Using COBOL, how can I create a line sequential .dat file if a file of the specified name does not exist?
Use either
OPEN OUTPUT fd
which will create a new file every time erasing any existent data, or add the OPTIONAL keyword to the FILE-CONTROL SELECT phrase.
ENVIRONMENT DIVISION.
INPUT-OUTPUT SECTION.
FILE-CONTROL.
select OPTIONAL data-file
assign to file-name
organization is line sequential
file status is data-file-status.
and use OPEN I-O data-file. COBOL will prep an empty file for first write if need be.
See http://opencobol.add1tocobol.com/#does-opencobol-support-isam for an arbitrary example (not line sequential, but should get the point across).
Listing (and output) included here for completeness:
OCOBOL >>SOURCE FORMAT IS FIXED
*> ***************************************************************
*><* ================
*><* indexing example
*><* ================
*><* :Author: Brian Tiffin
*><* :Date: 17-Feb-2009
*><* :Purpose: Fun with Indexed IO routines
*><* :Tectonics: cobc -x indexing.cob
*> ***************************************************************
identification division.
program-id. indexing.
environment division.
configuration section.
input-output section.
file-control.
select optional indexing
assign to "indexing.dat"
organization is indexed
access mode is dynamic
record key is keyfield of indexing-record
alternate record key is splitkey of indexing-record
with duplicates
.
*> ** OpenCOBOL does not yet support split keys **
*> alternate record key is newkey
*> source is first-part of indexing-record
*> last-part of indexing-record
*> with duplicates
data division.
file section.
fd indexing.
01 indexing-record.
03 keyfield pic x(8).
03 splitkey.
05 first-part pic 99.
05 middle-part pic x.
05 last-part pic 99.
03 data-part pic x(54).
working-storage section.
01 display-record.
03 filler pic x(4) value spaces.
03 keyfield pic x(8).
03 filler pic xx value spaces.
03 splitkey.
05 first-part pic z9.
05 filler pic x value space.
05 middle-part pic x.
05 filler pic xx value all "+".
05 last-part pic z9.
03 filler pic x(4) value all "-".
03 data-part pic x(54).
*> control break
01 oldkey pic 99x99.
*> In a real app this should well be two separate flags
01 control-flag pic x.
88 no-more-duplicates value high-value
when set to false is low-value.
88 no-more-records value high-value
when set to false is low-value.
*> ***************************************************************
procedure division.
*> Open optional index file for read write
open i-o indexing
*> populate a sample database
move "1234567800a01some 12345678 data here" to indexing-record
perform write-indexing-record
move "8765432100a01some 87654321 data here" to indexing-record
perform write-indexing-record
move "1234876500a01some 12348765 data here" to indexing-record
perform write-indexing-record
move "8765123400a01some 87651234 data here" to indexing-record
perform write-indexing-record
move "1234567900b02some 12345679 data here" to indexing-record
perform write-indexing-record
move "9765432100b02some 97654321 data here" to indexing-record
perform write-indexing-record
move "1234976500b02some 12349765 data here" to indexing-record
perform write-indexing-record
move "9765123400b02some 97651234 data here" to indexing-record
perform write-indexing-record
move "1234568900c13some 12345689 data here" to indexing-record
perform write-indexing-record
move "9865432100c13some 98654321 data here" to indexing-record
perform write-indexing-record
move "1234986500c13some 12349865 data here" to indexing-record
perform write-indexing-record
move "9865123400c13some 98651234 data here" to indexing-record
perform write-indexing-record
*> close it ... not necessary, but for the example
close indexing
*> clear the record space for this example
move spaces to indexing-record
*> open the data file again
open i-o indexing
*> read all the duplicate 00b02 keys
move 00 to first-part of indexing-record
move "b" to middle-part of indexing-record
move 02 to last-part of indexing-record
*> using read key and then next key / last key compare
set no-more-duplicates to false
perform read-indexing-record
perform read-next-record
until no-more-duplicates
*> read by key of reference ... the cool stuff
move 00 to first-part of indexing-record
move "a" to middle-part of indexing-record
move 02 to last-part of indexing-record
*> using start and read next
set no-more-records to false
perform start-at-key
perform read-next-by-key
until no-more-records
*> read by primary key of reference
move "87654321" to keyfield of indexing-record
*>
set no-more-records to false
perform start-prime-key
perform read-previous-by-key
until no-more-records
*> and with that we are done with indexing sample
close indexing
goback.
*> ***************************************************************
*><* Write paragraph
write-indexing-record.
write indexing-record
invalid key
display
"rewrite key: " keyfield of indexing-record
end-display
rewrite indexing-record
invalid key
display
"really bad key: "
keyfield of indexing-record
end-display
end-rewrite
end-write
.
*><* read by alternate key paragraph
read-indexing-record.
display "Reading: " splitkey of indexing-record end-display
read indexing key is splitkey of indexing-record
invalid key
display
"bad read key: " splitkey of indexing-record
end-display
set no-more-duplicates to true
end-read
.
*><* read next sequential paragraph
read-next-record.
move corresponding indexing-record to display-record
display display-record end-display
move splitkey of indexing-record to oldkey
read indexing next record
at end set no-more-duplicates to true
not at end
if oldkey not equal splitkey of indexing-record
set no-more-duplicates to true
end-if
end-read
.
*><* start primary key of reference paragraph
start-prime-key.
display "Prime < " keyfield of indexing-record end-display
start indexing
key is less than
keyfield of indexing-record
invalid key
display
"bad start: " keyfield of indexing-record
end-display
set no-more-records to true
not invalid key
read indexing previous record
at end set no-more-records to true
end-read
end-start
.
*><* read previous by key of reference paragraph
read-previous-by-key.
move corresponding indexing-record to display-record
display display-record end-display
read indexing previous record
at end set no-more-records to true
end-read
.
*><* start alternate key of reference paragraph
start-at-key.
display "Seeking >= " splitkey of indexing-record end-display
start indexing
key is greater than or equal to
splitkey of indexing-record
invalid key
display
"bad start: " splitkey of indexing-record
end-display
set no-more-records to true
not invalid key
read indexing next record
at end set no-more-records to true
end-read
end-start
.
*><* read next by key of reference paragraph
read-next-by-key.
move corresponding indexing-record to display-record
display display-record end-display
read indexing next record
at end set no-more-records to true
end-read
.
end program indexing.
*><*
*><* Last Update: 20090220
which outputs:
Reading: 00b02
12345679 0 b++ 2----some 12345679 data here
97654321 0 b++ 2----some 97654321 data here
12349765 0 b++ 2----some 12349765 data here
97651234 0 b++ 2----some 97651234 data here
12345679 0 b++ 2----some 12345679 data here
97654321 0 b++ 2----some 97654321 data here
12349765 0 b++ 2----some 12349765 data here
97651234 0 b++ 2----some 97651234 data here
12345679 0 b++ 2----some 12345679 data here
97654321 0 b++ 2----some 97654321 data here
12349765 0 b++ 2----some 12349765 data here
97651234 0 b++ 2----some 97651234 data here
Seeking >= 00a02
12345679 0 b++ 2----some 12345679 data here
97654321 0 b++ 2----some 97654321 data here
12349765 0 b++ 2----some 12349765 data here
97651234 0 b++ 2----some 97651234 data here
12345679 0 b++ 2----some 12345679 data here
97654321 0 b++ 2----some 97654321 data here
12349765 0 b++ 2----some 12349765 data here
97651234 0 b++ 2----some 97651234 data here
12345679 0 b++ 2----some 12345679 data here
97654321 0 b++ 2----some 97654321 data here
12349765 0 b++ 2----some 12349765 data here
97651234 0 b++ 2----some 97651234 data here
12345689 0 c++13----some 12345689 data here
98654321 0 c++13----some 98654321 data here
12349865 0 c++13----some 12349865 data here
98651234 0 c++13----some 98651234 data here
12345689 0 c++13----some 12345689 data here
98654321 0 c++13----some 98654321 data here
12349865 0 c++13----some 12349865 data here
98651234 0 c++13----some 98651234 data here
12345689 0 c++13----some 12345689 data here
98654321 0 c++13----some 98654321 data here
12349865 0 c++13----some 12349865 data here
98651234 0 c++13----some 98651234 data here
Prime < 87654321
87651234 0 a++ 1----some 87651234 data here
12349865 0 c++13----some 12349865 data here
12349765 0 b++ 2----some 12349765 data here
12348765 0 a++ 1----some 12348765 data here
12345689 0 c++13----some 12345689 data here
12345679 0 b++ 2----some 12345679 data here
12345678 0 a++ 1----some 12345678 data here
on any first runs, where indexing.dat does not exist. Subsequent runs have the same output with:
rewrite key: 12345678
rewrite key: 87654321
rewrite key: 12348765
rewrite key: 87651234
rewrite key: 12345679
rewrite key: 97654321
rewrite key: 12349765
rewrite key: 97651234
rewrite key: 12345689
rewrite key: 98654321
rewrite key: 12349865
rewrite key: 98651234
prepended, as the WRITE INVALID KEY clause triggers a REWRITE to allow overwriting key and data.