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BASIC-SO
Disk
File
Input/Output
It's
not
quite so simple to print
or
do calculations with numeric values from random
I/O
files, however, because they are represented as strings. You must convert each
string
that
represents a number to its corresponding numeric value, either integer,
single-precision floating-point,
or
double-precision floating point. The CVI, CVS,
and CVD functions, respectively, perform this conversion.
Assume that there
is
an additional 2-character field following the 9-character social
security number that represents the number of hours worked in the previous week.
The following statements open the file, define the fields, read the first (or next)
record, and increment the variable
HW
by the number of hours worked:
10
OPEN
"R",
#3,
":F1
:PERSON"
20
FIELD
#3,
20
AS N$, 9 AS SS$, 2 AS H$
30
GET
#3
40
PRINT N$; SS$;
"HOURS
=";
CVI (H$)
50
HW = HW + CVI (H$)
60
PRINT
"TOTAL
HOURS
=";
HW
The two characters defined as H$ are converted to an integer.
To
convert a string to
a numeric variable, the number of string characters must equal the number
of
bytes
required to store the corresponding numeric data type:
Integer
Single-precision value
Double-precision value
CVI
CVS
CVD
2 bytes
4 bytes
8 bytes
If
the number
of
string characters represented by the argument of the function
is
fewer
than
required,
an
ILLEGAL FUNCTION CALL error message
is
printed and
execution halts.
If
the string variable is longer than required, the extra characters are
ignored.
Writing to a Random
1/0
File
To
store data in a random disk file, you follow the same sequence of
OPEN
and
FIELD statements as when reading from a random disk file. The
PUT
statement
writes the
128
bytes
of
the
I/O
buffer to the disk file.
The FIELD statement, however, defines fixed-length fields in the
I/O
buffer. To get
the data to be written to disk into the
I/O
buffer, there are two special assignment
statements: LSET and RSET. LSET left-aligns a variable
in
the buffer field, and
RSET right-aligns it.
In
both
cases, if the variable
is
shorter than the field, it
is
pad-
ded with ASCII blanks
(20H);
if
the variable
is
longer than the field, it
is
truncated.
For
example, assume you have opened a file for random
I/O,
assigned it file number
3, and defined the first
20
characters as N$. You must use LSET
or
RSET to place
values in the buffer:
10 OPEN
"R",
#3,
":F1
:PERSON"
20
FIELD
#3,
20
AS N$
30
LSET N$ =
A$
40
PUT #3,1
This technique may be used for storing variables for use in another program, linked
to
the first program with the RUN statement (see description
of
RUN, chapter 6).
Whatever the length
of
A$, its leftmost
20
characters are now identified as N$.
If
A$
is
less
than
20
characters long, the remaining characters
of
N are set
to
ASCII blanks
(20H). Statement 40 writes all
128
bytes
of
the
I/O
buffer to disk file number
#3.
5-7