Difference between revisions of "Variables and Data Types"

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The basic data types in Pascal include:
 
The basic data types in Pascal include:
  
* integer
+
* integer / word / longint
 +
* word
 
* real
 
* real
 
* char
 
* char
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More information on Pascal data types:
 
More information on Pascal data types:
  
* The '''integer''' data type can contain integers from <tt>-32768</tt> to <tt>32767</tt>. 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 '''integer''' data type can contain whole numbers. the size of an integer depends on the compiler and the processor. On PCs before the 80386, "integer" meant 16-bit  whole numbers in the range from <tt>-32768</tt> to <tt>32767</tt>. 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, <tt>2147483647</tt> to <tt>-2147483648</tt>.
 +
* The '''word''' data type is a 16-bit unsigned integer, which has a range of <tt>0</tt> to <tt>65535</tt>.
 
* The '''real''' data type has a range from <tt>3.4x10<sup>-38</sup></tt> to <tt>3.4x10<sup>38</sup></tt>, 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 <tt>E</tt> separating the mantissa from the exponent. Thus, <tt>452.13</tt> is the same as <tt>4.5213e2</tt>
 
* The '''real''' data type has a range from <tt>3.4x10<sup>-38</sup></tt> to <tt>3.4x10<sup>38</sup></tt>, 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 <tt>E</tt> separating the mantissa from the exponent. Thus, <tt>452.13</tt> is the same as <tt>4.5213e2</tt>
 
* The '''char''' data type holds characters. Be sure to enclose them in single quotes, like so: <tt>'a' 'B' '+'</tt> 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 '''char''' data type holds characters. Be sure to enclose them in single quotes, like so: <tt>'a' 'B' '+'</tt> 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.

Revision as of 19:06, 29 December 2019

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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:

  • integer / word / longint
  • word
  • real
  • char
  • boolean

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 whole numbers. the size of an integer depends on the compiler and the processor. On PCs before the 80386, "integer" meant 16-bit whole numbers in the range 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, 2147483647 to -2147483648.
  • The word data type is a 16-bit unsigned integer, which has a range of 0 to 65535.
  • 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. Note: some Unicode characters require two WideChars. See UTF-16.
  • 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

integer types
Type Range Bytes
Byte 0 .. 255 1
Shortint -128 .. 127 1
Smallint -32768 .. 32767 2
Word 0 .. 65535 2
Integer smallint or longint 2 or 4
Cardinal longword 4
Longint -2147483648 .. 2147483647 4
Longword 0..4294967295 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 types
Type Range Significant digits Bytes
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
Currency -922337203685477.5808 922337203685477.5807 8


boolean types
Type Bytes Ord(True)
Boolean 1 1
ByteBool 1 Any nonzero value
WordBool 2 Any nonzero value
LongBool 4 Any nonzero value
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