(rasa

Doing things with words

Most computers can examine the words stored in vanables and do
vanous things with them. They can check the contents of a variable and
see 1 itcontalnsa particular word or letter. This is useful for checking the
| words input by someone using the program. Computers canalso
rearrange the lettersor words ina difierent order and add them to letters
inother vanables. Below you can find out how you do these things in

| BASIC
but not the ZX81,

4
| osi x you can leave -;. h
10 AS-"I AM STUPID outthe PRINT LEFTSIBS,4)

20 BS- "ONLY FOOLS THINK" ONLY
30 CS-BS:" "- AS PRINT LEFTSIBS,4) -
RUN

ONLY FOOLS THINK | AM STUPID

On most computers,

— You can add the contents You canalso add parts of
of two variables like this. variables, like this.
You need the space LEFT$(B$,4) means take the
between guotation first four letters from theleft
marks to leave a space ofB$.

This tells the computer to take

To tell the computer to use
letters from the right you

sa TD ONE o ee o o. —PmČmmemeAe£ee—, »- s-—— -

the middle letters. The first

use RIGHT$ with the name number tells it where to start
- olthestring and the and the second tells it how
'. numbers of letters you want. many letters to take.

Note for Sinclair
users

PRINT A$(6 TO 11)
STUPID
PRINT B$(14 TO 16)

10 KS-"DING DONG
20 PRINT LEN'KS)
RUN

10

This means take

You canalso find out the letters numbers

length ofa string -the
number of letters, spaces
and symbols it contains. To
do this you use LEN, short
for length.

oči

IF A$< "COMPUTER BOOK"
what is LEFT$ (A$,8)?

Codemaker program

This program automatically puts words into code. Similar, but much more complex

nam are used by intelligence services to write and crack codes.

sČaBI z ena dij to understand this program is to write a secret message on a piece of
per, then wor through the lines of the program carrying out the computer's tasks

Your message and writing them down.

S LET C$-"" | oa

1 LET D$-"" Sets up empry string variables
10 PRINT "TYPE IN A SHORT MESSAGE"

20 INPUT M$

30 PRINT "NOW TYPE IN A SECRET This means the lengih of your
NUMBER BETWEEN 2 AND";LEN(M$)- 1 message minus |.

40 INPUT N N (your secret number) letters from

50 LET A$- RIGHT$(M$,N) P——— the right of MS.
a H The length of M$ minus N numbe
60 LET B$-LEFTSMS.LENOMS)-N) ] —— rartom balet oMi(Le ineros
M$- A$4B$ — of the letters)
80 FOR I—1 TO LEN(M$) STEP s zalij Replaces the lerters in MS with AS BS.
90 LET C$-C$- MID$M3, 1, 1) From | to the number of letters in your message
100 NEXT | poe a aa na tni
110 FOR J<2 TO LEN(M$) STEP 2 sorepeated ine PYtukas cms lebar finan
position | of

(iz LET D$-D$ MID$(M4,J, 1) OZ oa a

po NEXT J message up in twos, i.e. 2, 4, 6, etc.

LET M$<C$:D$ -—— Works in the same way as the | loop.
Replaces the letters in MŠ again.

Suppose your message is In lines 50 and 60 the computer uses your secret number
"Move tonight" and your to divide the message. A! line 50 it takes four letters (rom
secret number is 4. These the right of the message and puts them in A$. Ar line 60it
arestored in M$ andN. puts the rest of the letters in B$.

— $

m uz
a
TP, mi?
At line 70 itadds A$ andB$. Each time the | loop repeats it puts an odd-numbered
This puts the letters fromthe letter in C$ (e.g. |, H, M, etc.). Each time the ] loop

message at the repeats it puts an even numbered letter in D$ (e.g. G. T,
moj Ee ši O, etc.). Then it adds C$ and D$ to make the coded

message.

tront.

. > | a z
. .. . di — % s Ed

DA

a oj vegi V> SK IS vUg vz vsega kd BA zi DEI!