8 File Storage
The data list is a collection of items separated by commas. The items can be variables, array identifiers, and numeric or string expressions. Including the optional END causes an EOF mark to be printed at the end of the data; otherwise, an EOR mark is placed after the data list is printed.
Printing begins at the position of the data pointers (which is after the data item most recently stored or retrieved) or at the beginning of the file if nothing has been stored or retrieved. The record pointer can also be repositioned to the beginning of the file (see page 231 ).
Here is a simple program which creates a file named CLASS and prints the names and grades of five students:
10 CREATE "CLASS",1 20 ASSIGN #1 TO "CLASS" 30 FOR I=1 TO 5 40 INPUT "STUDENT'S NAME?";N$,"TEST SCORE?";S 50 PRINT #1;N$,S 60 NEXT I 70 PRINT #1;END 80 ENDLine 50 prints students' names and grades, alternately, in the file. Line 70 places an EOF mark after the five sets of data are printed. The EOF prevents reading data beyond its position.
When printing a long string, it might possibly be too long to be contained in one logical record. In this case, the string is automatically broken up and stored into a series of logical records. This requires an additional two words each time the string crosses over into another logical record. The parts of the string are identified at the first record, intermediate records, and the last record.
Data can be stored using the PRINT# statement in a file created with the SAVE statement. SAVE, in effect, performs a serial print into a file.
Here are two examples:
100 PRINT #3;Apples,Bananas,Carrots 110 PRINT #3;Donuts,Eggs(*)These two statements store values for all five variables into file #3. The EOR which was placed after the data when line 100 was executed is overwritten when line 110 is executed. Another EOR is printed after the data in line 110. Remember, an EOR signifies that there is no more data between the data pointers and the end of the record.
The serial PRINT# statement can also be used to generate program lines into a file. Such a file can be retrieved with GET.
Here are two examples:
50 P$="COUNTR" 60 CREATE P$,3,50 70 ASSIGN #1 TO P$ 80 PRINT #1;"10 FOR I=1 TO 10","20 PRINT I","30 NEXT I","40 END" 90 GET P$,10,10 RUN 1 2 3 4 5 6 7 8 9 10Executing LIST produces:
10 FOR I=1 TO 10 20 PRINT I 30 NEXT I 40 ENDBelow you find two examples concerning printing of User Defined Type variables.
In the example below, the Comment$ member variable from the Phone1 variable is PRINTed.
Phone1.Comment$="*Fancy Comment*" PRINT #3;Phone1.Comment$In addition to accessing single variables, you can specify the whole variable at once.
The example below prints all member variables of Phone1:
PRINT #3;STRUCT Phone1
READ# file number; variable list
Before you can use data which has been stored in a data file with a PRINT# statement, you must read the data back into the computer memory. The data is not erased from the file, it is merely copied into the variables specified in the same order in which it was stored with the PRINT# statement. The User Defined Types can be used in the same way as with PRINT#. Variables do not have to have the same names specified in the PRINT# statement. Reading begins after the last item printed or read on the specified file. To begin reading from the beginning of the file, you must reposition the record pointer (see page 231 ) or do another ASSIGN.
As an example, the data printed in the previous example in a file named CLASS can be read by using this program:
10 ASSIGN #1 TO "CLASS" 20 PRINT " NAME GRADE" 30 FOR I=1 TO 5 40 READ #1;Name$,Score 50 PRINT Name$,Score 60 NEXT I 70 ENDNotice that the serial READ# statement must specify the types of data (data elements or string variable) in the order in which they were originally stored in the file. Line 40 reads a string variable and then a numeric variable. This program can run only when the order of the data on file is known. Here is the printout:
NAME GRADE Charlie Brown 79 Casey Jones 99 Sean Jackson 91 Jack Allison 83 Sam Amigo 95The variables into which you read data items need not have the same names used when the items were printed on the file. Although the variable name changes (from N$ and S when stored, to Name$ and Score when retrieved), the order in which the two data types are accessed is the same.
When a serial READ# statement encounters the EOF mark previously placed by the last PRINT# statement, the program ends and an error indicates the end of the file. The program can be written to run without displaying an error by using the ON END# statement, described later in this chapter.
READ# file number ,record number
A serial PRINT# or READ# statement can then be executed to access the beginning of the specified record, rather than the beginning of only the first record in the file.
To see how this works, first use the next program to store consecutive values beginning from the 8th record of a file named NUMBERS:
10 CREATE "NUMBERS",15 20 ASSIGN #1 TO "NUMBERS" 30 READ #1,8 40 FOR Value=1 TO 300 50 PRINT #1;Value 60 NEXT Value 70 ENDThe ASSIGN statement sets the record pointer to the beginning of the first record in the file. The pointer is then repositioned to the beginning of the eighth record by the READ# statement. The FORNEXT and PRINT statements fill the file with the numbers 1 through 300, starting at the eighth record.
Now use the following program to read and display the data, beginning at record 14:
10 DIM A(7) 20 ASSIGN #1 TO "NUMBERS" 30 READ #1,14 40 FOR I=1 TO 12 50 READ #1;A(*) 60 DISP A(*) 70 NEXT I 80 END RUN 193 194 195 196 197 198 199 200 record 14 201 202 203 204 205 206 207 208 209 210 211 212 213 214 215 216 217 218 219 220 221 222 223 224 225 226 227 228 229 230 231 232 record 15 233 234 235 236 237 238 239 240 241 242 243 244 245 246 247 248 249 250 251 252 253 254 255 256 257 258 259 260 261 262 263 264 record 16 265 266 267 268 269 270 271 272 273 274 275 276 277 278 279 280 281 282 283 284 285 286 287 288The ASSIGN statement automatically sets the record pointer to the beginning of the first record. The pointer is then repositioned to the beginning of record 14 by line 30. The serial READ# statement begins reading data from that point on.
NOTE: Reading record 17 causes an error if more than 12 values are read. Record 17 contains only 12 values (289-300).
Since each real-precision value uses 8 bytes of memory, 32 values can be printed into a 256-byte record. On the file NUMBERS, for example, the following values are stored on these corresponding records:
|Record No.||Full-precision Values|
|1 through 7||(none)|
|8||1 through 32|
|9||33 through 64|
|10||65 through 96|
|11||97 through 128|
|12||129 through 160|
|13||161 through 192|
|14||193 through 224|
|15||225 through 256|
|16||257 through 288|
|17||289 through 300|
Data read must correspond to the type (numeric or string) that was printed. However, a numeric data item need not be one of the same precision. Precision is automatically converted. You can also print an array and read back simple variables or other arrays and vice versa.