Variables and Data Types
1D - Variables and Data Types (author: Tao Yue, state: changed)
Variables are similar to constants, but their values can be changed as the program runs. Variables must first be declared in Pascal before they can be used:
var IdentifierList1 : DataType1; IdentifierList2 : DataType2; IdentifierList3 : DataType3; ...
IdentifierList is a series of identifiers, separated by commas (,). All identifiers in the list are declared as being of the same data type.
The basic data types in Pascal include:
Standard Pascal does not make provision for the string data type, but most modern compilers do. Experienced Pascal programmers also use pointers for dynamic memory allocation, objects for object-oriented programming, and many others, but this gets you started.
More information on Pascal data types:
- The integer data type can contain integers from -32768 to 32767. This is the signed range that can be stored in a 16-bit word, and is a legacy of the era when 16-bit CPUs were common. For backward compatibility purposes, a 32-bit signed integer is a longint and can hold a much greater range of values.
- The real data type has a range from 3.4x10-38 to 3.4x1038, in addition to the same range on the negative side. Real values are stored inside the computer similarly to scientific notation, with a mantissa and exponent, with some complications. In Pascal, you can express real values in your code in either fixed-point notation or in scientific notation, with the character E separating the mantissa from the exponent. Thus, 452.13 is the same as 4.5213e2
- The char data type holds characters. Be sure to enclose them in single quotes, like so: 'a' 'B' '+' Standard Pascal uses 8-bit characters, not 16-bits, so Unicode, which is used to represent all the world's language sets in one UNIfied CODE system, is not supported.
- The WideChar is a two-byte character (an element of a DBCS: Double Byte Character Set) and can hold a unicode character.
- Free Pascal supports the Delphi implementation of the PChar type. PChar is defined as a pointer to a Char type, but allows additional operations. The PChar type can be understood best as the Pascal equivalent of a C-style null-terminated string, i.e. a variable of type PChar is a pointer that points to an array of type Char, which is ended by a null-character (#0). Free Pascal supports initializing of PChar typed constants, or a direct assignment. For example, the following pieces of code are equivalent:
program one; var P : PChar; begin P := 'This is a null-terminated string.'; WriteLn (P); end.
program two; const P : PChar = 'This is a null-terminated string.'; begin WriteLn (P); end.
- Free Pascal supports the String type as it is defined in Turbo Pascal: a sequence of characters with an optional size specification. It also supports ansistrings (with unlimited length) as in Delphi. And can be declared as:
variable_name : string; // if no length is given, it defaults to 255 variable_name : string[length]; // where: 1 < length <= 255
- The predefined type ShortString is defined as a string of size 255.
- AnsiStrings are strings that have no length limit. They are reference counted and are guaranteed to be null terminated. Internally, an ansistring is treated as a pointer: the actual content of the string is stored on the heap, as much memory as needed to store the string content is allocated.
- Widestrings (used to represent unicode character strings) are implemented in much the same way as ansistrings: reference counted, null-terminated arrays, only they are implemented as arrays of WideChars instead of regular Chars.
- The boolean data type can have only two values: TRUE and FALSE
An example of declaring several variables is:
var age, year, grade : integer; circumference : real; LetterGrade : char; DidYouFail : Boolean;
From the FPC manual
|Byte||0 .. 255||1|
|Shortint||-128 .. 127||1|
|Smallint||-32768 .. 32767||2|
|Word||0 .. 65535||2|
|Integer||smallint or longint||2 or 4|
|Longint||-2147483648 .. 2147483647||4|
|Int64||-9223372036854775808 .. 9223372036854775807||8|
|QWord||0 .. 18446744073709551615||8|
Free Pascal does automatic type conversion in expressions where different kinds of integer types are used.
|Real||platform dependent||???||4 or 8|
|Single||1.5E-45 .. 3.4E38||7-8||4|
|Double||5.0E-324 .. 1.7E308||15-16||8|
|Extended||1.9E-4932 .. 1.1E4932||19-20||10|
|Comp||-2E64+1 .. 2E63-1||19-20||8|
|ByteBool||1||Any nonzero value|
|WordBool||2||Any nonzero value|
|LongBool||4||Any nonzero value|