Pascal Programming/Variables and Constants
Like all programming languages, Pascal provides some means to modify memory. This concept is known as variables. Variables are named chunks of memory. You can use them to store data you cannot predict.
Constants, on the other hand, are named pieces of data. You cannot alter them during run-time, but they are hard-coded into the compiled executable program. Constants do not necessarily occupy any dedicated unique space in memory, but facilitate writing clean and understandable source code.
In Pascal, before you are even allowed to use any variable or constant you have to declare them, like virtually any symbol in Pascal. A declaration makes a certain symbol known to the compiler and possibly instructs it to make the necessary provisions for their effective usage, that means – in the context of variables – earmark some piece of memory.
A declaration is always a two-tuple , to be more specific, variables are declared like and constant declarations are tuples. A tuple is an ordered collection. You may not reverse or rearrange its items without the tuple rendering to be different.
|After you have declared an identifier to refer to one thing, you may not re-declare the same identifier to refer to another (or same) thing (“shadowing” may apply, but more on that later).|
Identifiers are names denoting constants, types, bounds, variables, procedures, and functions. They must begin with a letter, which may be followed by any combination and numbers of letters and digits. The spelling of an identifier is significant over its whole length. Corresponding upper-case and lower-case letters are considered equivalent.
Letters refers to the modern Latin alphabet, that is all letters you use in writing English words, and digits are Western Arabic digits.
As you infer from the quote’s last sentence, the casing of letters does not matter:
fOO are both the same identifier, just different representations.
Identifiers are used simply by writing them out at a suitable position.
In the age Pascal was developed in, computer memory was a precious resource. In order to build a working compiler, however, the notion of significant characters was introduced. A significant character of an identifier is a character that contributes to distinguishing two identifiers from one another.
Some programming languages had a limit of 8 (eight) characters. This led to very cryptic identifiers. Today, however, the limit of significant characters is primarily governed by usability: The programmer eventually has to type them out if no IDE supports some auto-completion mechanism. The FPC, for example, has a limit of 127 characters:
Identifiers consist of between
127significant characters (letters, digits and the underscore character), of which the first must be a letter (
Z), or an underscore (
|You are still allowed to write identifiers longer than 127 characters, however, the compiler only looks at the first 127 characters and discards the remaining characters as irrelevant.|
_, too, is an ISO 10206 (“Extended Pascal”) extension, but – unlike the FPC – it imposes the restriction that an identifier may neither begin or end an identifier, nor may two underscores appear one another.
Variables are declared in a dedicated section, the
program varDemo(input, output); var number: integer; begin write('Enter a number: '); readLn(number); writeLn('Great choice! ', number, ' is awesome.'); end.
When the compiler processes the
var-section it will set as much memory aside as is required by its associated data type.
Here, we instruct the compiler to reserve space for an
integer is a data type that is part of the programming language, thus it is guaranteed to be present regardless of the used compiler.
It stores a subset of ℤ, the set of integers, like for example
Data type refers to the combination of a permissible range of values and permissible operations on this range of values.
Pascal defines some basic data types as part of the language.
integer there are also:
- A character, like a Latin letter or Western Arabic digit, but also spaces and other characters.
- A subset of ℚ, that is – due to computer’s binary nature – the set of rational numbers. Examples are
73728.5(216 + 213 + 2−1).
- A Boolean value, that is
Each data type defines how data are laid out in memory. In a high-level language, such as Pascal, it is not of the programmer’s concern how exactly the data are stored, but the processor (i. e. in most cases a compiler) has to to define it.
We will revisit all data types later on.
Reading from the consoleEdit
As you may have noticed, the example above contains
readLn(number) and the program header also lists
ReadLn will (try to) read data from the (optionally named) source and store the (interpreted) values into the supplied parameters discarding any line-end characters.
If the source is not specified, like it is the case here,
input is assumed, thus
readLn(number) is equivalent to
readLn(input, number), but shorter.
When the program is run, it will stop and wait for the user to input a number, that is a literal that can be converted into the argument’s data type.
You have to indicate in your program’s accompanying documents – the user manual – how and when the user needs to input data. Later we will learn how to treat erroneous input, but this is too complex for now.
There can be as many
var-sections as necessary, but they may not be empty.
There is also a shorthand syntax for declaring many variables of the same type:
var foo, bar, x: integer;
This will declare three independent variables, all of the
integer data type.
Nonetheless, different types have to appear in different declarations:
var x: integer; itIsSunnyInPhiladelphia: Boolean;
program constDemo(output); const answer = 42; begin writeLn('The answer to the Ultimate Question of ', 'Life, the Universe, and Everything, is: ', answer); end.
As already mentioned in the introduction, a constant may never change its value, but you have to modify the source code. Consequently, the name of a constant cannot appear on the left-hand side of an assignment.
There are some already predefined constants:
- This is the maximum
integervariable could assume. There is no minimum integer constant, but it is guaranteed that a
integervariable can at least store the value
- Likewise, this is the maximum
charvariable could assume, where maximum refers to the ordinal value using the built-in
- Are defined by the “Extended Pascal” standard.
- Refer to Boolean values.
Pascal was designed, so – among other considerations – it could be compiled in one pass, from top to bottom: The reason being to make compiling fast and simple. Distinguishing between variables and constants allows the processor to simply substitute any occurrence of a constant identifier to be replaced by its value. Thus, a constant does not need any special treatment like a variable, yet allows the programmer to reuse reappearing data.
- Jensen, Kathleen; Wirth, Niklaus. Pascal – user manual and report (4th revised ed.). doi:10.1007/978-1-4612-4450-9. ISBN 978-0-387-97649-5.
- Michaël Van Canneyt (September 2017). "§1.4". Free Pascal Reference guide. version 3.0.4. p. 15. ftp://ftp.freepascal.org/pub/fpc/docs-pdf/ref.pdf. Retrieved 2019-12-14.