FOR (F.)
This is used with NEXT and, sometimes, with STEP to
form loops.
Example: 10 FOR A=1 TO 10: ? A NEXT A
A will start off as 1, the current value of A will
be printed, and A will then be incremented by 1 to equal 2. This will
repeat until A=10 when the program will print this value and finish.
STEP changes the size of the increment and, if it is a negative
number, will also decrement the value.
Examples:
10 FOR S=0
TO 255 STEP 5: SO.0,S,10,10: NEXT S
10 FOR S=255 TO 0 STEP-5: SO.0,S,10,10: NEXT S
If no other commands are included in the FOR/NEXT
loop, then the computer will run the loop while doing nothing. This is
usually termed a 'wait' or 'timing' statement. You can have a loop
inside a loop (termed 'nesting').
Example:
10 FOR S=250
TO 0 STEP -10: SO.0,S,10,10: FOR WAIT= 1 TO 50: NEXT WAIT: NEXT S
It is not recommended to use too many nested loops.
Sometimes an error 13 will occur (no matching FOR statement). This can
be caused by using the same variable too many times. Just change the
variable (say, S to SND) to clear this. Of course, you will need to
check that your loops do coincide, that the first FOR corresponds with
the last NEXT. In the last example if you change NEXT WAIT: NEXT S to
NEXT S: NEXT WAIT it will not run properly.
This can be a very powerful statement in graphics. Instead of a series
of PLOT/DRAWTO, it is possible to use a FOR/ NEXT loop, incrementing X
or Y, and one PLOT/DRAWTO saving lots of memory.
Example:
10 GR8: C.1:
FOR X=0 TO 155 STEP 5: PL X,0: DR. X,319: NEXT X
FRE
This will test for, and return, the amount of free
user RAM in bytes. It takes the form of ? FRE(0), where 0 is a dummy
variable. It can also be used in the deferred mode.
Example: 10 IF FRE(0)<X THEN ?
FRE(0);" BYTES LEFT'
GET (GE.)
This is usually used in graphics with the POSITION
statement and will return the data under the cursor and place it in a
variable. This will be a character or colour, depending on the
graphics mode. NOTE: If you wish to PRINT after GET it is necessary to
PUT the data back as quite often the pixel value has been corrupted.
By OPENing a channel, it is possible to read a byte from a file
designated by the channel and store it in the variable. This is
normally done when retrieving screen displays or other large chunks of
data.
Example: 10 GET #1,X
GOSUB (GOS.)
This statement will send the program to a
sub-routine. This is a self contained routine outside the main
program. The sub-routine must finish with RETURN to go back to the
main sequence. Care must be taken when using nested GOSUBs or a GOTO
in the routine. GOSUB uses the system stack to remember where to
return to. POP will clear the stack if the sub-routine has not been
allowed to finish normally, but this is a sign of bad programming and,
after debugging, is not recommended. Calling an often used routine
through GOSUB will save memory, but you must use it often to be of
value.
Example: 10 GOSUB 100: GOSUB SOUND1
GOTO (G.)
Similar to GOSUB but does not need a RETURN. Again,
be careful of creating endless loops with too many GOTOs.
Example: 10 GOTO 100: GOTO SOUNDI
GRAPHICS (GR)
Used to select one of the 16 graphic modes. Apart
from GR.0, these modes will have a text window at the bottom unless 16
has been added to the GR. number. By adding 32 to the GR. mode, you
can set up the text window without clearing the screen or switching
off the sound.
Examples
GRAPHICS 0
GR.1+16
GR.17
GR. 39
GR..ZERO
IF
A
conditional statement used with THEN. When the IF part is true, the
THEN part is executed. If it is not true, the THEN part is ignored. If
any other commands follow IF/ THEN on the same command line, they will
only be executed when the conditional statement is true. Nesting can
occur in this statement.
Example:
10 IF X=5
THEN ? "5"
20 IF X=5 THEN IF Y=5 THEN GOTO 100
In line 10 X
must equal 5 for the PRINT to occur. In line 20 (nesting) both X and Y
must equal 5 for the GOTO to occur.
NOTE GOTO or
GOSUB is not necessary in this statement, as a line number or
reference would suffice.
Example:
10 IF X=5 THEN 100
Make sure that there is a THEN for each FOR
INPUT (I.)
This statement looks for a keyboard response from
the user. It will place this response into a variable or string, to be
used later in the program. Unfortunately the program will halt until
this response is entered with RETURN. Several responses to one INPUT
can be entered at once by using commas. When used with an IOCB
number(#) it will request data (string or numbers) from a specified
device, providing that IOCB# has been OPENed first.
Example:
10 INPUT X
INPUT A$
INPUT X,A$,Y
INPUT #2,A$
Warning: An input without a variable may not cause
an error report. Also, an input of more than 128 bytes will overwrite
the start of page 6, the so called 'safe area'.
INT
This returns the integer (the whole number preceding
the decimal point) and sign of a number. Unlike calculators, it will
return the whole number nearest to the complete number given.
Example:
A=INT(3.44):
REM *** Returns 3
A=INT(-3.99):
REM *** Returns -4
LEN
This will give the length of a string in bytes.
Until something has been stored in a string, its length will be 0.
Example: A$="PAGE 6": ? LEN(A$):
REM *** Will return 6
LET (LE.)
This will define a variable or variable name. It can
be left out (implied LET). However, if you wish to use a command word
as a variable, you must use LET.
Example:
LET X=1.2
X=1.2: REM *** Implied LET
LET SOUND=1000: REM *** Compulsory LET
LIST (L)
This is a way of saving a program in a form that can be loaded
(using ENTER) without clearing memory, either to cassette or to disk.
LIST by itself will list out the resident progam, to the screen, in
full. If line numbers are specified then only those lines will be
listed. Likewise, if the printer is specified the program (or required
lines) will be printed.
Example:
LIST
"C:"
LIST "D:PART2.BAS"
LIST 10
LIST 20,100
LIST
"P."
LIST "P.",10;50
LOAD (L)
This will load a program from disk into RAM. It will
clear any existing memory and will need the command RUN to start the
program.
Example: LOAD "D:PART1.BAS"
LOCATE (LOC.)
For some reason, this is seldom seen now in
programs, maybe because people do not understand it It is really quite
simple.
Example: LOCATE X,Y,Z
This will position the cursor (visible or invisible)
at point X,Y and place the information stored there in the variable Z.
This will be a number, either O-255 for a text character or 0-4 for a
colour. This is the same as using POSITION X,Y: GET #6,Z. The only
problem is that a PRINT or GET after LOCATE may corrupt the data under
the cursor. You may need to reposition the cursor and PUT the data
back.
LOG
This will return the natural logarithm of a variable
or expression.
Example: L=LOG(55.2)
LPRINT (LP.)
This will open a channel to the line printer and
will print in direct or deferred modes. As with ?, LP. by itself will
skip a line.
Example: LP."PRINTER"
NEW
This will clear the contents of RAM in either direct
or deferred mode. Unfortunately it will clear all dimensioned tables.
NEXT (N.)
Used with FOR please refer to that section.
NOT
A logical operator. It will return a 1 if the result
is not true. Be careful of ? A=NOT B, it will put the computer to
sleep. Be sure to use a space after NOT as confusion could occur with
NOTE.
Example: A=NOT E
NOTE (NO.)
This is used in disk operations to open a channel to
the drive and store the current sector and byte in the variables. The
DOS manual gives an example program of this command. The first
variable is the IOCB#, the second is the sector, and the third is the
byte.
Example: NOTE #2,SEC,BYTE
ON
Used mainly with GOSUB or GOTO, but can be used with
other commands. When the variable value is known, the program will go
to the subroutine that corresponds to the value.
Example: 10 ON X GOTO 100,200,300,400
If X=3 then the program will branch to the third
choice, ie. line 300. This will be a topic for 'First Steps' later in
the year.
continued next issue
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