More About Input (Advanced) - Commodore 1541 User Manual

but many programs cheat on safety a bit and use
800 INPUT#5,NAME$ ,STREET$ ,CITY$
810 GOSUB 59990:REM CHECK FOR DISK ERRORS
This is done primarily when top speed in the program is essential, and there is little or n
risk of reading improper data from the file .
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MORE ABOUT INPUT# (FOR ADVANCED USERS)
Troublesome Characters
After you begin using data files regularly , you may encounter two Basic error
messages more or less frequently . They are " STRING TOO LONG ERROR " and "FILE
DATA ERROR ". Both are likely to halt your program at an Input# statement, but may
also have been caused by errors in a Print# statement when the file was written .
"STRING TOO LONG " ERRORS
A Basic string may be up to 255 characters long , although the longest string you can
enter via a single Input statement is just under 2 lines of text (4 on the VIC 20) . This lower
limitation is due to the 88 character size of the Input buffer in Commodore's serial bus
computers . The same limit applies to Input# statements .
If a single data element (stri
ng or
number) being read from a disk file into an Input# statement contains more than 87
characters, Basic will halt with a " STRING TOO LONG ERROR ". To prevent this
error, be sure to limit each string to under 88 characters , and separate all file data items
with carriage returns (See the next section for a cure once the error has occurred.)
"FILE DATA " ERRORS
The other error message " FILE DATA ERROR" is caused by attempting to
read a
non-numeric character into a numeric variable. To a computer, a number is the characters
0 through 9, the
" + "
ana " - " signs, the decimal point(.), the SPACE character,
and
the letter " E" used in scientific notation. If any other character appears in an Input# to
a
numeric variable, " FILE DATA ERROR" will be displayed and the program will hall.
The usual causes of this error are a mismatch between the order in which variables
are
written to and read from a file, a missing carriage return within a Print# statement that
writes more than one data item , or a data item that includes either a comma or a colon
without a preceding quotation mark . Once a file data error has occurred , you should
correct it by reading the data item into a string variable, and then converting it back
to
a
number with the Basic Val() statement after removing non-numeric characters with the
string functions described in your computer users manual.
COMMAS (.) AND COLONS (:)
As suggested before, commas and colons can cause trouble in a file, because theY
delimit (end) the data element in which they appear and cause any remaining characters in
the data element to be read into the next Input# variable. (They have the same effect in
an
Input statement, causing the common " EXTRA IGNORED" error message.) However.
sometimes we really need a comma or colon within a data element, such as a name written
as " Last, First" . The cure is to precede such data elements with a quotation mark . After
a
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quotation mark, in either an Input or Input# statement, all other characters except a
carriage return or another quotation mark are accepted as part of the current data element.
eXAMPLES :
To force a quotation mark into a data element going to a file , append a CHR$(34) to
the start
of tbe..fiata element. For example:
PRINT#2,CHR$(34) + " STRASMA, JIM"
or
PRINT#2,CHR$(34);"STRASMA ,
JIM"
If you do this often, some space and time may be saved by previously defining a variable
as
equal to CHR$(34) as we did earlier with CHR$(13):
20 QT$= CHR$(34)
400 PRINT#5,QT$ +NAME$
In each case, the added quotation mark will be stripped out of the data by the Input or
Input# statement, but the comma or colon will remain safely part of the data.
NUMERIC DATA STORAGE ON DISKETTE
Inside the computer, the space occupied by a numeric variable depends only on its
type. Simple numeric variables use 7 bytes (character locations) of memory . Real array
variables use 5 byte§ per array element, and integer array elements use 2 bytes each . In
contrast, when a numeric variable or any type is written to a file, the space it occupies
depends entirely on its length, not its type .
Numeric data is written to a file in the form of a string, as if the Str$() function had
been performed on it . The first character will be a blank space if the number is positive ,
and
a minus sign ( - ) if the number is negative. Then comes the number, digit by digit.
The last character is a cursor right character.
This format allows the disk data to be read back into a string or numeric variable
later. It
is, however, somewhat wasteful of disk space, and it can be difficult to anticipate
the space required by numbers of unknown length . For this reason, some programs
convert all numeric variables into strings before writing them to diskette, and use string
functions to remove any unneeded characters in advance . Doing so still allows those data
elements to be read back into a numeric variable by Input# later, although file data errors
~y
be avoided by reading all data in as strings , and converting to numbers after the
tnforrnation is inside the computer.
For example, "N$ = MID$(STR$(N),2) " will convert a positive number N into a
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1ring N$ without the usual leading space for its numeric sign . Then instead of writing
i>RINT#5 ,N , you would use PRINT#5 ,N$ .
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