Reading File Data: Using Get - Commodore 1541 User Manual

READING FILE DATA: USING GET#
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Get# statement retrieves data from the disk drive, one character at a time.
Like
the similar keyboard Get statement in Basic, it only accepts a single character into
specified variable. However, unlike the Get statement, it doesn 't just fall through to th
a
n~xt
statement if there is no data to be gotten. The primary use of Get# is to retrieve fro;
diskette any data that cannot be read into an Input# statement, either because it is too Ion
to fit in the input buffer or because it includes troublesome characters .
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FORMAT FOR THE GET# STATEMENT:
GET#file#, variable list
where "file#" is the same file number given in the desired file's current Open statement
and "variable list" is one or more valid Basic variable names . If more than one
dat~
element is to be input by a particular Get# statement, each variable name must
be
separated from others by a comma.
In practice, you will almost never see a Get or Get# statement containing more than
one variable name. If more than one character is needed, a loop is used rather than
additional variables . Also as in the Input# statement, it is safer to use string variables
when the file to be read might contain a non-numeric character.
Data in a Get# statement comes in byte by byte, including such normally invisible
characters as the Carriage Return, and the various cursor controls . All but one will be read
properly . The exception is CHR$(0), the ASCII Null character. It is different from an
empty string(oneofthe form A$= '"'),even though empty strings are often referred to as
null strings. Unfortunately, in a Get# statement, CHR$(0) is converted into an empty
string. The cure is to test for an empty string after a Get#, and replace any that are found
with CHR$(0) instead. The first example below illustrates the method.
EXAMPLES :
To read a file that may contain a CHR$(0), such as a machine language program fi le,
we could correct any CHR$(0) bytes with
1100 GET#3,G$ :1F G$= ""THEN G$=CHR$(0)
If an overlong string has managed to be recorded in a file, it may be safely read back
into the computer with Get# , using a loop such as this
3300 B$ = ""
3310GET#l ,A$
3320 IF A$< >CHR$( 13) THEN B$ = B$
+
A$:GOTO 3310
The limit for such a technique is 255
characters. It will ignore CHR$(0), but that may
be:
an advantage in building a text string .
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Get# may be especially useful in recovering damaged files , or files with unknown
nts
The Basic reserved variable ST (the file STatus variable) can be used to indicate
' ° "te . -
whC"
all of a properly-closed file has been read .
500 GET#2,S$
510 SU=ST:REM REMEMBER FILE STATUS
520 PRINT S$; .
530 IF SU =0 THEN 500:REM IF THERE'S MORE TO BE READ
540 IF SU< > 64 THEN PRINT " STATUS ERROR : ST= ";SU
eopying ST into SU is often
an unneccessary prec~ution, but must
be done if any other
file-handling statement appears between the one which
r~ad fro~
the file and the one that
toops back to read again . For example, it would be required 1f lane 520 was changed to
520 PRINT# I ,S$;
()dterwise, the file status checked in line 530 would be that of the write file, not the read
file.
POSSIBLE VALUES OF THE FILE STATUS VARIABLE "ST" ,
AND l'HEIR MEANINGS
IFST=
0
All is OK
THEN
I Receiving device wa not available (time out on talker)
2 Tran milling device was not available (time out on listener)
4 Cassette data file block was too short
8 Cassette data file block was too long
16
Unrecoverable read error from cas ette , verify error
32 Cassette checksum error--0ne or more faulty characters were read
64 End of file reached (EOI detected)
128 Device not present, or end of tape mark found on cassette
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