Internal tables

Internal Tables Internal tables provide a means of taking data from a fixed structure and storing it in working memory in ABAP. The data is stored line by line in memory, and each line has the same structure. In ABAP, internal tables fulfill the function of arrays. Since they are dynamic data objects, they save the programmer the task of dynamic memory management in his or her programs. You should use internal tables whenever you want to process a dataset with a fixed structure within a program. A particularly important use for internal tables is for storing and formatting data from a database table within a program. They are also a good way of including very complicated data structures in an ABAP program.

Data Type of an Internal Table The data type of an internal table is fully specified by its line type, key, and table type.

Line Type The line type of an internal table can be any data type. The data type of an internal table is normally a structure. Each component of the structure is a column in the internal table. However, the line type may also be elementary or another internal table.

Key The key identifies table rows. There are two kinds of key for internal tables - the standard key and a user-defined key. You can specify whether the key should be UNIQUE or NON-UNIQUE. Internal tables with a unique key cannot contain duplicate entries. The uniqueness depends on the table access method.

At tables with structured row type, the standard key is formed from all character-type columns of the internal table. If a table has an elementary line type, the default key is the entire line. The default key of an internal table whose line type is an internal table, the default key is empty. At tables with nonstructured row type, the standard key consists of the entire row. If the row type is also a table, an empty key is defined.

The user-defined key can contain any columns of the internal table that are no internal table themselves, and do not contain internal tables. References are allowed as table keys. Internal tables with a user-defined key are called key tables. When you define the key, the sequence of the key fields is significant. You should remember this, for example, if you intend to sort the table according to the key.

Table type

The table type determines how ABAP will access individual table entries. Internal tables can be divided into three types: Standard tables have an internal linear index. From a particular size upwards, the indexes of internal tables are administered as trees. In this case, the index administration overhead increases in logarithmic and not linear relation to the number of lines. The system can access records either by using the table index or the key. The response time for key access is proportional to the number of entries in the table.

The key of a standard table is always non-unique. You cannot specify a unique key. This means that standard tables can always be filled very quickly, since the system does not have to check whether there are already existing entries.

Sorted tables are always saved sorted by the key. They also have an internal index. The system can access records either by using the table index or the key. The response time for key access is logarithmically proportional to the number of table entries, since the system uses a binary search. The key of a sorted table can be either unique or non-unique. When you define the table, you must specify whether the key is to be UNIQUE or NON-UNIQUE. Standard tables and sorted tables are known generically as index tables.

Hashed tables have no linear index. You can only access a hashed table using its key. The response time is independent of the number of table entries, and is constant, since the system access the table entries using a hash algorithm. The key of a hashed table must be unique. When you define the table, you must specify the key as UNIQUE. Generic Internal Tables Unlike other local data types in programs, you do not have to specify the data type of an internal table fully. Instead, you can specify a generic construction, that is, the key or key and line type of an internal table data type may remain unspecified. You can use generic internal tables to specify the types of field symbols and the interface parameters of procedures . You cannot use them to declare data objects.

Internal Tables as Dynamic Data Objects Internal tables are always completely specified regarding row type, key and access type. However, the number of lines is not fixed. Thus internal tables are dynamic data objects, since they can contain any number of lines of a particular type. The only restriction on the number of lines an internal table may contain are the limits of your system installation. The maximum memory that can be occupied by an internal table (including its internal administration) is 2 gigabytes. A more realistic figure is up to 500 megabytes. An additional restriction for hashed tables is that they may not contain more than 2 million entries. The line types of internal tables can be any ABAP data types - elementary, structured, or

internal tables. The individual lines of an internal table are called table lines or table entries. Each component of a structured line is called a column in the internal table.

Choosing a Table Type The table type (and particularly the access method) that you will use depends on how the typical internal table operations will be most frequently executed.

Standard tables This is the most appropriate type if you are going to address the individual table entries using the index.

Index access is the quickest possible access. You should fill a standard table by appending lines (ABAP APPENDstatement), and read, modify and delete entries by specifying the index (INDEX option with the relevant ABAP command). The access time for a standard table increases in a linear relationship with the number of table entries. If you need key access, standard tables are particularly useful if you can fill and process the table in separate steps. For example, you could fill the table by appending entries, and then sort it. If you use the binary search option (BINARY) with key access, the response time is logarithmically proportional to the number of table entries.

Sorted tables This is the most appropriate type if you need a table which is sorted as you fill it. You fill sorted tables using the INSERTstatement. Entries are inserted according to the sort sequence defined through the table key. Any illegal entries are recognized as soon as you try to add them to the table. The response time for key access is logarithmically proportional to the number of table entries, since the system always uses a binary search. Sorted tables are particularly useful for partially sequential processing in a LOOP if you specify the beginning of the table key in the WHEREcondition.

Hashed tables This is the most appropriate type for any table where the main operation is key access. You cannot access a hashed table using its index. The response time for key access remains constant, regardless of the number of table entries. Like database tables, hashed tables always have a unique key. Hashed tables are useful if you want to construct and use an internal table which resembles a database table or for processing large amounts of data.

Creating Internal Tables

You define internal tables first as an abstract data type in the program or ABAP Dictionary, and then as a data object based on that, or they are declared directly as a fully specified data object. When you create an internal table as a data object, you should ensure that only the administration entry which belongs to an internal table is declared statically. The size of table headers for initial tables is currently 8 bytes. This should be heeded whenever internal tables occur as components of complex data objects.

Also, empty tables can use up a relatively high amount of storage space as components of tables. The size of the entire storage space required for an internal table is not defined in the declaration – as is the case for data objects of the type string or xstring. Table rows are added to and deleted from the table dynamically at runtime by the various statements for adding and deleting records.

Internal Tables Internal tables are dynamic variable data objects. Like all variables, you declare them using the DATA statement.

You can also declare static internal tables in procedures using the STATICSstatement, and static internal tables in classes using the CLASS-DATAstatement.

This description is restricted to the DATAstatement. However, it applies equally to the STATICS and CLASS-DATA statements.

Referring to Known Table Types Like all other data objects, you can declare internal tables using the LIKE or TYPE addition of the DATA statement.

DATA itab TYPE type|LIKE obj [WITH HEADER LINE].

Here, the LIKE addition refers to an existing table object in the same program. The TYPE addition can refer to an internal type in the program declared using the TYPES statement, or a table type in the ABAP Dictionary.

You must ensure that you only refer to tables that are fully typed. Referring to generic table types (ANY TABLE, INDEX TABLE) or not specifying the key fully is not allowed (for exceptions, refer to Special Features of Standard Tables).

The WITH HEADER LINE addition is obsolete; you should no longer use it. Also see the keyword documentation.

The optional addition WITH HEADER LINE declares an extra data object with the same name and line type as the internal table. This data object is known as the header line of the internal table. You use it as a work area when working with the internal table (see Using the Header Line as a Work Area).

When you use internal tables with header lines, you must remember that the header line and the body of the table have the same name. If you have an internal table with header line and you want to address the body of the table, you must indicate this by placing brackets after the table name (itab[]).

Otherwise, ABAP interprets the name as the name of the header line and not of the body of the table.

You can avoid this potential confusion by using internal tables without header lines. In particular, internal tables nested in structures or other internal tables must not have a header line, since this can lead to ambiguous expressions.

TYPES vector TYPE SORTED TABLE OF i WITH UNIQUE KEY table_line.

DATA: itab TYPE vector, jtab LIKE itab WITH HEADER LINE. * MOVE itab TO jtab. <- Syntax error!

MOVE itab TO jtab[].

The table object itab is created with reference to the table type vector. The table object jtab has the same data type as itab. jtab also has a header line. In the first MOVE statement, jtab addresses the header line. Since this has the data type I, and the table type of itab cannot be converted into an elementary type, the MOVE statement causes a syntax error. The second MOVE statement is correct, since both operands are table objects.

Declaring New Internal Tables You can use the DATA statement to construct new internal tables as well as using the LIKE or TYPEaddition to refer to existing types or objects. The table type that you construct does not exist in its own right; instead, it is only an attribute of the table object. You can refer to it using the LIKE addition, but not using TYPE. The syntax for constructing a table object in the DATA statement is similar to that for defining a table type in the TYPESstatement.

DATA itab TYPE|LIKE tabkind OF linetype WITH key [INITIAL SIZE n] [WITH HEADER LINE].

As illustrated when you define a table type yourself, the type constructor tabkind OF linetype WITH key defines the table type tabkind, the line type linetype, and the key key of the internal table itab. Since the technical attributes of data objects are always fully specified, the table must be fully specified in the DATAstatement. You cannot create generic table types (ANY TABLE, INDEX TABLE), only fullytyped tables (STANDARD TABLE and TABLE, SORTED TABLE, HASHED TABLE). You must also specify the key and whether it is to be unique (for exceptions, refer to Special Features of Standard Tables).

As in the TYPES statement, you can, if you wish, allocate an initial amount of memory to the internal table using the INITIAL SIZEaddition. You can create an internal table with a header line using the WITH HEADER LINE addition. The header line is created under the same conditions as apply when you refer to an existing table type.

DATA itab TYPE HASHED TABLE OF spfli WITH UNIQUE KEY carrid connid.

The table object itab has the type hashed table, a line type corresponding to the flat structure SPFLI from the ABAP Dictionary, and a unique key with the key fields CARRID and CONNID. The internal table itab can be regarded as an internal template for the database table SPFLI. It is therefore particularly suitable for working with data from this database table as long as you only access it using the key.