HANDS ON

by Viktor

The End of the Beginning (or vice versa?)

READ,DATA & Restore

This group of instructions can be extremely useful in certain programming situations, for instance, where a large number of values are to be assigned to variables before the main body of the program can be run. For example, in a program to play music you might want to hold the frequencies of, say, 85 notes in an array. After dimensioning the array, by the command DIM A(84), the values could be assigned as follows:

20 A(0)=123:A(1)=125:A(2)=128. . . . . . .A(84)=222

It is much simpler, however, to put the values in DATA statements and then READ them all into the array in one go:

10 DATA 123, 125, 128 . . . . . . . .
20 DATA . . . . . . . . 216,220,222
30 FOR J=0 TO 84: READ A(J): NEXT

Note that because the array contains a member A(0), you can always dimension it to one less than the number of array members needed

When a BASIC program is run, the interpreter looks right through the program and notes the position of all items included in the DATA statements. The DATA statement pointer is then set to the beginning of the list. As each READ statement is executed, the pointer is moved onto the next item. If at any time you want the program to start at the beginning of the list, you just use the command Restore. You may also want to READ from various positions down the list. As long as this coincides with a line number you can use Restore X (where X is the relevant line number)

A very good example of this was in the Hangman program in the last issue of Micropower, where the author needed first to refer to Numeric data string at line 8000 and then to String data starting at line 9000.

It is possible but not advisable to mix numeric and string items in DATA statements e.g.

10 DATA "Fred",5,"Jim",7,8,"Harry"
20 READ A$,X,B$,Y,Z,C$

However, if you make an error and try to read one type of data into the other type of variable you will get a ‘SYN’ error – and serves you right !!