Dynamic Programming in ABAP – Introduction to Field Symbols
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Dynamic Programming in ABAP – Part 1 – Introduction to Field Symbols
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Field symbol is a placeholder for data object, which points to the value present at the memory address of a data object. It does not reserve any physical memory space when we declare them. It only points to a data object at run time. Field symbols are of two types:
Typed Field Symbol
Generic Field Symbol
Typed Field Symbol – Typed field symbol can be declared as
DATA: var TYPE i VALUE 2.FIELD-SYMBOLS: <fs_num> TYPE i.
ASSIGN var TO <fs_num>.
WRITE: / <fs_num>.<fs_num> = 4.
WRITE: / var.The output will be 2 and 4.
NOTE
Typed field symbols can point to the data objects of specified type only.
After assigning a data object to a field symbol, if we make any changes to the field symbol value, then the value of corresponding data object is also updated.
Field symbol as a replacement of Work area
Modifying internal table records – We can declare a field symbol of type any structure, which we can use while looping through an internal table.
DATA: lt_mara TYPE STANDARD TABLE OF mara.FIELD-SYMBOLS: <fs_mara> TYPE mara.
SELECT * FROM mara INTO TABLE lt_mara UP TO 10 ROWS.LOOP AT lt_mara ASSIGNING <fs_mara>.<fs_mara>-matkl = 'DEMO'.
ENDLOOP.NOTE
If we change any field of structure in field symbol, the corresponding field in internal will get updated. We don’t need to write the MODIFY statement which we would have written if we had used work area. This is because work area stores a copy of the internal table row, whereas field symbol directly references the internal table row.
Hence processing of internal table with field symbol is faster than the processing of internal table with work area.
Appending to internal table – Now suppose we want to append some values to one internal table, then we can use field symbol as below:
DATA: lt_mara TYPE STANDARD TABLE OF mara.FIELD-SYMBOLS: <fs_mara> TYPE mara.
APPEND INITIAL LINE TO lt_mara ASSIGNING <fs_mara>.IF <fs_mara> IS ASSIGNED.<fs_mara>-matnr = 'MAT1'.
<fs_mara>-matkl = '001'.
UNASSIGN <fs_mara>.
ENDIF.APPEND INITIAL LINE TO lt_mara ASSIGNING <fs_mara>.IF <fs_mara> IS ASSIGNED.<fs_mara>-matnr = 'MAT2'.
<fs_mara>-matkl = '001'.
UNASSIGN <fs_mara>.
ENDIF.After executing this, the internal table will hold two rows.
NOTE
Always perform a check on field symbol that if it is assigned before doing any operation to avoid short dump. Also after doing the operation, unassign the field symbol.
Reading internal table – We can read a record of internal table using field symbol as below:
READ TABLE lt_mara ASSIGNING <fs_mara> WITH KEY matnr = 'MAT1'.Generic Field Symbol
Dynamic programming is actually implemented using generic field symbols. The most commonly used generic types are TYPE ANY and TYPE ANY TABLE.
FIELD-SYMBOLS: <fs_str> TYPE ANY.
FIELD-SYMBOLS: <fs_tab> TYPE ANY TABLE.
Here we can assign any data object to TYPE ANY field symbol whereas TYPE ANY TABLE field symbol is used for assigning any internal table.
TYPE ANY
Let us assign a work area of type MARA to a TYPE ANY field symbol and then populate the work area using field symbol.
FIELD-SYMBOLS: <fs_str> TYPE ANY.
FIELD-SYMBOLS: <fs_data> TYPE ANY.
DATA: lw_mara TYPE mara.ASSIGN lw_mara TO <fs_str>.
IF <fs_str> IS ASSIGNED.ASSIGN COMPONENT 'MATNR' OF STRUCTURE <fs_str> TO <fs_data>.
IF <fs_data> IS ASSIGNED.<fs_data> = 'MAT001'.
UNASSIGN <fs_data>.
ENDIF.UNASSIGN <fs_str>.
ENDIF.NOTE
After assigning lw_mara to <fs_str>, we cannot directly use the ‘-‘ operator on field symbol to access the fields of MARA structure i.e. <fs_str>-matnr would produce syntax error. This is because the field symbol type is declared only at runtime not at compile time.
So to access the matnr field with field symbol, first we need to assign that field component to a different field symbol and then use the new field symbol to update the matnr field as show in above code snippet.
After execution of above code snippet, the value of lw_mara-matnr would be MAT001.
TYPE ANY TABLE
We can assign any internal table to this field symbol. Let us analyze the below code snippet to understand how we could use such field symbol.
FIELD-SYMBOLS: <fs_tab> TYPE ANY TABLE.
FIELD-SYMBOLS: <fs_str> TYPE any.
FIELD-SYMBOLS: <fs_data> TYPE any.
DATA: lt_mara TYPE STANDARD TABLE OF mara.DATA: lw_mara TYPE mara.ASSIGN lt_mara TO <fs_tab>.
SELECT * FROM mara INTO TABLE lt_mara UP TO 10 ROWS.LOOP AT <fs_tab> ASSIGNING <fs_str>.IF <fs_str> IS ASSIGNED.ASSIGN COMPONENT 'MATKL' OF STRUCTURE <fs_str> TO <fs_data>.
IF <fs_data> IS ASSIGNED.IF <fs_data> EQ '01'.*********** Do some processing *********
ENDIF.UNASSIGN <fs_data>.
ENDIF.ENDIF.ENDLOOP.Reading internal table using generic field symbol
FIELD-SYMBOLS: <fs_tab> TYPE ANY TABLE.
FIELD-SYMBOLS: <fs_str> TYPE any.
DATA: lt_mara TYPE STANDARD TABLE OF mara.ASSIGN lt_mara TO <fs_tab>.
SELECT * FROM mara INTO TABLE lt_mara UP TO 10 ROWS.READ TABLE <fs_tab> ASSIGNING <fs_str> WITH KEY ('MATNR') = 'MAT001'.NOTE
Since <fs_tab> is a generic field symbol, its type will be known only at runtime, hence we cannot directly write the fields of MARA structure after WITH KEY, instead we have to write the field name within parenthesis as shown above.
In ABAP, this parenthesis indicates the compiler that the value of the operand will be decided at runtime, hence we don’t get any compilation error.
In my next blog i have explained about data references and its significance in dynamic programming. Below is the link for same.
https://blogs.sap.com/2017/09/11/dynamic-programming-in-abap-part-2-introduction-to-data-reference/
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19 Comments
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Matthew Billingham
September 5, 2017 at 7:43 am
I'm not a fan of prefixes related to typing generally, but having FS before each field symbol seems particularly useless. The names are embedded in angle brackets - we know it's an FS without the need for any prefix. Concentrate on meaningful names rather than prefixes that add zero value.
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Bhakti joshi
April 3, 2020 at 7:05 am
its for readability.we shud not invent new things just for the heck of it. readability in a program is very important for the portability and management of code.
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Sandra Rossi
April 3, 2020 at 8:14 am
What “should not invent new things” means, here?
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Bhakti joshi
April 3, 2020 at 5:03 pm
i mean the idea of ditching fs_ from the name of field symbol is against universally accepted standards and its better not to divert bcz it might reduce readability and clarity in complex coding.
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Sandra Rossi
April 3, 2020 at 5:08 pm
I've been telling developers to not use <FS_...> for 20 years. I think all the projects I was working in did not prefix them with FS. Sorry, it's not a universal rule. I don't know who invented this rule of prefixing field symbols with FS. I think those people who invented the prefix FS for field symbols were crazy
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Matthew Billingham
April 3, 2020 at 6:09 pm
"i mean the idea of ditching fs_ from the name of field symbol is against universally accepted standards"
With the greatest possible respect - this is not the case. But just to be sure I checked - twice - and it turns out - I am right. It really isn't the case..
I've been programming in ABAP since 1997. I've also defined the standards for numerous multinationals. Never was <fs_...> permitted. In code reviews, the code would be sent back and the programmer told not to be so silly. Precisely because clarity and readability are so important.
I've also managed to get the current SAP and DSAG recommendations for variable naming included at a couple of global companies. These recommendations follow the general consensus across the entire IT current programming community that clarity and meaningful names are vital to readability, and prefixes reduce clearly and readability and so should be avoided.
Do you also prefix forms with F_
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Bhakti joshi
April 3, 2020 at 7:48 pm
Thanks for sharing ur experience. I’ve always seen field symbols with fs_ in more than 15 years of experience seeing multitude of global MNC systems across geographies . If there are any std sap blogs / document / examples without this convention then it will be good to refer. Maybe within scn site itself . tat is in case it’s worth spending any more time. thanks
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Sandra Rossi
April 4, 2020 at 6:25 am
Avoid prefixes (documentation “Clean ABAP”, by SAP): https://github.com/SAP/styleguides/blob/master/clean-abap/CleanABAP.md#avoid-encodings-esp-hungarian-notation-and-prefixes
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Patrick Van Nierop
April 4, 2020 at 10:19 am
Personally I don't even consider the 'fs_' part to be a prefix. It's completely useless in my eyes as the '<' & '>' clearly indicate that you're looking at a field-symbol.
A prefix in a field-symbol would be adding a 's_', 't_',.. etc to the name of the field-symbol. At the very least that would add some meaning :).
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