Part 6
Data in programs

Subjects covered...

	READ, DATA, RESTORE

In some of the previous programs we saw that information, or data, can
be entered directly into the +3 using the INPUT statement. Sometimes
this can be very tedious, especially if a lot of the data is repeated
every time the program is run. You can save a lot of time by using the
READ, DATA and RESTORE commands. For example:

	10 READ a,b,c
	20 PRINT a,b,c
	30 DATA 1,2,3

A READ statement consists of READ followed by a list of the names of
variables, separated by commas. It works rather like an INPUT
statement, except that instead of getting you to type in the values to
give to the variables, the +3 looks up the values in the DATA
statement.

Each DATA statement is a list of expressions - numeric or string
expressions - separated by commas. You can put them anywhere you like
in a program, because the +3 ignores them except when it is doing a
READ. You must imagine the expressions from all the DATA statements in
the program as being put together to form one long list of expressions
- the DATA list. The first time the +3 goes to READ a value, it reads
the first expression from the DATA list; the next time, it reads the
second; and thus as it meets successive READ statements, it works its
way through the DATA list. (If it tries to read past the end of the
DATA list, then it reports an error.)

Note that it's a waste of time putting DATA statements in a direct
command, because READ will not find them. DATA statements must go in a
program.

Let's see how all this works in the program you've just typed in. Line
10 tells the +3 to read three pieces of data and assign them to the
variable 'a', 'b' and 'c'. Line 20 then say PRINT these variables. The
DATA statement in line 30 provides the values of 'a', 'b' and 'c' for
line 10 to read.

The information in DATA can be part of a FOR...NEXT loop. Type in...

	10 DATA 2,4,6,8,10,12
	20 FOR n=1 TO 6
	30 READ d
	40 PRINT d
	50 NEXT n

Note from the above two programs that a DATA statement can appear
anywhere - before or after the READ statement.

When the above program is run, the READ statement moves through the
DATA list with each pass of the FOR...NEXT loop.

DATA statements may also contain string variables. For example...

	10 FOR a=1 TO 7
	20 READ n$
	30 PRINT n$
	40 DATA "Bob","Edith","Carole","Jacquie","Gavin","Charles","Holly"
	50 NEXT a

The +3 doesn't have to READ the DATA statements in order - it can be
made to 'jump about' between DATA statements by using the RESTORE
command. The form of the command is...

	RESTORE xxx

...where 'xxx' is the line number of the DATA statement to be READ
from. If you use the command RESTORE on its own (without a line
number) the +3 will jump to the first DATA statement in the program.

Type in and run the following program...

	 10 DATA 1,2,3,4,5
	 20 DATA 6,7,8,9
	 30 GO SUB 110
	 40 GO SUB 110
	 50 GO SUB 110
	 60 RESTORE 20
	 70 GO SUB 110
	 80 RESTORE
	 90 GO SUB 110
	100 STOP
	110 READ a,b,c
	120 PRINT a'b'c
	130 PRINT
	140 RETURN

The command 'GO SUB 110' calls a subroutine which READs the next three
items of DATA and the PRINTs them. Notice how the RESTORE command
affects which items are read.

Delete line 60 and run this program again to see what happens.