username and password match, and so on. Judgment in turn has two types of statements: conditional statements and select statements.

Conditional Statements

Below is a simple conditional statement structure:

if 条件1 then
    dosth()
elseif 条件2 then
    dosth2()
else
    dosth3()
end if

For the if statement, it executes like this: if the 条件 expression evaluates to true, then the code block after then is executed; if it's false, the statements after else are executed.

Here's an example:

Comparing the Size of Two Numbers

Code function: compare the size of two numbers and output the larger one.

dim a as integer
a = 1
dim b as integer
b = 2

if a > b then
    print a
else
    print b
end if

Code execution result:

2

Checking the Username

Code function: check whether the username is an administrator

dim username as string
username = inputbox("enter your username")
if username = "admin" then
    print "you are super user"
else
    print "you are not super user"
end if

Run result:

Input: admin

Output: you are super user

Naming Months by Season

Code function: output the corresponding months based on the season entered by the user

dim season as string
season = inputbox("please enter season")
if season = "spring" then
    print "Jan, Feb, Mar"
elseif season = "summer" then
    print "Apr, May, June"
elseif season = "autumn" then
    print "July, Aug, Sept"
elseif season = "winter" then
    print "Oct, Nov, Dec"
end if

Run result:

Input: spring

Output: Jan, Feb, Mar

Select Statements

In the examples above, there's one that names months by season. There are only four seasons in a year, so it's not a big problem—you can write four if else if statements to check. But what if we reverse it and name the season by month? Would we really have to write 12 statements?

To solve the problem of overly verbose code, VB uses the select statement, which is similar to the switch statement in C, except VB's select statement is more powerful!

Syntax:

Value selection: that is, performing different operations based on different values of a variable.

select case 变量
    case 1
        call dosth1()
    case 2
        call dosth2()
    case 3
        call dosth3()
    case else
        call dosth4()
end select

How does the select case statement run? VB first evaluates the test expression, then compares the expression's value with the value of each Case. If they are equal, it executes the statement block associated with that Case. If more than one Case matches the test expression, only the first matching Case has its associated statement block executed; if none of the values in the expression list match the test expression, the statements in the Case Else clause are executed.

You can set multiple comparison values for each Case statement

' 飞机棋游戏
dim number as integer
number = int(rnd * 6) + 1
select case number
    case 1,6
        print "起飞"
    case 2,3,4,5
        print "不起飞"
end select

You can also set multiple comparison values by specifying a range

dim score as integer
score = inputbox("请输入考试分数")
select case score
    case 90 to 100
        print "优秀"
    case 80 to 90
        print "良好"
    case 60 to 80
        print "及格"
    case else
        print "不及格"
end select

Conditional selection, a unique feature of VB's select statement, and a very handy one. It's equivalent to a simplified version of the if else if structure.

dim score as integer
score = inputbox("请输入考试分数")
select case sorce
    case is < 60
        print "不及格"
        exit select
    case is > 60 and is < 80
        print "及格"
        exit select
    case is > 80 and is < 90
        print "良好"
        exit select
    case is > 90
        print "优秀"
end select

Due to time and space constraints, I won't give more examples. A more detailed introduction to the syntax will be expanded in the next update, so please stay tuned~

end

That's all for program execution order in this article. Loops and jumps will be covered in the following articles. Hope you'll keep following along~

About

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