How computers compare things (predi?

YESTERDAY

One of the most useful thingsa computer can do isto compare/-wen tu
pieces of information and then do difterent things according | WEAR MY

tothe results To do this you use the wordslF. THEN

The computer can do several different
tests on information to compare it. The
symbols for the tests are shown above. lt
can test to see iftwo pieces of data are
egual, different, or if one is greater or less
than the other.

Here is a program using IF and THEN.At
hne 20 the computer stores the word you
input in variable WS. Then, at lines 30 and
40 it checks to see if the word in W$ isthe
same as "rain" or "sunny'". Ifitis, it prints

6

5 Ageprogram
10 PRINT "HOW OLD ARE YOU"
20 INPUT A

30 IF A>16 THEN PRINT "OLD"

40 IF A<16 THEN PRINT "YOUNG"

50 IF A-16 THEN PRINT "JUST RIGHT"
RUN

HOW OLD ARE YOU

216

JUST RIGHT

In the age program, the computer
compares input A with the bgure 16. Ifitis
bigger than 16 it pnints "old". Ifit is smaller
|. ipnnts "young" and ifit1s 16 it prints"just

|
'

MNlProgram puzzle - Can you write a program lo ge! the computer to ask you sums, the

narrac" ar cve vou the nght answer?

SCARF

2

IF AsB THEN PRINT "THEY ARE EOUAL'"
IF A>B THEN PRINT "A IS BIGGER"

IF A<B THEN PRINT "A [S SMALLER"

IF A<>B THEN PRINT "THEY ARE NOT
EOUAL"

These lines show how you use the
symbols with IF and THEN to make the
computer compare two pieces of data.
You can compare any kind of data -
words, numbers and variables, l.e. the
contents of memory spaces, too.

out one ofthe responses. l you put ina
different word at line 20 nothing will
happen. You could change the words in
lines 30 and 40, though, then try inputting
one of the new words.

French lesson
10 PRINT "HOW DO YOU SAY RED IN
FRENCH"

20 INPUT A$
30 IF A$-"ROUGE" THEN PRINT "CORRECT"

40 IF A$<>"ROUGE" THEN PRINT "NO,
ROUGE'"

RUN
HOW DO YOU SAY RED IN FRENCH

?BLEU
NO, ROUGE

right". ln the other program the computer
prints out one of two different responseS
depending on whether A$ eguals"rouge"

or not.
n either print

Branching programS

IF As6 THEN LET A$-'SIX"
IF XeY-—2 THEN LET Z-0

computer to go
to ling

30.

You can give the computer almost any
instruction after the word THEN, as shown
above. A useful instruction is to make itgo
to another line. (On most computers, but

Maths program

10 PRINT "TYPE IN A NUMBER"

20 INPUT A

30 PRINT "TYPE IN ANOTHER NUMBER'"
40 INPUT B

50 PRINT "DO YOU WANT TO"

60 PRINT "ADD, SUBIRACT, MULTIPLY"
66 PRINT "DIVIDE OR STOP"

70 INPUT C$

80 IF C$-"ADD" THEN PRINT A1B

90 IF C$<"SUBTRACT" THEN PRINT A-B
100 IF C$-"MULTIPLY" THEN PRINT A4B
110 IF C$s"DIVIDE" THEN PRINT A/B
120 IF C$-"STOP" THEN STOP

130 GOTO 10

Age guessing program

This program will go on repeating itself

until G- 14. When G- l4the computer will

pass over lines 30 and 40 and print

Pd Ram di NR
7 These two hnes |

make the computer
S

branch to other part
(the program

not the ZX81, you can leave out the word
GOTO.) You usually need a STOP
instruction in programs with GOTO, or the
computer wil] go on repeating the
program endlessly.

TME PROGRAM WILL.
ONLY STOP WHEN YOU

RUN

TYPE IN A NUMBER
nI

TYPE IN ANOTHER NUMBER
7184

DO YOU WANT TO

ADD, SUBTRACT, MULTIPLY
DIVIDE OR STOP -

?

In this program the numbers you type in
are stored in A and B and your instructions
are stored in C$. At lines 80 to 120the
computer compares C$ with five diflerent
words, and when it finds the right word, it
carrjes out the instruction. lt passes over
all the lines which are not true.

"correct". Can you alter the program so
that it gives you some clues, as shown in
the picture on the right?

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