programming-for-dummies

224
Passing Parameters
This is how the preceding subprogram works:
✦ The first line defines the subprogram name — ConvertC2F — and its
parameter list as accepting one Single variable called Temperature. To
specify that this parameter will be passed by reference, this BASIC language
example uses the ByRef keyword.
✦ The second line plugs the value of the Temperature variable into the
conversion equation and stores the result back in the Temperature
variable, erasing the preceding value that was stored there.
✦ The third line ends the subprogram. At this point, the modified value of
the Temperature variable is sent back to the main program to use.
Every programming language uses different ways to identify when a parameter
will be passed by reference. The BASIC language uses the ByRef keyword
whereas the C language uses the ampersand symbol (&) to identify parameters
passed by reference. In the following C example, the a parameter is
passed by value but the x parameter is passed by reference:
subprogram_example (int a, float &x);
If you had the following BASIC subprogram:
SUB ConvertC2F (ByRef Temperature as Single)
Temperature = ((9/5) * Temperature) + 32
END SUB
You could call that subprogram like this:
DIM Temp AS SINGLE
Temp = 12
PRINT “This is the temperature in Celsius = “; Temp
ConvertC2F (Temp)
PRINT “This is the temperature in Fahrenheit = “; Temp
END
Running this program would produce the following:
This is the temperature in Celsius = 12
This is the temperature in Fahrenheit = 53.6
Notice that right before calling the ConvertC2F subprogram, the value of
the Temperature variable is 12, but the ConvertC2F subprogram changes
that value because the subprogram was passed the Temperature value by
reference. What happens if you run the same program except change the
subprogram to accept parameters passed by value instead, such as