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,


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.

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