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Constants

Summary:

• Definition
• Examples

See also: Variables, Records, Data Types.

Definition

Purpose:

A constant defines a read-only value identified by a name.

Syntax:

Notes:

1. identifier is the name of the constant to be defined.
2. datatype can be any data type except complex types like TEXT or BYTE.
3. value can be an integer literal, a decimal literal, or a string literal. value cannot be NULL.

Usage:

You can declare a constant to define a static value that can be used in other instructions. Constants can be global, local to a module, or local to a function.

When declaring a constant, the data type specification can be omitted. The literal value automatically defines the data type:

01 CONSTANT c1 = "Drink" -- Declares a STRING constant
02 CONSTANT c2 = 4711    -- Declares an INTEGER constant

However, in some cases, you may need to specify the data type:

01 CONSTANT c1 SMALLINT = 12000 -- Would be an INTEGER by default

01 CONSTANT n = 10
02 DEFINE a ARRAY[n] OF INTEGER

Constants can be used at any place in the language where you normally use literals:

01 CONSTANT n = 10
02 FOR i=1 TO n

Warnings:

1. A constant cannot be used in the ORDER BY clause of a static SELECT statement, because the compiler considers identifiers after ORDER BY as part of the SQL statement (i.e. column names), not as constants.
     CONSTANT position = 3
     SELECT * FROM table ORDER BY position
2. Automatic date type conversion can take place in some cases:
     CONSTANT c1 CHAR(10) = "123"
     CONSTANT c2 CHAR(10) = "abc"
     DEFINE i INTEGER
     FOR i=1 TO c1 # Constant "123" is be converted to 123
     FOR i=1 TO c2 # Constant "abc" is converted to zero!
3. Character constants defined with a string literal that is longer than the length of the datatype are truncated:
     CONSTANT s CHAR(3) = 'abcdef'
     DISPLAY s # Displays "abc"
4. The compiler throws an error when the symbol used as a constant is not defined:
     DEFINE s CHAR(c) # Error, c is not defined!
5. The compiler throws an error when the symbol used as a constant is a variable:
     DEFINE c INTEGER
     DEFINE s CHAR(c) # Error, c is a variable!
6. The compiler throws an error when you try to assign a value to a constant:
     CONSTANT c INTEGER = 123
     LET c = 345 # Error, c is a constant!
7. The compiler throws an error when the symbol used is not defined as an integer constant:
     CONSTANT c CHAR(10) = "123"
     DEFINE s CHAR(c) # Error, c is not an integer!

Tips:

1. Define common special characters with constants:
     CONSTANT C_TAB  = '\t'
     CONSTANT C_CR   = '\r'
     CONSTANT C_LF   = '\n'
     CONSTANT C_CRLF = '\r\n'

Examples

Example 1:

01 CONSTANT c1 ="Drink",   # Declares a STRING constant
02          c2 = 4711,     # Declares an INTEGER constant
03          n = 10,        # Declares an INTEGER constant
04          x SMALLINT = 1 # Declares a SMALLINT constant
05 DEFINE a ARRAY[n] OF INTEGER
06 MAIN
07   CONSTANT c1 = "Hello"
08   DEFINE i INTEGER
09   FOR i=1 TO n
10       ...
11   END FOR
12   DISPLAY c1 || c2  # Displays "Hello4711"
13 END MAIN