PREFIX(prefix{:nbr_of_char_replaced})

The PREFIX keyword is used to partially rename the fields in an externally described file.The characters specified in the first parameter are prefixed to the names of all fields defined in all records of the externally-described file. The characters can be specified as a name, for example PREFIX(F1_), or as a character literal, for example PREFIX('F1_'). A character literal must be used if the prefix contains a period, for example PREFIX('F1DS.') or PREFIX('F1DS.A'). To remove characters from the beginning of every name, specify an empty string as the first parameter: PREFIX('':number_to_remove). In addition, you can optionally specify a numeric value to indicate the number of characters, if any, in the existing name to be replaced. If the 'nbr_of_char_replaced' is not specified, then the string is attached to the beginning of the name.

If the 'nbr_of_char_replaced' is specified, it must be a numeric constant containing a value between 0 and 9 with no decimal places. For example, the specification PREFIX(YE:3) would change the field name 'YTDTOTAL' to 'YETOTAL'. Specifying a value of zero is the same as not specifying 'nbr_of_char_replaced' at all.

The 'nbr_of_char_replaced' parameter is not used when applying the prefix to an alias name. See the ALIAS keyword for information on how the PREFIX keyword interacts with the ALIAS keyword.

Rules:
  • To explicitly rename a field on an Input specification when the PREFIX keyword has been specified for a file you must choose the correct field name to specify for the External Field Name (positions 21 - 30) of the Input specification. The name specified depends on whether the prefixed name has been used prior to the rename specification.
    • If there has been a prior reference made to the prefixed name, the prefixed name must be specified.
    • If there has not been a prior reference made to the prefixed name, the external name of the input field must be specified.
    Once the rename operation has been coded then the new name must be used to reference the input field. For more information, see External Field Name of the Input specification.
  • The total length of the name after applying the prefix must not exceed the maximum length of an RPG field name. If the file is a global file defined without the LIKEFILE or QUALIFIED keywords, the total length of the name must not exceed 14 characters, the length of the Name entries of the Input and Output specifications.
  • The number of characters in the name to be prefixed must not be less than or equal to the value represented by the 'nbr_of_char_replaced' parameter. That is, after applying the prefix, the resulting name must not be the same as the prefix string.
  • If the prefix is a character literal, it can contain a period or end in a period. In this case, the field names must all be subfields of the same qualified data structure. The data structure must be defined as a qualified data structure. For example, for PREFIX('F1DS.'), data structure F1DS must be define as a qualified data structure; if the file has fields FLD1 and FLD2, the data structure must have subfields F1DS.FLD1 and F1DS.FLD2. Similarly, for PREFIX('F2DS.A'), data structure F2DS must be a qualified data structure; if the file has fields FLD1 and FLD2, the data structure must have subfields F2DS.AFLD1 and F2DS.AFLD2.
  • If the prefix is a character literal, it must be uppercase.
  • If an externally-described data structure is used to define the fields in the file, care must be taken to ensure that the field names in the file are the same as the subfield names in the data structure. The following table shows the prefix required for an externally-described file and externally-described data structure for several prefixed versions of the name "XYNAME". When the "Internal name" column contains a dot, for example D1.NAME, the externally-described data structure is defined as QUALIFIED, and the PREFIX for the File specification must contain a dot.
    PREFIX for file PREFIX for externally-described data structure Internal name
    PREFIX(A) PREFIX(A) AXYNAME
    PREFIX(A:2) PREFIX(A:2) ANAME
    PREFIX('D.') None D.XYNAME
    PREFIX('D.' : 2) PREFIX('' : 2) D.NAME
    PREFIX('D.A') PREFIX(A) D.AXYNAME
    PREFIX('D.A' : 2) PREFIX(A : 2) D.ANAME
    PREFIX('':2) PREFIX('' : 2) NAME

Examples:

The following example adds the prefix "NEW_" to the beginning of the field names for file NEWFILE, and the prefix "OLD_" to the beginning of the field names for file OLDFILE. 
Fnewfile   o    e             disk    prefix(NEW_)
Foldfile   if   e             disk    prefix(OLD_)
C                   READ      OLDREC
C                   EVAL      NEWIDNO = OLD_IDNO
C                   EVAL      NEWABAL = OLD_ABAL
C                   WRITE     NEWREC
The following example uses PREFIX(N:2) on both file FILE1 and the externally-described data structure DS1. The File-specification prefix will cause the FILE1 fields XYIDNUM and XYCUSTNAME to be known as NIDNUM and NCUSTNAME in the program; the Data-specification prefix will cause the data structure to have subfields NIDNUM and NCUSTNAME. During the READ operation, data from the record will be moved to the subfields of DS1, which can then be passed to the subprocedure processRec to process the data in the record. 
Ffile1     if   e             disk    prefix(N:2)
D ds1           e ds                  extname(file1) prefix(N:2)
C                   READ      file1
C                   CALLP     processRec (ds1)
The following example uses prefix 'MYDS.' to associate the fields in MYFILE with the subfields of qualified data structure MYDS. 
 Fmyfile    if   e             disk    prefix('MYDS.')
 D myds          e ds                  qualified extname(myfile)
The next example uses prefix 'MYDS2.F2':3 to associate the fields in MYFILE with the subfields of qualified data structure MYDS2. The subfields themselves are further prefixed by replacing the first three characters with 'F2'. The fields used by this file will be MYDS2.F2FLD1 and MYDS2.F2FLD2. (Data structure MYDS2 must be defined with a similar prefix. However, it is not exactly the same, since it does not include the data structure name.) 
 A          R REC
 A            ACRFLD1       10A
 A            ACRFLD2        5S 0
 Fmyfile2   if   e             disk    prefix('MYDS2.F2':3)
 D myds2         e ds                  qualified extname(myfile)
 D                                     prefix('F2':3)