Learning to Program with the Cybiko Handheld Computer Using B2C

Chapter 12 : Functions & Subroutines

You may have noticed in the last example program that we had to create two sets of If/then/else/endif statements to handle the conversion of a grade into a letter grade.  This duplication of code is a bad practice.  If there are several of these blocks of code, and we find a bug in one of them, we have to update all of them.  For example, what if, in our example program, we decided that the grade cutoffs were 90, 80, 70, and 60?  We would have to make the modification in each of the two blocks of If statements.  This could lead to extra work, or even errors in our program if we forget to make the same changes in both places.

Fortunately, B2C has a concept called a Subroutine.  In the old days, a program was called a routine.  A Subroutine therefore is a routine within a routine.  You can declare a subroutine with the word Sub:

Sub [routine-name] [(var as type, var as type, …)]
   'statements
End Sub

The Parameters portion of the Subroutine is a list of variables (declared much in the same way as the Dim statement).  These variables are the inputs to the subroutine.  Once declared, a subroutine can be called (or invoked) with the Call statement

[Call] routine-name [(parameters)]

The keyword Call is optional.

Here is an example using our grade-averaging program:

Sub letterGrade(grade as int)
    if grade >=94 then
       print grade, "=A"
    elseif grade>=86 then
       print grade,"=B"
    elseif grade>=80 then
       print grade, "=C"
    elseif grade>=74 then
       print grade, "=D"
    else
       print grade, "=F"
    end if
End Sub

You may exit a Subroutine early by executing the command Exit Sub.

Another way to solve this problem is with a Function.  A function is like a subroutine, but it returns a value.  In our example a function could return the letter grade, given the numeric grade.  To return a value, you set the name of the function to the value, just like an assignment statement.  As with the Subroutine, you may exit a function early with the Exit Function command.

Function letterGrade(grade as int) as char
    if grade >=94 then
       letterGrade = 65 ‘ascii characters but we didn’t teach it
    elseif grade>=88 then
       letterGrade = 66
    elseif grade>=80 then
       letterGrade = 67
    elseif grade>=74 then
       letterGrade = 68
    else
       letterGrade = 70
    end if
End Function

Example program:

' chapter 11 example program
' sum and average of n grades
' grades are from 0-100
Sub letterGrade(grade as double)
    if grade >=94 then
       print grade, "=A"
    elseif grade>=86 then
        print grade,"=B"
    elseif grade>=80 then
       print grade, "=C"
    elseif grade>=74 then
       print grade, "=D"
    else
       print grade, "=F"
    end if
End Sub
 
dim sum    'this variable will store the sum of the n grades
dim avg    'this variable will store the average of the n grades
dim n as int   ' the number of grades to average
 
print "How many grades?"
input n
dim grade[n]  ' an array of n grades
 
 ' ---INPUT ---
for i=0 to n-1                 'get the inputs
    print "Enter grade ", i   ' notice that we indent in loops
    input grade[i]                'get the grades from the student
next
 
'--- PROCESS ---
' compute the sum and the average
for i=0 to n-1
    print "Grade ", i, ": ", grade[i]
    call letterGrade(grade[i])
    sum = sum + grade[i]   'notice we accumulate the sum
next
avg = sum/n
 
'--- OUTPUT ---
print "The average of your"
print n, " grades is ", avg
call letterGrade(avg)
 
print "Press <Enter> to continue"
dim tmp ' a temporary variable
input tmp 'wait for the user to press enter

  .