Tricks with loops
UR Pattern repeat program
Here are some more pro S
Ra la a na ami poo Below you can find out how This program uses nested loops to repeat a small ali
These are called nested loops peat several things at the same ume screen. The program looka guite complicated but if you read oni
carefully and work out what each line does, you will soon see howit
iN ested loops works. The shape of the pattern is decided by random numbers and
na klali wil] be different each time you run the program.

graphics use larger
random numbers
e g on BBC micro
change figure in
hnes 101040, to 60

| S CLS
«20 FOR ]<l TO 34 20 LET B-INT(RND(l)474-1) These lines choose the random
((30 PRINT 1, ]. Jicop 30 LET C-INT(RND(I)a6--1) more bemin A. k. CandD.
Y 40 NEXT] "— 45 40 LET D-INT(RND(l)a4-1) |
50 NEXTI »— ?P 80 INPUT "HOW MANY POINTS o ea a
60 END - ACROSS THE SCREEN"; W NR ( JO Eo nam
| 60 INPUT "HOW MANY UP";V The | loop counts the number of umes the
This program has an | loop and a] loop. Out the value for J each time. The picture 65 CLS pattern is repeated up the screen. Each time, |
The] loop is nested inside the | loop and above shows the result of this program. 70 FOR I<0 TO V STEP V/6 iz increased by the height of he screen (V)
for each time that the | loop is carned out, The commas spaced the figures out like 80 FOR J<0 TO W STEP W/6 divided o ARMANI
the ] loop is repeated three times, printing this. 90 PLOT JtA,14B)
(0 PLOT GtA,14O) ropom IT
Computer clock 110 PLOT JtC,I4D).'— jioop iho Cirrent vakone for | sad ] pis (he random
5 CLS 120 PLOT AO ški numbers.
130 NEXT ] o are The ] loop counts the number of times the
To set the time 140 NEXT 1 pattern is repeated across ihe screen. lt works
150 END in the same way as lhe | loop.

use this delay loop:
54 FOR Z<1 TO 100

3šht

pi im.

Ba828885

udgea

Both partsofa
nested loop must
be inside the other

|. Inside a computer there is an electronic one. s

| "elock" which sets the rhythm forallthe The seconds loop is carned out 59times

.. computer's work. The clock pulsesat for each minute loop. If you try this Imagine that the computer On the first run through the program land] are 0 sothe

.. between one and four million pulsesa program ona computer it might run very has chosen the random computer plots the first pattern of dots using only the

. second. This program makes the fast at first. You need to put in an extra numbers 2, 5, 3 and l and random numbers. Line 130 sends it back to Bine le nexi

'. computer behave like a digital clock. "delay loop", then set it by changing the that the width and height of  valueforj which is ] 460/6, i.e. 10. Thenit p rkp
figure in the loop so your computer clock the screen are both 60. second pattern using the random numbers p

|. h has nested loops, one to count the

seconds and one to count the minutes. ujeks" at the same rate as a real one. This repeats the pattern along the screen.

K ' IH you try this program
and it does not work,
try itagaln with

smaller numbers

ii
[
;

Mo
20 for V and W.
o s ušuie /::
o ; S!
Re4
"THEN | — the computer
0 NEX 1 imi [( This program takes The computer repeats the] loop six umes, Om poto rms using 10 er]
100 TNIBEEL. , long.long time. 7 each time adding lOtoJandsop zdi - increasing ] by 10 each time as belore.
110 You can make it pattern further along the screen. Itthen: and 29
5 s none] prana oe nimi
the number in line s ASI uz write a pattem
p 500 or even 2, ogre pa lj me hints to help you on page 4%.

| shape over the screen?