Small Basic Curriculum: Lesson 1.5: Branching and Subroutines

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Small Basic > Curriculum >** **Online > Lesson 1.5: Branching and Subroutines

 

Estimated time to deliver this lesson: 1 hour. (Budget more time to practice delivery and familiarize yourself with the lesson.)

 

Code Branches and Subroutines

In this lesson, you will learn how to:

  • Branch your code using the If Else EndIf key words;
  • Branch your code by using Goto statements; and
  • Create subroutines by using Sub and EndSub statements.

Branching

As you know, the computer runs a program by reading and processing the statements line by line, one at a time.

Sometimes, you may want the computer to break the flow and jump to another line of code while the program is running.

The following is an example of Conditional Branching using the condition in the If statement.

If the remainder doesn't = 0 then the program branches to Else.

TextWindow.Write("Enter a number: ")number = TextWindow.ReadNumber()  remainder = Math.Remainder(number, 2)  If remainder = 0 Then  TextWindow.WriteLine("The number is even.")Else  TextWindow.WriteLine("The number is odd.")EndIf

 

You can instruct the computer to process a line of code out of sequence if you use the Goto statement.

j = 1  lineQ:TextWindow.WriteLine(j)j =  j + 1  If j < 30 Then  Goto lineQEndIf

 

Branching in Small Basic Programs

Let’s examine the Goto statement and its various parts by writing a program.

j = 1  lineQ:TextWindow.WriteLine(j)  j =  j + 1If j < 10 Then  Goto lineQEndIf

 

In this program, the lineQ: statement is called a label, which is similar to a bookmark. You can add as many labels as you want and name them whatever you want, as long as you don’t use the same name more than once.

The Goto statement instructs the computer to run the statements after the lineQ: label again only if the condition in the If statement is true.

This is the output you will see:

http://msdn.microsoft.com/gg712878.Picture1(en-us,MSDN.10).png

In the first line of this program, you create a variable that is named j, and you set its value to 1.

Then you create a label that is named lineQ: with a colon (:) at the end.

In the next line, you tell the computer to display the value of the j variable on the screen.

Then you increase the value of the j variable by 1.

In the fourth line, you determine whether the value of the j variable is smaller than 10.

--If it is, you tell the computer to repeat the lines of code that follow the lineQ: label. In other words, you tell the computer to display the value of the j variable, increase its value by 1, and then determine whether that value is smaller than 10.

--If the value of the j variable is not smaller than 10, you tell the computer to continue to the next part of the program (or to stop running the program if no more code exists).

You can also use the Goto statement to make a program run forever.

Let’s see how Goto statements work by adding one to a familiar program.

start:TextWindow.WriteLine("How many members are in your family?")number = TextWindow.ReadNumber()  remainder = Math.Remainder(number, 2)  If remainder = 0 Then  TextWindow.WriteLine("Your family has an even number of members.")Else  TextWindow.WriteLine("Your family has an odd number of members.")EndIfGoto start

 

This program will continue to run until someone clicks the Close (X) button in the top-right corner of the text window.

This is the output you will see:

http://msdn.microsoft.com/gg712878.Picture2(en-us,MSDN.10).png

Warning: If you use Goto statements a lot, your code will be difficult to understand and to correct. Although these statements are useful sometimes, you should try to structure your programs so that you rarely use Goto statements.

Subroutines in Small Basic Programs

When we write programs, we often want the computer to run certain statements more than once. You can avoid writing the same statements over and over if you use subroutines in your programs.

By using a subroutine,  you can run one or more statements with a single instruction. To create a subroutine, you use the Sub keyword, and then you give the subroutine a specific name. You end the subroutine by using the EndSub keyword.

Look at the following subroutine named PrintHour, which opens a text window and displays the current hour.

Sub PrintHour  TextWindow.WriteLine(Clock.Hour)EndSub

The code inside the subroutine does not run until it is called by writing its name followed by (), like in the full program below.

PrintHour() Sub PrintHour  TextWindow.WriteLine(Clock.Hour)EndSub

Let’s gain a better understanding of subroutines by writing another program…

While i < 5  TextWindow.Write("Enter Dividend: ")  dividend = TextWindow.ReadNumber()      TextWindow.Write("Enter Divisor: ")  divisor = TextWindow.ReadNumber()      Divide()    TextWindow.WriteLine("Your answer is: " + answer)    i = i + 1EndWhile  Sub Divide  answer = dividend / divisor    EndSub

 

In this program, we use the Divide( ) statement to run (or “call”) the subroutine Divide from any location within the program.

This is the output you will see:

http://msdn.microsoft.com/gg712878.Picture3(en-us,MSDN.10).png

If you use subroutines, your programs will be easier to read and understand than if you use Goto statements.

In this program, you write the Divide subroutine once, but you can run it from anywhere in the program.

When you instruct the computer to run a subroutine, you use a statement that contains the name of the subroutine followed by a set of open and close parentheses. When you use this type of statement, you are calling the subroutine.

Let’s Summarize…

Congratulations!

Now you know how to:

  • Branch your code using the** If Else EndIf** key words;
  • Create a branch by using a Goto statement; and
  • Create a subroutine by using a Sub..EndSub statement.

Show What You Know

Write a program that opens a text window and then performs the following steps:

  1. Asks the user for the name, temperature, rain status, and wind status of 10 cities.
  2. Uses branching and subroutines to determine and display the total number of:
    • Cold Cities
    • Cool Cities
    • Warm Cities
    • Hot Cities
    • Rainy Cities
    • Windy Cities

To see the answers to these questions, go to the Answer Key page or Import using its Program ID: KQN133-0.

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