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Contents / Part 1 / Previous chapter / Next chapter / Index
Most of the rest of this book is concerned with introducing the various parts of the BBC BASIC language which the computer understands and other features of the machine. But first, here are a few complete programs which you can try to type in yourself. They must be typed in as accurately as possible and can then be run. If a program fails to run properly, then almost certainly you have typed a line in incorrectly - for instance, you may have typed ; when you should have typed : or typed O instead of 0.
Most of the sample programs are too big to fit on the screen in one go. If you LIST a program you have typed in, for example to check that you have made no mistakes, you may find that the lines you want to look at disappear off the top of the siren. To prevent this you can specify the range of lines you want to be listed. For example
LIST 100,200
will only list those lines numbered between 100 and 200.
Alternatively you can enter "page mode" by pressing, CTRL N (hold down CTRL and press N). In this mode the listing will stop after every "page" and will continue only when you press the SHIFT key. "Paged mode" is switched off by pressing CTRL O and you should always remember to do this after you have listed the program.
Typing in programs will help you to get a feel for the keyboard and, if you save them on cassette after you have satisfied yourself that they do run properly, will enable you to start to build up a library of them.
Learning to use the computer is a little like learning to drive a car - when you first start you find that there are an enormous number of things to think about at once. Many of the things you come across from now on will be bewildering at first, But as you get further into the book and as you gain experience in using BASIC, the various parts of the jig-saw puzzle should begin to fall into place. So don't worry if, for instance, some of the comments about the following programs are difficult to understand at first.
Note: In the program listings which follow, extra spaces have been inserted between the line numbers (10, 20, etc) and what follows on each line. This is to improve the readability of the programs. However, although it will do no harm, there is no reason to type in any spaces after the line number. For example in the first program, called POLYGON, when entering line 250, all you need type is
250MOVE 0,0
This program draws polygons (many sided shapes) in random colours.
Lines 120 to 180 move to a random place on the screen which will be the centre (origin) of the next shape. Lines 210 to 290 calculate the X and Y co-ordinates of each "corner" of the polygon and store the values in two 'arrays' for future use. In addition the shape is filled with black triangles (lines 260 and 200) thus making it appear that the new polygon is in front of older ones. Lines 310 to 370 draw all the lines that make up the polygon.
Lines 50 to 70 set the actual colours of logical colours 1, 2 and 3 to red, blue and yellow. You can change these if you wish to use other colours.
10 REM POLYGON
20 REM JOHN A COLL
30 REM VERSION 1 / 16 NOV 81
40 MODE5
50 VDU 19,1,1,0,0,0
60 VDU 19,2,4,0,0 0
70 VDU 19,3,3,0,0,0
80 DIM X(10)
90 DIM Y<10)
100
110 FOR C=l TO 2500
120 xorigin=RND(1200)
130 yorigin=RND(1000)
140 VDU29,xorigin;yorigin;
150 radius=RND(300)+50
160 sides=RND(8)+2
170 MOVE radius,0
180 MOVE 10,10
190
200 GCOL 0,0
210 FOR SIDE=1 TO sides
220 angle=(SIDE-1)*2*PI/sides
230 X(SIDE)=radius*COS(angle)
240 Y(SIDE)=radius*SIN(angle)
250 MOVE0,0
260 PLOT 85,X(SIDE), Y(SIDE)
270 NEXT SIDE
280 MOVE0,0
290 PLOT 85,radius,0
300
310 GCOL 0,RND(3)
320 FOR SIDE=1 TO sides
330 FOR line=SIDE TO sides
340 MOVE X(SIDE), Y(SIDE)
350 DRAW X(line), Y(line)
360 NEXT line
370 NEXT SIDE
380 NEXT C
You may like to try this alternative for line 200
200 GCOL 0, RND(4)
This program plots a set of "blocks" on the screen which might represent prices over a twelve month period. In its present form it will only work on a Model B Computer. With minor modifications (eg changing line 100 to MODE 5) it can be made to run on Model A. In this example the height of the bars is randomly selected at line 170. Lines 180 to 270 then draw a "solid" bar and the edges are marked in black by lines 290 to 330. Lines 340 and 350 print out one letter representing the month of the year at the bottom of each bar.
Notice that lines 60 and 70 set up two of the function keys.
Key 0 resets the computer to MODE 7 and then lists the program. Key 9 can be used to run the program.
10 REM MONTHLY
20 REM JOHN A COLL
30 REM VERSION 1 / 16 NOV 81
40 REM MODEL B
50
60 *KEY 0 "MODE7 |M LIST|M"
70 *KEY 9 "RUN |M"
80 M$="JFMAMJJASOND"
90 C=0
100 MODE 2
110 VDU 5
120 VDU 29,0;100;
130
140 FOR X=0 TO 1100 STEP 100
150 GCOL 0,C MOD 7+1
160 C=C+1
170 H=RND(400)+300
180 MOVE X,0
190 MOVE X,H
200 PLOT 85,X+100,0
210 PLOT 85,X+100,H
220 MOVE X+70,H+50
230 MOVE X,H
240 PLOT 85,X+170,H+50
250 PLOT 85,X+100,H
260 PLOT 85,X+170,50
270 PLOT 85,X+100,0
280 GCOL0,0
290 MOVEX,H
300 DRAW X+100,H
310 DRAW X+170,H+50
320 MOVE X+100,H
330 DRAW X+100,0
340 MOVE X+10,50
350 PRINT MID$(M$,C,1)
360 NEXT
370
380 GCOL 4,1
390 PRINT TAB(0,16)"----------------"
400 VDU4
410 PRINTTAB(3,0)"critical level"
The height of each bar is set randomly by the variable H. If you want to put in your own values (data), then type the following extra lines. Line 170 must also be deleted by typing 170 followed by RETURN.
50 DIM data(12)
82 FOR J=l TO 12
84 PRINT "Input data for month" MID$(M$,J,1);
86 INPUT data(J)
88 NEXT J
89 INPUT "CRITICAL LEVEL", level
155 H=data(C+1)
390 MOVE 0, level:PRINT"-------------------"
This program can be used to solve equations of the form
Y=Ax²+Bx+C
The "roots of the equation" are printed to two decimal places. The number of decimal places is set by line 90.
The main program between lines 110 and 170 uses three procedures - one for each of the three types of result. The main program is surrounded by
REPEAT
'
'
'
UNTIL FALSE
which has the effect of keeping the program going for ever - or until the ESCAPE key is pressed.
Line 170 PRINT''' prints three blank lines to separate one set of results from the next.
10 REM QUADRAT
20 REM JOHN A COLL BASED ON A PROGRAM
30 REM BY MAX BRAMER, OPEN UNIVERSITY
40 REM VERSION 1.0 / 16 NOV 81
50 REM SOLVES AN EQUATION OF THE FORM
60 REM A*X^2 + B*X +C
70 ON ERROR GOTO 350
80 MODE 7
90 @%=&2020A
100 REPEAT
110 PRINT "What are the three coefficients ";
120 INPUT A,B,C
130 DISCRIM=B^2-4*A*C
140 IF DISCRIM<0 THEN PROCcomplex
150 IF DISCRIM=0 THEN PROCcoincident
160 IF DISCRIM>0 THEN PROCreal
170 PRINT'''
180 UNTIL FALSE
190 END
200
210 DEF PROCcomplex
220 PRINT "Complex roots X=";-B/(2*A)
230 PRINT " +/ "; SQR(-DISCRIM) /(2*A) "i"
240 ENDPROC
250
260 DEF PROCcoincident
270 PRINT"Co-incident roots X=";B/(2*A)
280 ENDPROC
290
300 DEF PROCreal
310 X1=(-B+SQR(DISCRIM))/(2*A)
320 X2=(-B-SQR(DISCRIM))/(2*A)
330 PRINT "Real distinct roots X=";X1;" and X=";X2
340 ENDPROC
350 @%=10:REPORT:PRINT
>RUN
What are the three coefficients ?1,-1, -2
Real distinct roots X=2.00 and X=-1.00
What are the three coefficients ?3,3,3
Complex roots X=-0.50 +/- 0.87i
What are the three coefficients ?1,2,1
Co-incident roots X=1.00
What are the three coefficients ?
Escape
>
This program draws a pattern (lines 80 to 140) and then changes foreground and background colours with a half second pause between each change.
10 REM FOURPNT / DRAWS A PATTERN WITH 4 POINTS
20 REM JOHN A COLL
30 REM VERSION 1 / 16 NOV 81
40 REM MODEL A
50 MODE 4
60 VDU 29,640;512;
70
80 FOR A=0 TO 500 STEP 15
90 MOVE A-500,0
100 DRAW 0,A
110 DRAW 500-A,0
120 DRAW 0,-A
130 DRAW A-500,0
140 NEXT A
150
160 FOR B=0 TO 7 :REM CHANGE THE COLOUR
170 FOR C=1 TO 3
180 T=TIME :REM WAIT A WHILE
190 REPEAT UNTIL TIME-T>50
200 VDU 19,3,C,0,0,0
210 VDU 19,0,6,0,0,0
220 NEXT C
230 NEXT B
This program builds up a changing pattern by overdrawing lines on the screen.
The main program between lines 90 and 140 loops for ever and calls various subroutines as necessary. The use of subroutines with implied GOTO (e.g. line 170) results in a structure which is not easy to follow! It would be better to use "structures" such as procedures (see page 86).
10 REM TARTAN
20 REM BASED ON RESEARCH MACHINES DEMO
30 REM VERSION 1.0 / 16 NOV 81
40 MODE 2: REM ALSO WORKS IN MODE 5
50 R=l: D=l: X=0
60 Y=RND(1280)
70 MOVE X,Y
80
90 REPEAT
100 ON D GOSUB 160,260,350,430
110 IF RND(1000)<10 THEN R=D-1
120 GCOL R,(D*1.7)
130 DRAW X,Y
140 UNTIL FALSE
150
160 X=X+1024-Y
170 IF X>1280 THEN 220
180 Y=1024
190 D=2
200 RETURN
210
220 Y=1024/1280-X
230 X=1280: D=4
240 RETURN
250
260 Y=Y-1280+X
270 IF Y<0 THEN 310
280 X=1280: D=3
290 RETURN
300
310 X=1280+Y
320 Y=0: D=1
330 RETURN
340
350 X=X-Y
360 IFX<0 THEN 400
370 Y=0: D=4
380 RETURN
390
400 Y=-X: X=0: D=2
410 RETURN
420
430 Y=Y+X
I40 IF Y>1024 THEN 480
450 X=0: D=1
460 RETURN
470
480 X=Y-1024
490 Y=1028: D=3
500 RETURN
This program produces a pattern by drawing hundreds of lines. Random colours are selected by lines 60 and 70. Line 80 moves the "origin (middle) of the picture to the middle of the screen". It runs on Model B only.
10 REM PERSIAN
20 REM ACORN COMPUTERS
30 REM VERSION 2 / 16 NOV 81
40 MODE1
50 D%=4
60 VDU 19,2,RND(3)+1,0,0,0
70 VDU 19,3,RND(3)+4,0,0,0
80 VDU 29,640;512;
90 J1%=0
100 FOR K%=500 TO 380 STEP -40
110 REPEATJ2%=RND(3):UNTILJ2%<>J1%
120 J1%=J2%
130 GCOL 3, J1%
140 FOR I%=-K% TO K% STEP D%
150 MOVE K%,I%
160 DRAW -K%,-I%
170 MOVE I%,-K%
180 DRAW -I%,K%
190 NEXT
200 NEXT
This program calculates the square root of a number by repeating a simple operation (lines 90 and 200) until the calculated result stays steady. The program also indicates how long the calculation takes.
This program illustrates an important mathematical technique - but of course you don't have to work out square roots this way, the (unction SQR is provided in BASIC (see page 355).
10 REM SQROOT
20 REM VERSION 1.0 / 16 NOV 81
30 REM TRADITIONAL ITERATION METHOD
40 REM TO CALCULATE THE SQUARE ROOT
50 REM OF A NUMBER TO 3 DECIMAL PLACES
60 MODE 7
70 ON ERROR GOTO 300
80 @%=%2030A
90 REPEAT
100 count=0
110 REPEAT
120 INPUT "What is your number ",N
130 UNTIL N>0
140 DELTA=N
150 ROOT=N/2
160 T=TIME
170 REPEAT
180 count=count+1
190 DELTA=(N/ROOT-ROOT)/2
200 ROOT=ROOT+DELTA
210 UNTIL ABS(DELTA) <0.001
220 T=TIME-T
230 PRINT
240 PRINT "Number ",N
250 PRINT "Root ",ROOT
260 PRINT "Iterations", count
270 PRINT "Time",T/100;" seconds"
280 PRINT''
290 UNTIL FALSE
300 @%=10:PRINT:REPORT:PRINT
>RUN
What is your number ?34
Number 34.000
Root 5.831
Iterations 5.000
Time 0.070 seconds
What is your number ?125
Number 125.000
Root 11.180
Iterations 6.000
Time 0.080 seconds
What is your number?
This program prints a "path in the grass".
It is a fine example of a "non-structured" use of BASIC, you might like to try and "structure" it.
90 REM BRIAN2
100 REM (C) BRIAN R SMITH 1980
110 REM ROYAL COLLEGE OF ART, LONDON
120 REM VERSION 1.0 / 16 NOV 81
130 INPUT "NUMBER OF CYCLES e.g. 1 to 5 ",T
140 INPUT "BACKGROUND SYMBOL e.g. + ",D$
150 INPUT "MOTIF (<20 CHR$.)",A$
160 INPUT "TEXT AFTER DESIGN", B$
170 CLS
180 F=1
190 READ A,G,S,C,D,N
200 H=(D-C)/N
210 X=0
220 J=1
230 X=X+S
240 Y=SIN(X)
250 Y1=1+INT((Y-C)/H+0.5)
260 I=0
270 I=I+1
280 IF I=Yl THEN 310
290 PRINT D$;
300 GOTO 420
310 Z=Z+F
320 IF Z>0 THEN 350
330 F=-F
340 GOTO 450
350 IF Z<=LEN(A$) THEN 390
360 F=-F
370 Z=Z-1
380 GOTO 310
390 S$=LEFT$(A$,Z)
400 PRINT S$;
410 I=I+Z
420 IF I<40 THEN 270
430 PRINT
440 GOTO 230
450 J=J+1
460 IF J>T THEN 490
470 Z=Z+1
480 GOTO 310
490 FOR K=1 TO 39
500 PRINT D$;
510 NEXT K
520 PRINT
530 PRINT B$
540 DATA 0,6.4,0.2,-1,1,20
>RUN
NUMBER OF CYCLES e.g.1 to 5 ?3
BACKGROUND SYMBOL e.g. + ?.
MOTIF (<20 chrs.)?Hello David !!
TEXT AFTER DESIGN?That's all foLks
This program draws a sine wave on the screen. The computer can draw dotted lines and the feature is used to fill in one part of the sine wave (line 130).
The computer can also print letters anywhere on the screen not just on a 40 by 32 grid. Lines 190 to 220 print a message in the shape of another sine curve.
10 REM SINE
20 REM JOHN A COLL
30 REM VERSION 2 / 16 NOV 81
40 REM MODEL A
50 MODE4
60 VDU5
70 GCOL 0,1
80 VDU19,1,1,0,0,0
90 MOVE 16,400
100
110 FOR X=0 TO 320
120 IF X<150 THEN MOVE 4*X+16,400
130 PLOT 21,4*X+16,300*SIN(X/48)+ 400
140 NEXT
160 GCOL 0,1
170 AS="SINE WAVES ARE FAR MORE INTERESTING . . . . ."
180
190 FOR X=1 TO 39
200 MOVE X*1280/40,300*SIN(X/6)+512
210 PRINT MID$(AS,X,1)
220 NEXT
230
240 VDU4
250 END
Here is an example of an 'assembly' language program embedded within a BASIC program between the two brackets [ and ] which enables you to type in double height letters on the screen.
10 REM DOUBLE HEIGHT IN TELETEXT
20 WIDTH 36: MODE7
30 VDU 28,0,23,39,0
40 write=!&20E AND &FFFF
50 DIM PROG 100
60 FOR PASS=0 TO 1
70 P%=PROG
80 [
90 OPT PASS*3
100 CMP #&D : BNE noter
110 PHA : JSR write
120 LDA #&8D : JSR write
130 LDA #&0A : JSR write
140 LDA #&08 : JSR write
150 LDA #&8D : JSR write
160 PLA : RTS
170 . noter CMP #&20 : BCS legal
180 JMP write
190 . leqal PHA : JSR write
200 LDA #&0B : JSR write
210 LDA #&08 : JSR write
220 PLA : PHA : JSR write
230 LDA #&0A : JSR write
240 PLA : RTS
250 ]
260 NEXT PASS
270 !&20E=!&20E AND &FFFF0000 OR PROG
280 END
Line 270 changes the "write character" routine indirection vector so that all output is sent to the new routine given above. This routine tests for a "return" code (line 100) and if it finds one it issues Teletext double height control codes on to the next two lines. Otherwise the routine just prints the characters on two lines one above the other so as to produce a double height character. This routine has a quite different effect in non-Teletext modes. Try it. Press BREAK after you have finished with this program.
Before we leave this section, here are a few points about entering lines into BASIC.
10 space RETURN
to separate program sections. Some of the programs above have such blank lines. Because of this you should avoid using COPY past the true end of a line.
The BBC Microcomputer User Guide was written by John Coll and edited by David Allen for the British Broadcasting Corporation.
Optical character recognition and original formatting effort by Mark Usher.
HTML version maintained by: Kade "Archer" Hansson; e-mail: archer@dialix.com.au
Last updated: Monday 12th February 2001