[internal tables] TYPES or DATA

Question

Hi,
starting with ABAP and try to understand internal tables. The structure
of an internal table isn't quiet clear. They have a work area and a table body,
but why do I need a additionally type of line?
Is this content, my tutorial uses DATA to define the itab and TYPES to define
the workarea (type of line). I don't really understand the different between those
commands.
Does anyone got a good web blog to understand internal tables completly?
thx
chris

Answer 1

Hi,
The Statements TYPES and DATA
Each ABAP program define its own data types using the statement.
TYPES dtype TYPE type ...
and declare its own variables or instance attributes of classes using the statement
DATA var {TYPE type} ...
Within the program or a class, you can also define local data types and variables within procedures. Local variables in procedures obscure identically-named variables in the main program or class.
When creating data types and data objects, there are a number of naming convention that also apply for other local program definitions, such as procedures. These are described in detail in the keyword documentation.
The Additions TYPE and LIKE
The additions TYPE type and LIKE dobj are used in various ABAP statements. The additions can have various meanings, depending on the syntax and context.
·        Definition of local types in a program
·        Declaration of data objects
·        Dynamic creation of data objects
·        Specification of the type of formal parameters in subroutines
·        Specification of the type of formal parameters in methods
·        Specification of the type of field symbols
Constructing New Data Types
The TYPE addition allows you to construct new data types in the TYPES, DATA; CONSTANTS; and STATICSstatements. In the TYPES statement, these are local data types in the program. In the other statements, they are attributes of new data objects, meaning that the newly defined data types are not free-standing. Rather, they are linked to database objects.This means that you can refer to them using the LIKEaddition, but not using TYPE.
To construct new data types, the addition TYPE can be used with the following type constructors:
·        Construction of reference types
REF TO type|dobj
·        Construction of structured data types
BEGIN OF struc_type.
END OF struc_type.
·        Construction of table types
tabkind OF linetype
These data types only exist during the runtime of the ABAP program.
Referring to Known Data Types or Data Objects
Using the additions TYPE or LIKE in the TYPESstatement, local data types in a program can be referred to known data types or data objects. This is mainly the case with user-defined elementary data types. If you declare variables using the additions TYPE type or LIKE dobj with statement DATA, the data type of var is already fully defined before the declaration is made.
The known types or data that are referred to must be visible at the point where the data type or variable is declared.
A known data type can be any of the following:
·        A predefined ABAP type to which you refer using the TYPE addition
·        An existing local data type in the program to which you refer using the TYPE addition
·        The data type of a local data object in the program to which you refer using the LIKE addition
·        A data type in the ABAP Dictionary to which you refer using the TYPE addition. To ensure compatibility with earlier releases, it is still possible to use the LIKE addition to refer to database tables and flat structures in the ABAP Dictionary. However, you should use the TYPE addition in new programs.
The LIKE addition takes its technical attributes from a visible data object. As a rule, you can use LIKE to refer to any object that has been declared using DATA or a similar statement, and is visible in the current context.  The data object only has to have been declared. It is irrelevant whether the data object already exists in memory when you make the LIKE reference.
·        In principle, the local data objects in the same program are visible. As with local data types, there is a difference between local data objects in procedures and global data objects. Data objects defined in a procedure obscure other objects with the same name that are declared in the global declarations of the program.
·        You can also refer to the data objects of other visible ABAP programs. These might be, for example, the visible attributes of global classes in class pools. If a global class cl_lobal has a public instance attribute or static attribute attr, you can refer to it as follows in any ABAP program:
DATA dref TYPE REF TO cl_global.
DATA:  f1 LIKE cl_global=>attr,
       f2 LIKE dref->attr.
You can access the technical properties of an instance attribute using the class name and a reference variable without first having to create an object. The properties of the attributes of a class are not instance-specific and belong to the static properties of the class.
TYPES: BEGIN OF struct,
         number_1 TYPE i,
         number_2 TYPE p DECIMALS 2,
       END OF struct.
DATA:  wa_struct TYPE struct,
       number    LIKE wa_struct-number_2,
       date      LIKE sy-datum,
       time      TYPE t,
       text      TYPE string,
       company   TYPE s_carr_id.
This example declares variables with reference to the internal type STRUCT in the program, a component of an existing data object wa_struct, the predefined data object SY-DATUM, the predefined ABAP type t and STRING, and the data element S_CARR_ID from the ABAP Dictionary.
Referring to Generic Data Types
If you refer to one of the generic predefined ABAP types of fixed length (c, n, p, x) in the TYPES or DATA statement, you must specify the undefined technical attributes.
TYPES|DATA var[(length)] TYPE type ...
TYPES|DATA var TYPE type ...
DATA: text1,
      text2 LENGTH 2,
      text3 TYPE c LENGTH 3,
      pack TYPE p DECIMALS 2 VALUE '1.225'.
This example creates three character variables with field lengths of one, two, and three bytes respectively, and a packed number variable with field length 8 bytes and two decimal places. If the attribute Fixed point arithmetic is set, the value of pack is 1.23.
This example shows how to declare elementary data objects with reference to predefined ABAP types.
PROGRAM demo_elementary_data_objects.
DATA text1  TYPE c LENGTH 20.
DATA text2  TYPE string.
DATA number TYPE i.
text1 = 'The number'.
number = 100.
text2 = 'is an integer.'.
WRITE: text1, number, text2.
This program produces the following output on the screen:
The number              100 is an integer.
In this example, the data objects text1, text2 and number are declared with the DATA statement. The technical attributes are determined by referring to the predefined ABAP types c, string, and I. Values from unnamed literals are assigned to the data objects. The contents of the named data objects are displayed on the list.
Specifying a Start Value
When you declare an elementary fixed-length variable, the DATAstatement automatically fills it with the type-specific initial value as listed in the table in the Predefined ABAP Types section.
However, you can also specify a starting value of a fixed-length elementary variable (also within a structure declaration) using the VALUE addition in the DATAstatement:
DATA var ... VALUE val|{IS INITIAL}.
Specifying start values:
DATA: counter TYPE p VALUE 1,
      date    TYPE d VALUE '19980601',
      flag    TYPE n VALUE IS INITIAL.
After this data declaration, the character string flag contains its type specific
Initial value ‘0’.
Regarsd