Basic Pascal Tutorial/Chapter 1/Identifiers

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1B - Identifiers (author: Tao Yue, state: changed)

Identifiers are names that allow you to reference stored values, such as variables and constants. Also, every program must be identified (get it?) by an identifier.

Rules for identifiers:

• Must begin with a letter (a..z or A..Z, Pascal is case insensitive) from the English alphabet or an underscore (_).
• Can be followed by zero or more letters (a..Z), digits (0..9), or underscores (_), in any combination.
• Cannot be the same as a keyword such as begin, for, case, absolute etc.
• May not contain special characters, such as:

 ~ ! @ # $ % ^ & * ( ) + ` - = { } [ ] : " ; ' < > ? , . / | \ (or the space character)

Reserved words

Several identifiers are reserved in Pascal -- you cannot use them as your own identifiers. According to the FPC Reference they are grouped in:

• Turbo Pascal reserved words
• Delphi reserved words
• FPC reserved words

Turbo Pascal reserved words

absolute	and	array	asm	begin	break	case	const
constructor	continue	destructor	div	do	downto	else	end
file	for	function	goto	if	implementation	in	inherited
inline	interface	label	mod	nil	not	object	of
on	operator	or	packed	procedure	program	record	reintroduce
repeat	self	set	shl	shr	string	then	to
type	unit	until	uses	var	while	with	xor

Delphi reserved words

The Delphi (II) reserved words are the same as the pascal ones, plus the following ones:

as	class	except	exports	finalization	finally	initialization
is	library	on	property	raise	threadvar	try

Free Pascal reserved words

On top of the Turbo Pascal and Delphi reserved words, Free Pascal also considers the following as reserved words:

dispose

exit

false

new

true

break

continue

Also, Pascal has several pre-defined identifiers. You can replace them with your own definitions, but then you'd be deleting part of the functionality of Pascal.

abs	arctan	boolean	char	cos	dispose	eof	eoln
exp	false	input	integer	ln	maxint	new	odd
ord	output	pack	page	pred	read	readln	real
reset	rewrite	round	sin	sqr	sqrt	succ	text
true	trunc write	writeln

Pascal is not case sensitive! MyProgram, MYPROGRAM, and mYpRoGrAm are equivalent. But for readability purposes, it is a good idea to use meaningful capitalization!

There are two possible methods you could choose to apply to your identifiers: CamelCase and underscore as space. CamelCase, as it appears, means that separate words in an identifier are capitalized, so that you have newPerson or NewPerson instead of newperson. Using underscore as space means you separate words in an identifier with underscores, so that you have new_person instead of newperson. Or you could combine the two, so that you have new_Person or New_Person instead of newperson.

Identifiers can be any length, but many Pascal compilers will only look at the first 32 characters or so. That is,

    ABCDEFGHIJKLMNOPQRSTUVWXYZABCDEFAlphaBeta
    ABCDEFGHIJKLMNOPQRSTUVWXYZABCDEFGammaDelta

may be equivalent to some Pascal compilers because the differences begin in the 33rd character. Free Pascal limits identifiers to 127 characters.

This is extremely generous. The original Pascal compiler for the CDC 6000 mainframe only noticed the first 10 characters of an identifier. This was because the CDC had a 60 bit word, and by using 6 bit characters (all upper case letters plus digits and some punctuation) an identifier could fit in one word. You could have more than 10 characters in an identifier, but only the first 10 counted, so ThisIsObviouslyAVeryLongName and ThisIsObviouslyAnEvenLongerName would be considered the same.

To make your code compilable by all compilers, use a reasonable length for identifiers -- up to 15 characters. That way, you'll also save on typing.

While it is a good idea to make identifiers to be mnemonic with the use of longer names, there is nothing wrong with using very short identifiers in specific uses. it is extremely common to use I, J, and K as the control variable in a for loop.