Commodore VIC-20 - Page 145

Chapter 4 Advanced BASIC Progromming 131
This has two implications.
1. The subroutine must receive parameters from the calling program.
For example, if a message asks the operator to enter a number, the
calling program should pass the minimum and maximum allowed
numbers to the subroutine as parameters.
2. The subroutine must return the operator's response to the calling
program. This variable may be a character (for example, Y
or N), a
word (such as
yes or no), or a number.
A subroutine that prompts for a reply
ofY for "yes" or N for "no"uses a
PRINT statement to ask the question, followed by a GET to receive a
one-character response. Since you may have many questions in a program
which require a response of "yes" or "no," the subroutine should also allow
for a prompt to be passed to the subroutine from the main program in a
string variable. Here are the necessary statements.
3000 REM RSK R QUESTION AND RETURN A RESPONSE OF Y OR N IN YN.
3010 PRINT ":']"
302£1 PRINT "DO YOU WRNT TO MRKE ANY CHANGES? "i
3030 GET YNS: IF YN'()"N" AND YN$()"Y" THEN 3030
3040 PRINT YNS
3050 RETURN
The string variable QU$ must be set in the program that calls the
subroutine. The subroutine
is generalized; that is, it displays any prompt.
sent to it by the main program. The response
is returned to the main
program in the string variable
YN$.
Now consider dialog that allows an operator to enter a number.
Assume that the main program passes to the subroutine the lowest allowable
numeric entry in LO% and the highest in HI%. Once the operator enters a
number within range, the subroutine will return the entered number in
NM%. Here
is the subroutine that gets the keyboard entry, checks it against
LO% and HI% values, and then passes it back to the main program in
NM%.
3~0e REM ASK FOR A NUMERIC 8E~!CTION
3510 REM RETURN THE NUMBER IN NM~
3520 REM REM NM~ MUST IE <- HI~ AND >- ~O~
3530 PRINT QUI;