I learned about data objects, their definition and declaration.
DATA OBJECTS
Defining Data Objects:
The
physical units with which ABAP statements work at runtime are called internal program
data
objects.
The contents of a data object occupy memory space in the program. ABAP
statements
access
these contents by addressing the name of the data object. For example,
statements can
write
the contents of data objects in lists or in the database, they can pass them to
and receive
them
from routines, they can change them by assigning new values, and they can
compare them
in
logical expressions.
Each
ABAP data object has a set of technical
attributes, which are fully defined at all times when
an
ABAP program is running. The technical attributes of a data object are:
Data type, field length,
and
number of decimal places.
The
modifiable data objects are variables, field strings, and internal tables.
A
field string is the ABAP/4 equivalent of a structure. An internal table is the
ABAP/4 equivalent of an array. When the program starts, the memory allocation
for each data object occurs in the roll area of the program. While the program
is running, we can read the contents of a non-modifiable data object or put
data into a modifiable data object and then retrieve it. When the program ends,
the system frees the memory for all data objects and their contents are lost.
Data objects have three levels of visibility: local,
global, and external. The visibility of a data object indicates from where in
the program the data object is accessible.
Locally visible data objects are accessible only from inside the
subroutine in which they are defined. Globally visible objects can be accessed
from anywhere within the program. Externally visible objects are accessible
from outside of the program by another program.
The following figure displays these three levels
of visibility pictorially.
The local data objects in subroutine 1A are visible only from
within that subroutine. Any statement outside of it cannot access them.
Similarly, the local data objects in subroutines 1B and 2A are not accessible
from anywhere but within those subroutines.
Any statement in program 1, regardless of where the statement
appears, can access the global data objects in program 1. Similarly, any
statement in program 2 can access the global data objects in program 2.
The external data objects could be accessed from any statement in
program 1 or program 2. In actuality, whether they can be or not depends on the
type of external memory area used and the relationship between the two
programs.
Declaring Data Objects
Apart from the interface parameters of
routines, we declare all of the data objects in an ABAP program or routine in its declaration
part. The declarative
statements establish the data type of the object, along with any missing technical attributes, such as
its length or the number of decimal places. This all takes place before the program is actually
executed. The exception to this are internal tables.
Defining Literals (non-modifiable data objects)
A literal is a non-modifiable data object.
Literals can appear anywhere in a program, and they are defined merely by
typing them where needed. There
are four types: character string, numeric, floating-point, and hexadecimal.
Character String Literals
Character string literals are case-sensitive character strings of any length enclosed
within single quotes. For
example, 'JACK' is a character string literal
containing only uppercase characters. 'Caesar the cat' is a character string literal containing a mixture of upper-
and lowercase characters.
Because a character string literal is enclosed by
quotes, we cannot use a single quote by itself in the value of the literal. To
represent a single quote, we must use two consecutive single quotes.
For example, the statement write 'Caesar''s tail'. will write Caesar's tail, but the statement write 'Caesar's tail' will cause a syntax
error because it contains a single quote by itself.
Numeric Literals
Numeric literals are hard-coded numeric values with an optional leading sign.
They are not usually enclosed in quotes. However, if a numeric literal contains
a decimal, it must be coded as a character string enclosed within single
quotes. If it is not, the syntax error Statement x is not defined. Please check
your spelling. will
occur.
For example, 256 is a numeric literal, as is -99. '10.5' is a numeric literal that contains a
decimal point, so it is enclosed by single quotes. A literal can be used as the
default value for a variable, or it can be used to supply a value in a
statement.
Examples of invalid literals are: 99- (trailing
minus sign), "Confirm" (enclosed in double quotes),
and 2.2 (contains a decimal but is not
enclosed in quotes).
Floating-Point Literals
Floating-point literals are specified within single quotes as '<mantissa>E<exponent>'. The mantissa can be specified with a leading sign
and with or without decimal places, and the exponent can be signed or unsigned, with or
without leading zeros.
For example, '9.99E9', '-10E-32', and '+1E09' are valid floating-point literals.
Hexadecimal Literals
A hexadecimal literal is specified within single
quotes as if it were a character string. The permissible values are 0-9 and A-F. There must be an even number of
characters in the string, and all characters must be in uppercase.
Examples of valid hexadecimal literals are '00', 'A2E5', and 'F1F0FF'.
Examples of invalid hexadecimal literals are 'a2e5' (contains lowercase
characters), '0' (contains an uneven number of characters), "FF"(enclosed in double
quotes), and x'00' (should not have a preceding x).
Right and Wrong Ways to
Code Literals
Right
|
Wrong
|
Explanation
|
-99
|
99-
|
Trailing sign not permitted on a
numeric unless it's within quotes.
|
'-12'
|
Numerics within quotes can have
leading'12-'or trailing sign.
|
|
'12.3'
|
12.3
|
Numerics containing a decimal
point must be enclosed in single quotes.
|
'Hi'
|
"Hi"
|
Double quotes are not permitted.
|
Right
|
Wrong
|
Explanation
|
'Can''t'
|
'Can't'
|
To represent a single quote, use
two consecutive single quotes.
|
'Hi'
|
'Hi
|
Trailing quote is missing.
|
'7E1'
|
7E1
|
Floating-point values must be
enclosed within quotes.
|
'7e1'
|
Lowercase e is allowed for
floating-point literals.
|
|
'0A00'
|
'0a00'
|
Lowercase in hexadecimal literals
gives incorrect results.
|
'0A00'
|
'A00'
|
An uneven number of hexadecimal
digits gives incorrect results.
|
'0A00'
|
X'0A00'
|
A preceding or following
character is not permitted for hexadecimal literals.
|
