HP 50g Graphing manual VFY Verify Program Argument, Techniques used in VFY, VFY program listing

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NAMES is demonstrated in the program VFY.

VFY (Verify Program Argument)

VFY verifies that an argument on the stack is either a name or a list that contains exactly two names.

Level 1	→	Level 1
'name'	→	'name'
{ valid list }	→	{ valid list }
{ invalid list }	→	{ invalid list } (and error message in status area)
invalid object	→	invalid object (and error message in status area)

Techniques used in VFY

Utility programs. VFY by itself has little use. However, it can be used with minor modifications by other programs to verify that specific object types are valid arguments.

CASE…END case structure). VFY uses a case structure to determine if the argument is a list or a name.

Structured programming. If the argument is a list, VFY calls NAMES to verify that the list contains exactly two names.

Local variable structure. VFY stores its argument in a local variable so that it can be passed to NAMES if necessary.

Logical function. VFY uses NOT to display an error message.

Required Programs

NAMES

NAMES verifies that a list argument contains exactly two names.

VFY program listing

Program:	Comments:
«
DUP	Copies the original argument to leave on
	the stack.
DTAG	Removes any tags from the argument for
	subsequent testing.
→ argm	Stores the argument in local variable argm.
«	Begins the defining procedure.
CASE	Begins the case structure.
argm TYPE 5. SAME	Tests if the argument is a list.
THEN	If so, puts the argument back on the stack
argm NAMES	and calls NAMES to verify that the list is
END	valid, then leaves the CASE structure.

226 RPL Programming Examples