Using a LOOKUP attribute in a RECORDOF function is useful when fields were present in the original and later dropped or when you want to write to a file that matches the layout of an existing file, but you don't know the layout.

The LOOKUP attribute in the RECORDOF function takes a filename rather than a dataset. The result is expanded at compile time to the record layout stored in the named file's metadata. There are several forms of this construct:

RECORDOF('myfile', LOOKUP);
RECORDOF('myfile', defaultstructure, LOOKUP);
RECORDOF('myfile', defaultstructure, LOOKUP, OPT);

You can also specify a DATASET as the first parameter instead of a filename (a syntactic convenience) and the filename specified on the dataset will be used for the lookup.

The defaultstructure is useful for situations where the file layout information may not be available (for example, when syntax-checking locally or creating an archive). It is also useful when the file being looked up may not exist--this is where OPT should be used.

The compiler checks that the actual record structure retrieved from the distributed file system lookup contains all the fields specified, and that the types are compatible.

For example, to read a file whose structure is unknown other than that it contains an ID field, and create an output file containing all records that matched a supplied value, you could write:

myfile := DATASET('myinputfile', RECORDOF('myinputfile', { STRING id },
                                                        LOOKUP), FLAT);
filtered := myfile(id='123');
OUTPUT(filtered,,'myfilteredfile');