Tag Archives: constant

Harbour New Data types

Posted on by vivaclipper

Data type & Syntax extensions in Harbour

In addition to Clipper’s scalar ( Character, Number, Date, Logical, MEMO, Nil ) and complex ( array, CodeBlock )  data types; Harbour has extended data types: pointer as scalar and object and hach as complex type.

For standard data types please refer here and/or here.

In database files (tables) data types of fields are predefined in structure of table.

For extended field types please refer here.

For data items other than fields (such as variables and manifest constants); in general, type of data  determined automatically by system, when assigning a value. The first basic way of this, is assigning a “literal” value.

For a working sample about constants please refer here.

cString := "This is a string" // A character string enclosed by a string delimiter
nNumber := 123.45 // A numeric value combined digits, decimal point and a sign ( + / - )
lTrue   := .T. // A T (tYy) or F (fNn) letter enclosed by two periods (.)
aArray  := {} // Arrays can be assigned literally by enclosed with curly brace

In addition to this basic literal value notations, Harbour has also extended notations:

– Data Types determined by special prefixs

— 0x… : Hexadecimal constant

  nNumber := 0x0A  // 0x prefix implies the string as Hexadecimal String  
                   // and type of resulting value become as Numeric (N) 
  ? nNumber, VALTYPE( nNumber ) // 10 N

— 0d… date constant 

    dDate_1 := 0d20121225  // 0d prefix implies the string a date string 
                           // ( instead of using CTOD() )
                           // and type of resulting value become as Date (D) 
    ? dDate_1, VALTYPE( dDate_1 ) // 25.12.2012 D

– Special literal string formats

— d”…” : Date constant

dDate_2 := d"2012-12-26" ? dDate_2, VALTYPE( dDate_2 ) // 26.12.2012 D

— t”…” : Time constant

tTime_1 := dDate_2 + t”01:31:06″

? tTime_1, VALTYPE( tTime_1 ) // 26.12.2012 01:31:06.000 T

— e”…” : Escape sequences

Escape sequences are used to define certain special characters within string literals.

( Prefix by “\” escape sequence codes within that string )

The following escape sequences are available in C and C++ language :

Escape
sequence Description            Representation

   '     single quote          byte 0x27
   "     double quote          byte 0x22
   ?     question mark         byte 0x3f
         backslash             byte 0x5c

         null character        byte 0x00
   a     audible bell          byte 0x07
   b     backspace             byte 0x08
   f     form feed - new page  byte 0x0c
   n     line feed - new line  byte 0x0a
   r     carriage return       byte 0x0d
   t     horizontal tab        byte 0x09
   v     vertical tab          byte 0x0b

   nnn   arbitrary octal value byte nnn
   xnn   arbitrary hexadecimal value byte nn

   unnnn arbitrary Unicode value.
          May result in several characters. code point U+nnnn
   Unnnnnnnn arbitrary Unicode value.
           May result in several characters. code point U+nnnnnnnn

Note that all sequences not available in Harbour.

For the new complex data type Hash, there is a literally assigning way :

hHash := { => }    // => sign indicates the hash

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

PROCEDURE Main()
SET CENT ON
SET DATE GERM
CLS

* Data Types determined by special prefixs

** 0x... : Hexadecimal constant

nNumber := 0x0A // 0x prefix implies the string as Hexadecimal String 
// and type of resulting value become as Numeric(D)
? nNumber, VALTYPE( nNumber ) // 10 N
** 0d... date constant 

 dDate_1 := 0d20121225 // 0d prefix implies the string a date string 
                       // ( instead of using CTOD() )
                       // and type of resulting value become as Date (D) 

? dDate_1, VALTYPE( dDate_1 ) // 25.12.2012 D
* Special literal string formats
** d"..." : Date constant
dDate_2 := d"2012-12-26"
? dDate_2, VALTYPE( dDate_2 ) // 26.12.2012 D 

** t"..." : Time constant
tTime_1 := dDate_2 + t"01:31:06"
? tTime_1, VALTYPE( tTime_1 ) // 26.12.2012 01:31:06.000 T

** e"..." : Escape sequences 

? e"This is\na string\nformatted by\nEscape sequences \x21

/* The result : 
This is
a string
formatted by
Escape sequences !
*/
@ MAXROW(), 0 WAIT "EOF DTS_Exts.prg" 
RETURN // DTS_Exts.Main() 

*~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ DTS_Exts

Using code blocks, again

Posted on by vivaclipper

Using code blocks again (.pdf)

What a preprocessor is ?

Posted on by vivaclipper

What a preprocessor is, how will operate, and what benefits it will offer ?

Let’s Look at Clipper 5.0’s preprocessor

C5 Preprocessor

Posted on by vivaclipper

What a preprocessor is, how will operate, and what benefits  it will offer ?

Clipper 5.0’s Preprocessor

.ch Files

Posted on by vivaclipper

What are .ch files ?

ch means “C header” and as name implied, this coding way borrowed from C language.

Normally, that files written individually and are included into .prg file via #include directive.

In general, .ch files includes pre-processor directives for constant declarations, pseudo functions and commands; including #include directives for other files. These lines are general purpose code line and may or not may #included by more than one .prg files.

For example, say we have some values like :

pi := 22 / 7
MaxUserCount := 10

If we declare this constants as above in the .prg file, this two identifiers are variables.

We can declare that values in another way :

#define pi 22 / 7
#define MaxUserCount 10

In this case this two identifiers are constant, so their values has been fixed, no changeable afterward. #define is a pre-processor directive, these two identifiers are meta-constants and can be use in the code (.prg) file, after defined place ( if defined at top of file, can be use anywhere in that file).

I we want use that two value into other code (.prg) files, we need repeat above two lines (consider a quite large list) for all that code (.prg) files. And of course when a modification required, we need edit all code files.

In this case we build a .ch file that contains that definitions and add an #include directive to code files that need this definitions.

That’s all.

.ch file extension is a general tradition and doesn’t implies any specific action to compiler.

C5 Memory Management

Posted on by vivaclipper

Clipper Memory Management

Once upon a time ..

PCs has a 640 KB memory limit and Clipper programmers was must struggled with errors such as Memory overflow, Conventional memory exhausted, Stack overflow and so on…

This article mainly focused on this subject.

In our modern era we haven’t such problems, because we have gigabytes of memory installed in our computers. Morever OS’s offers to us immense memory management possibilities transparent to us. Like we have unlimited memory to use.

But nothing can be unlimited …

This article of Roger Donnay has very useful info about efficient ways to using memory; and especially variable handling.

Variable Terms

Posted on by vivaclipper

Argument :

Generally, a value or variable supplied in a function or procedure call, or an operand supplied to an operator. In function and procedure calls, arguments are often referred to as actual parameters.

See also : Parameter

Constant :

The representation of an actual value. For example, .T. is a logical constant, string is a character constant, 21 is a numeric constant. There are no date and memo constants.

Declaration :

A statement used by the compiler to define a variable, procedure, or function identifier. The scope of the declaration is determined by the position of the declaration statement in the source file.

See Also: Identifier, Scope

Dynamic Scoping :

A method of determining an item’s existence or visibility based on the state of a program during execution. Example: A Clipper public variable may or may not be visible within a particular function, depending on whether the variable has been created and whether a previously called function has obscured it by creating a private variable with the same name.

See Also: Lexical Scoping, Scope

Field Variable :

A variable that refers to data in a database field, as opposed to data in memory.

See Also: Local Variable, Memory Variable, Variable

Filewide Declaration :

A variable declaration statement that has the scope of the entire source file. Filewide declarations are specified before the first procedure or function declaration in a program file and the program file must be compiled with the /N option.

See Also: Scope, Storage Class

Identifier :

A name that identifies a function, procedure, variable, constant or other named entity in a source program. In Clipper language, identifiers must begin with an alphabetic character and may contain alphabetic characters, numeric characters, and the underscore character.

Initialize :

To assign a starting value to a variable. If initialization is specified as part of a declaration or variable creation statement, the value to be assigned is called an initializer.

See Also: Assignment

Lexical Scoping :

A method of determining an item’s existence, visibility, or applicability (i.e., the item’s scope) by it’s position within the text of a program.

See Also: Local Variable, Scope, Static Variable

Lexically Scoped Variable :

A variable that is only accessible in a particular section of a program, where that section is defined using simple textual rules. For example, a local variable is only accessible within the procedure that declares it.

See Also: Dynamic Scoping, Local Variable, Static Variable

Lifetime of a Variable :

The period of time during which a variable retains its assigned value. The lifetime of a variable depends on its storage class.

See Also: Scope, Visibility

Local Variable :

A variable that exists and retains its value only as long as the procedure in which it is declared is active (i.e., until the procedure returns control to a higher-level procedure). Local variables are lexically scoped; they are accessible by name only within the procedure where they are declared.

See Also: Dynamic Scoping, Lexical Scoping, Static Variable

Memory Variable :

In general, a variable that resides in memory, as opposed to a database field variable. Sometimes used specifically to refer to variables of the MEMVAR storage class (private and public variables), as opposed to static or local variables.

See Also: Field Variable, Private Variable, Public Variable, Variable

Parameter :

A identifier that receives a value or reference passed to a procedure or user-defined function. A parameter is sometimes referred to as a formal parameter.

See Also: Activation, Argument, Function, Procedure, Reference

Private Variable :

A variable of the MEMVAR storage class. Private variables are created dynamically at runtime using the PRIVATE statement, and accessible within the creating procedure and any lower-level procedures unless obscured by another private variable with the same name.

See Also: Activation, Dynamic Scoping, Function, Public Variable

Public Variable :

A variable of the MEMVAR storage class. Public variables are created dynamically at runtime using the PUBLIC statement, and are accessible from any procedure at any level unless obscured by a private variable with the same name.

See Also: Activation, Dynamic Scoping, Function, Private Variable

Reference :

A special value that refers indirectly to a variable or array. If one variable contains a reference to a second variable (achieved by passing the second variable by reference in a function or procedure call), operations on the first variable (including assignment) are passed through to the second variable. If a variable contains a reference to an array, the elements of the array can be accessed by applying a subscript to the variable.

See Also: Array Reference, Parameter

Static Variable :

A variable that exists and retains its value for the duration of execution. Static variables are lexically scoped; they are only accessible within the procedure that declares them, unless they are declared as filewide, in which case they are accessible to any procedure in the source file that contains the declaration.

See Also: Dynamic Scoping, Lexical Scoping, Local Variable

Storage Class :

Defines the two characteristics of variables: lifetime and visibility.

See Also: Lifetime, Scope, Visibility

Variable :

An area of memory that contains a stored value. Also, the source code identifier that names a variable.

See Also: Local Variable, Private Variable, Static Variable

Visibility :

The set of conditions under which a variable is accessible by name. A variable’s visibility depends on its storage class.

See Also: Dynamic Scoping, Lexical Scoping