Lesson 16 Sub Procedures

Sub procedures help to make programs smaller and easier to manage. A sub procedure begins with a Sub keyword and ends with an End Sub keyword. The program structure of a sub procedure is as follows:

Sub ProcedureName (parameter)
Statements
 End Sub

The parameter is a certain data that is passed into the sub procedure to perform a specified task.

Example 16.1

In this example, we create a sub procedure, to sum up two values that are specified as the parameters. The main program can reference a procedure by using its name together with the parameters in the parentheses.

Private Sub Form1_Load(sender As Object, e As EventArgs) Handles MyBase.Load
sum(5, 6)
End Sub 

Sub sum(a As Single, b As Single) 
MsgBox("sum="& a + b)
End Sub 

Running the program produces a message box

Example 16.2: Password Cracker

This is Visual Basic 2015 program that demonstrates how to crack passwords. It can generate possible passwords and compare each of them with the actual password; and if the generated password found to be equal to the actual password, login will be successful.

In this program, a timer is inserted into the form and it is used to do a repetitive job of generating the passwords.We create passwords generating procedure generate () and it is called by the  Timer1_Tick() event so that the procedure is repeated after every interval.

The interval of the timer can be set in its properties window. A value of 1 is 1 millisecond and a value of 1000 is 1 second. We shall set the Timer's interval at 100 which is equivalent to 0.1 seconds. The Timer1.Enabled property is set to false so that the program will only start generating the passwords after the user clicks on the Generate button. Rnd is a Visual Basic 2015 function that generates a random number between 0 and 1. Multiplying Rnd by 100 will produce a number between 0 and 100. Int is a Visual Basic 2015 function that returns an integer by ignoring the decimal part of that number.

Therefore, Int(Rnd*100) will produce a number between 0 and 99, and the value of Int(Rnd*100)+100 will produce a number between 100 and 199.Finally, the program uses If…Then…Else to check whether the generated password is equal the actual password or not. If they are equal, the passwords generating process will be terminated by setting the Timer1.Enabled property to false.

The Code

Public Class Form1
Dim password As Integer Dim crackpass As Integer

Private Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click
Timer1.Enabled = True
End Sub

Private Sub Timer1_Tick(sender As Object, e As EventArgs) Handles Timer1.Tick

generate()

If crackpass = password Then

Timer1.Enabled = False
Label1.Text = crackpass
MsgBox("Password Cracked!Login Successful!")
Else Label1.Text = crackpass
Label2.Text = "Please wait..."

End If

End Sub

Sub generate()
crackpass = Int(Rnd() * 100) + 100
End Sub


Private Sub Form1_Load(sender As Object, e As EventArgs) Handles MyBase.Load
password = 123
End Sub
End Class

The output