Sinclair ZX81 User manual

Sinclair ZX81 BASIC Programming
by Steven Vickers
Second Edition 1981
Copyright 1980 Sinclair Research Limited
Front cover illustration by John Harris of Young Artists,
specially comissioned by Sinclair Research Limited.

Chapter 1 - Setting up the ZX81
Unpack the ZX81, you will have found
1. This manual.
2. The computer. This has three jack sockets (marked 9V DC IN, EAR & MIC), one aerial socket, & an
exposed part of its circuit board where you can
plug extra equipment. It has no switches - to turn it on you just connect it to the power supply.
3. A power supply.
This converts mains electricity into the form that the ZX81 uses. If by accident you plug it into the wrong
socket in the computer you will do no damage. If you want to use your own power supply, it should give 9
volts DC at 700mA unregulated, & end in a 3.5mm jack plug with positive tip.
4. An aerial lead about 4 feet (120cm) long, which connects the computer to a television.
5. A pair of leads about a foot (30cm) long with 3.5mm jack plugs at both ends. These connect the
computer to a tape recorder.
You will also need a television - the ZX81 can work without one, but you won't be able to see what it's
doing! It must be a UHF television - if it's
not built to receive BBC2 (in the UK) then it's no good.
Later you will need a cassette recorder. This is because when you turn a ZX81 off, all the information
stored in it is permanently lost. The only way to keep it for later is by recording it on a cassette tape - you'll
see how
to do this in chapter 16. You can also buy tapes that other people have prepared, & so run their programs.
When you've got everything together (except the cassette recorder) connect them as shown on previous
page.
If your television has two aerial sockets, marked UHF & VHF, then use the UHF one.
Turn the power on, & switch on the television. You now need to tune the television in. The ZX81
operates on channel 36 UHF, & when it is first plugged in and properly tuned it gives a picture like this:

When using the computer, you will probably want to turn the volume right down.
If your television has a continuously variable tuning control, then you just have to adjust it until you get
this picture. Many televisions now have an individual push button for each station. Choose an unused one
(e.g. ITV2), & tune it in.
If you get stuck with the computer, remember that you can always reset the computer & get back this
picture by taking out the '9V DC IN' plug and putting it back again. This should be a last resort, because
you lose all the information in the computer.
Note: This description of the television applies to Britain, where there is a UHF system using 625 lines at
50 frames per second. This will work in some other countries (for instance most Western European
countries except France). The USA, uses VHF & 525 lines at 60 frames per second.
Now that you've set up the computer, you'll want to use it. If you already know the computer language
BASIC, then read appendix C & use the rest of the manual only to clarify the obscure points.
If you're a novice, then the main part of the manual has been written for you. Don't ignore the exercises;
many of them raise interesting points that are not dealt with in the text. Look through them, & do any that
take your fancy, or that seem to cover ground you don't understand properly.
Whatever else you do, keep using the computer. If you have a question 'What does it do if I tell it such &
such?' then the answer is easy: type it in & see. Whenever the manual tells you to type something in,
always ask yourself, 'What could I type instead?' & try out your replies. The more of your own stuff you
write, the better you will understand the ZX81. (This is called unprogrammed learning.) Regardless of what
you type in, you cannot damage the computer.

Chapter 2 - Telling the computer what to do
Turn the computer on (by plugging it in) and get the blank screen with the write-on-black K, as in the
picture in chapter 1. To make it do something, you have to type in a message that it understands; for
instance the message
PRINT 2+2
tells it to work out the sum 2+2 and display the answer on the TV screen.
A message like this, telling the computer to do something straight away, is a command; this particular
one is a PRINT command, but also a PRINTstatement. Calling it a PRINT statement just specifies its form
without referring to how the computer is going to use it. Thus every command takes the form of a
statement, but so do some other things - program lines do, as we shall see in chapter 8.
To type in this command,
1. First type PRINT. But, although as you can see the keyboard has a key for each letter, you do not spell
the word out P, R, I, N, T. As soon as you press P the whole word will come up on the screen, tohether
with a space to make things look nice, and the screen will look like this:
The reason for this is that at the beginning of each command the computer is expecting a keyword - a
word that specifies what kind of command it is. The keywords are written above the keys, and you will see
that 'PRINT' appears above the P key, so that to get 'PRINT' you have to press P.
The computer lets you know that it expects a keyword by the that you had to start off with. There is
almost always some white-on-black (inverse video) letter, either or (or, we shall see later, or ),
called the cursor. The means 'whatever key you press, I shall interpret it as a keyword'. As you saw,
after you had pressed P for PRINT, the changed to an .
This system of pressing just one key to get more than one symbol is used a lot on the ZX81. In the rest
of this manual, words with their own keys are printed in BOLD TYPE.
You must remember that it is useless trying to spell these words out in full, because the computer just
won't understand.
2. Now type 2. This should cause no problems. Again, you should see 2 appear on the screen, and the L
move along one place.
Note also how much space is automatically put in between PRINT & 2 to make it look neat. This is done
as much as possible, so that you hardly ever have to type a space. If you do type a space, it will appear on
the screen, but it will not affect the meaning of the message at all.
3. Now type +. This is a shifted character (they are marked in red - the colour of SHIFT itself on its key - in
the top right hand corner of each key), and to get '+' you must hold down the key and while you are
still doing that, press the key .
4. Now type 2 again. The screen will look like this:

5. Now - and you must always remember this - press NEWLINE, the key . This means 'message
complete', or 'all right,
computer, lets see some action'. The computer will now read the message, work out what has to be done,
and do it. In this case, the screen will change to
4 is the answer - but of course you do not need to buy a computer to work that out.
0/0 (Note how zero is written with a slash to distinguish it from capital O. This is fairly common in
computing circles.) is the report in which the computer tells you how it got on. The first 0 means 'OK, no
problems'. (In appendix B there is a list of the other report codes that can arise, for instance if something
goes wrong.) The second 0 means 'the last thing I did was line 0'. You will see later - when you come to
write programs - that a statement can be given a number & stored away for execution later: it is then a
program line. Commands do not actually have numbers, but for the sake of reports the computer pretends
that they are line 0.
You should imagine a report as hiding a cursor - if you press P for PRINT now, the report will
disappear and the screen will change to

The cursor can also be used for correcting mistakes: type ++2, to get
PRINT ++2
on the bottom line, Pretty incomprehensible stuff, and when you press NEWLINE you get
The is the syntax error marker (the syntax is the grammar of the message, saying which are allowed
and which are not), and shows that the computer got as far as 'PRINT', but after that decided that it was
not a proper message.
What you want to do of course is rub out the first +, and replace it by - let us say - 3. First you have to
move the cursor so that it is just to the right of the first +; there are two keys, and (shifted 5 and shifted
8), that move the cursor left and right. Holding SHIFT down, press the key twice. This moves the cursor
left two places to give you
PRINT ++2
Now press the RUBOUT key (shifted 0), and you will get
PRINT +2
RUBOUT rubs out the character (or keyword) immediately to the left of the cursor.
If you now press 3 this will insert a '3', again immediately to the left of the cursor, giving
PRINT 3+2
and pressing NEWLINE gives the answer (5).
The key (shifted 8) works just like the key, except that it moves the cursor right instead of left.

Summary
This chapter has covered how to type messages in for the ZX81, explaining
the single keystroke system for words,
the & cursors,
reports,
the syntax error marker,
& how to correct mistakes using , and RUBOUT.
The keyboard
Here is a picture of the keyboard.
Remember that to use SHIFT, you have to hold it down at the same time as you press another key. Do
not confuse digit 0 with letter O.

Chapter 3 - A History lesson
The messages that you type in are in a computer language called BASIC (standing for Beginners' All-
purpose Symbolic Instruction Code). It actually takes the computer quite a lot of effort to break down the
BASIC messages into its own rudumentary operations, but, after all, that's what it's paid to do. The BASIC
messages countain enough English words (like PRINT) to make them fairly easy for an English speaking
human to learn.
BASIC was designed at Dartmouth College in New Hampshire, USA in 1964, & since then it has come
to be by far the most widely used computer language by beginners & hobbyists. This is largely because it
is very well adapted for on-line use where the user types something in & the computer answers straight
away. There are other languages - such as ALGOL (in fact a whole family of ALGOLs) & PASCAL - with a
much neater structure & greater power than BASIC, but only a few - such as the relatively unknown APL &
POP-2 - are as easy to use on-line. Some others that must be mentioned are FORTRAN,
PL1 & COBOL.
Many personal computing magazines publish programs in BASIC, & are well worth looking through for
ideas. You will almost certainly have to adapt them slightly because every computer that uses the BASIC
language has its own dialect, different from all the others.

Chapter 4 - The Sinclair ZX81 as a calculator
Turn the computer on. You can now use it as a calculator, along the lines of chapter 2: type PRINT, then
whatever it is that you want working out, & then NEWLINE. (We shan't usually bother to tell you to type
NEWLINE.)
As you would hope, the ZX81 can not only add, but also subtract, multiply using a star * instead of the
usual times sign - this is fairly common on computers) & divide (using / instead of ). Try these out.
+, -, * and / are operations, & the numbers they operate on are their operands.
The computer can also raise one number to the power of another using the operation ** (Shifted H. Do
not type * - shifted B - twice): type
PRINT 2**3 (Remember the NEWLINE.)
& you will get the answer 8 (2 raised to the power 3, or 23, or 2 cubed)
The ZX81 will also work out combinations of the operations. For instance.
PRINT 20-2*3**2+4/2*3
gives the answer 8. It goes all round the houses to get this, because first it works out all the powers (**) in
order from left to right, & then all the multiplications & divisions (* & /), again from left to right, & then the
additions & subtractions (+ & -), yet again from left to right. Thus our example is worked out in the following
stages:
We formalize this by giving each operation a priority, a number between 1 & 16. The operations with
highest priority are evaluated first, & operations with equal priority are evaluated in order from left to right.
When - is used to negate something, as when you write -1, then it has priority 9. (This is unary minus, as
opposed to the binary minus in 3-1: a unary operation has one operand, while a binary operation has two.
Note that on the ZX81 you cannot use + as a unary operation.)
This order is absolutely rigid, but you can circumvent it by using brackets: anything in brackets is
evaluated first & then treated as a single number, so that
PRINT 3*2+2
gives the answer 6+2 = 6, but
** has priority 10
* and / have priority 8
+ & - have priority 6

PRINT 3*(2+2)
gives the answer 3*4 = 12.
A combination like this is called an expression - in this case, an arithmetic or numeric expression
because the answer is a number. In general, whenever the computer is expecting a number from you, you
can give it an expression instead and it will work out the answer.
You can write numbers with decimal points (use the full stop), & you can also use scientific notation - as
is quite common on pocket calculators. In this, after an ordinary number (with or without a decimal point),
you can write an exponent part consisting of the letter E, then maybe + or -, & then a number without a
decimal point. The E here means '*10**' ('times ten to the power of'), so that
(Try printing these out on the ZX81.)
The easiest way of thinking of this is to imagine the exponent part shifting the decimal point along to the
right (for a positive exponent) or to the left (for a negative exponent).
You can also print more than one thing at once, separating them either with commas (,) or semicolons (;
or shifted X). If you use a comma, then the next number will be displayed starting either at the left hand
margin, or in the middle of the line in the 16th column. If you use a semicolon, then the next number will be
displayed immediately following the last one.
Try
PRINT 1;2;3;4;5;6;7;8;9;10
&
PRINT 1,2,3,4,5,6,7,8,9,10
to see the differences. You can mix commas & semicolons within a single PRINT statement if you want.
Summary
Statements: PRINT, with commas & semicolons
Operations: +,-,*,/,**
Expressions, scientific notation
Exercises
1. Try
PRINT 2.34E0
PRINT 2.34E1
PRINT 2.34E2
and so on up to
PRINT 2.34E15
You will see that after a while the ZX81 also starts using scientific notation. This is because it never
takes more than 14 spaces to write a number in. Similarly, try
PRINT 2.34E-1
PRINT 2.34E-2
& so on.
2. Try
PRINT 1,,2,,3,,,4,,,,5
2.34E0 = 2.34 * 10**0 = 2.34
2.34E3 = 2.34 * 10**3 = 2340
2.34E-2 = 2.34 * 10**-2 = 0.0234 & so on.

A comma always moves you on a bit for the next number. Now try
PRINT 1;;2;;3;;;4;;;;5
Why is a string of semicolons no different from a single one?
3. PRINT gives only 8 significant digits. Try
PRINT 4294967295, 4294967295 -429E7
This proves that the computer can hold all the digits of 4294967295, even though it is not prepared to
display them all at once.
4. If you've got some log tables, then test out this rule:
Raising 10 to the power of a number is the same as taking the antilog of that number.
For instance, type
PRINT 10**0.3010
& look up the antilog of 0.3010. Why are the answers not exactly equal?
5. The ZX81 uses floating point arithmetic, which means that it keeps separate the digits of a number (its
mantissa) and the position of the point (the exponent). This is not always exact, even for whole numbers.
Type
PRINT 1E10+1-1E10,1E10-1E10+1
Numbers are held to about 9 1/2 digits accuracy, so 1E10 is too big to be held exactly right. The inaccuracy
(actually about 2) is more than 1, so the numbers 1E10 & 1E10+1 appear to the computer to be equal.
For an even more peculiar example, type
PRINT 5E9+1-5E9
Here the inaccuracy in 5E9 is only about 1, & the 1 to be added on in fact gets rounded up to 2. Here the
numbers 5E9+1 & 5E9+2 appear to the computer to be equal.
The larger integer (whole number) that can be held completely accurately is 232-1 (4,294,967,295).

Chapter 5 - Functions
Mathematically, a function is a rule for giving a number (the result) in exchange for another (the argument,
or operand) & so is really a unary operation. The ZX81 has some of these built into it & their names are the
words written under the keys. SQR, for instance, is the familiar square root function, and
PRINT SQR 9
gives 3, the square root of 9. (To get SQR, you first press the FUNCTION key - shifted NEWLINE. This
changes the cursor to . Now press the SQR key (H): SQR appears on the screen and the cursor changes
back to . The same method works for all the words that are written underneath the keys, almost all of
which are function names.)
Try
PRINT SQR 2
You can test the accuracy of the answer by
PRINT SQR 2*SQR 2
which ought to give 2. Note that both SQRs are worked out before the *, and in fact all functions (except
one - NOT) are worked out before the five operations +, -, *, / and **. Again, you can circumvent this rule
using brackets -
PRINT SQR (2*2)
gives 2.
Here are some functions (there is a complete list in appendix C). If your maths is not up to
understanding some of these, it does not matter - you will still be able to use the computer.
* The trigonometrical functions. These work in radians, not degrees.
Using the jargon of the last chapter, all these except PI & RND are unary operations with priority 11. (PI
& RND are nullary operations, because they have no operands.)
SGN The sign function (sometimes called signum to avoid confusion with SIN). The result is -1, 0 or +1
according as the argument is gative, zero or positive.
ABS The absolute value, or modulus. The result is the argument made positive, so that
ABS -3.2 = ABS 3.2 = 3.2
SIN *
COS *
TAN *
ASN arcsin *
ACS arccos *
ATN arctan *
LN natural logarithm (to base 2.718281828459045..., alias e)
EXP exponential function
SQR square root
INT integer part. This always rounds down, so INT 3.9 = 3 & INT -3.9 = -4. (An integer is a whole
number, possibly negative.)
PI = 3.1415265358979..., the girth in cubits of a circle one cubit across. PI has no argument. (Only
ten digits of this are actually stored in the computer, & only eight will be displayed.)
RND Neither has RND an argument. It yields a random number between 0 (which value it can take) & 1
(which it cannot).

The trigonometrical functions, & EXP, LN & SQR, are generally calculated to 8 digits accuracy.
RND & RAND: These are both on the same key, but whereas RND is a function, RAND is a keyword, like
PRINT. RAND is used for controlling the randomness of RND.
RND is not truly random, but follows a fixed sequence of 65536 numbers that happen to be so jumbled
up as to appear random (RND is pseudo-random). You can use RAND to start RND off at a definite place
in this sequence by typing RAND, & then a number between 1 & 65535, & then NEWLINE. It's not so
important to know where a given number starts RND off, as that the same number after RAND will always
start RND off at the same place. For instance, type
RAND 1 (& NEWLINE)
& then
PRINT RND
& type both these in turn several times. (Remember to use FUNCTION to get RND.) The answer from RND
will always be 0.0022735596, not a very random sequence.
RAND 0
(or you can miss out the 0) acts slightly differently: it judges where to srart RND off by how long the
television has been on, & this should be genuinely random.
Note: In some dialects of BASIC you must always enclose the arguments of a function on brackets. This is
not the case in ZX81 8K BASIC.
Summary
Starement: RAND
Functions: SGN. ABS, SIN, COS, TAN, ASN, ACS, ATN, LN, EXP, SQR, INT, PI, RND
FUNCTION
Exercises
1. To get common logorithms (to base 10), which are what you'd look up in log tables, divide the natural
logarithm by LN 10. For instance, to find log 2,
PRINT LN 2/LN 10
which gives the answer 0.30103.
Try doing multiplication & division using logs, using the ZX81 as a set of log tables in this way (for
antilogs, see chapter 2, exercise 3). Check the answer using * and / - which are easier, quicker, more
accurate, & much to be preferred.
2. EXP & LN are mutually inverse functions in the same sense that if you apply one & then the other, you
get back to your original number. For instance,
LN EXP 2 = EXP LN 2 = 2
The same also holds for SIN & ASN, for COS & ACS, & for TAN & ATN. You can use this to test how
accurately the computer works out these functions.
3. radians are 180°, so to convert from degrees to radians you divide by 180 & multiply by : thus
PRINT TAN (45/180*PI)
gives tan 45° (1).
To get from radians to degrees, you divide by & multiply by 180.
4. Try
PRINT RND
a few times to see how the answer varies. Can you detect any pattern? (Unlikely.)

How would you use RND & INT to get a random whole number between 1 & 6, to represent the throw of
a die? (Answer: INT (RND *6) +1.)
5. Test this rule:
Suppose you choose a number between 1 & 872 & type
RAND & then your number (& NEWLINE)
Then the next value of RND will be
(75 * (your number + 1) - 1)/65536
6. (For mathematicians only.)
Let p be a [large] prime, & let a be a primitive root modulo p.
Then if biis the residue of aimodulo p (1 bi< p-1), the sequence
bi-1 / p - 1
is a cylindrical sequence of p-1 distinct numbers in the range 0 to 1 (excluding 1). By choosing a suitably,
these can be made to look fairly random.
65537 is a Mersenne prime, 216-1. Use this & Gauss' law of quadratic reciprocity, to show that 75 is a
primitive root modulo 65537.
The ZX81 uses p=65537 & a=75, & stores some bi-1 in memory. The function RND involves replacing
bi-1 in memory by bi+1-1, & yielding the result (bi+1-1)/(p-1). RAND n (with 1 n 65535) makes biequal
to n+1.
7. INT always rounds down. To round to the nearest integer, add 0.5 first. For instance,
Compare these with the answers you get when you don't add 0.5
8. Try
PRINT PI, PI -3, PI -3.1, PI -3.14, PI -3.141
This shows how accurately the computer stores .
INT (2.9+0.5) = 3 INT (2.4+0.5) = 2
INT (-2.9+0.5) = -3 INT (-2.4+0.5) = -2

Chapter 6 - Variables
'My pocket calculator,' you will be saying, 'can store a number away & remember it later. Can your ZX81 do
that?'
Yes. In fact it can store away literally hundreds, using the LET statement. Suppose that eggs cost 58p a
dozen, & you want to remember this. Type
LET EGGS=58 (& NEWLINE, of course)
Now first, the computer has reserved a place inside itself where you can store a number, & second, it
has given this place the name 'EGGS' so you can refer to it later. This combination of storage space &
name is called a variable. Third, it has stored the number 58 in the space: we say that it has assigned the
value 58 to the variable [whose name is] EGGS. EGGS is a numeric variable, because its value is a
number.
Do you want to know how much eggs cost? Type
PRINT EGGS
If you want to know the cost of half a dozen eggs, then type
PRINT EGGS/2
In fact, should you want to know the square of the consine of the price of one egg, you can type
PRINT COS (EGGS/12)**2
'How very easy,' you must think, & you will be wondering what to do next, when in rushes your
housekeeper saying 'Glory be, eggs have just gone up to 61p a dozen.'
Well. There is no time to lose. Type
LET EGGS=61
This does not reserve any extra storage space, but replaces the old value of 58 with 61. Now you can
type
PRINT EGGS
confident in the expectation of getting the most up-to-date price available.
Now type
PRINT MILK
You will get a report 2/0, & looking up 2 in appendix B, you will see that it means 'variable not found' -
the computer hasn't tha faintest idea how much milk costs, because you haven't told it. Type
LET MILK=18.5
& all will be all right.
(Type
PRINT MILK
again.)
A variable need not be named after groceries - you can use any letters or digits as long as the first one
is a letter. You can put spaces in as well to make it easier to read, but they won't count as part of the
name.
For instance, these are allowed to be the names of variables:
TWO POUNDS OF APPLES BUT NOT GOLDEN DELICIOUS
RADIO 3

RADIO 33
X
K9P
but these are not:
3 BEARS (begins with a digit)
TALBOT? (? is not a letter or a digit)
. (inverse video characters not allowed)
FOTHERINGTON-THOMAS (- is not a letter or a digit)
Now type
CLEAR
&
PRINT EGGS
You will get report 2 (variable not found) again. The effect of CLEAR is to release all the storage space
that had been reserved for variables - then every variable is as though it had never been defined. Turning
the computer off & on will also do this - but then it doesn't remember anything at all when it is turned back
on.
Expressions can contain the name of a variable anywhere they can have a number.
Note: In some versions of BASIC you are allowed to omit LET & just type in (say)
EGGS=58
This is not allowed on the ZX81. In any case, you'd find it rather difficult to type in.
Also in some versions, only the first two characters in a name are checked, so that RADIO 3 & RADIO
33 would count as the same name; & in some others a variable name must be a letter followed by a digit.
Neither of these restrictions applies to the ZX81.
Yet again, in some versions of BASIC, if a variable has not yet appeared on the left-hand side of a LET
statement then it is assumed to have a value 0. As you saw above with PRINT MILK, this is not so on the
ZX81.
Summary
Variables
Statements: LET, CLEAR
Exercises
1. Why do variable names (that is to say, names of variables) have to begin with a letter?
2. If you're unfamiliar with raising to powers (**, shifted H) then do this exercise.
At its most elementary level, 'A**B' means 'A multiplied by itself B times', but obviously this only makes
sense if B is a positive whole number. To find a definition that works for other values of B, we consider the
rule
A ** (B+C) = A**B * A**C
You should not need much convincing that this works when B & C are both positive whole numbers, but
if we decide that we want it to work even when they are not, then we find ourselves compelled to accept
that
&
A**0= 1
A**(-B) = 1/A**B
A**(1/B) = the Bth root of A

If you've never seen any of this before then don't try to remember it straight away; just remember that
&
& maybe when you're familiar with these the rest will begin to make sense.
Experiment with all this by telling the computer to print various expressions containing **: e.g.
PRINT 3**(2+0),3**2*3**0
PRINT 4**-1,1/4
3. Type
LET E=EXP 1
Now E has the value 2.718281828..., the base of natural logarithms. Test the rule
EXP a number = E ** the number
for various numbers.
A**(B*C) = (A**B)**C
A**-1 = 1/A
A**(1/2) = the square root of A

Chapter 7 - Strings
One thing the ZX81 can do that pocket calculators cannot is deal with text. Type
PRINT "HI THERE. I AM YOUR ZX81." (" is shifted P.)
The chilling greeting inside the quotes is called a string (meaning a string of characters), & can contain
any characters you like except the string quote,". (But you can use the so-called quote image, "" (shifted
Q), & this will be printed as " by a PRINT statement.)
A common typing error with strings is to miss out one of the quotes - this will give you the marker.
If you are printing numbers out, you can use these strings to explain what the numbers mean. For
instance, type
LET EGGS=61
& then
PRINT "THE PRICE OF EGGS IS ";EGGS;" NEW PENCE A DOZEN."
(Don't worry about this going over the end of the line.)
This statement displays three things (PRINT items), namely the string "THE PRICE OF EGGS IS ", the
number 61 (the value of the variable EGGS), & then the string " NEW PENCE A DOZEN." In fact you can
PRINT any number of items you like, & any mixture of strings & numbers (or expressions), note how the
spaces in a string are just as much part of it as the letters. They are not ignored even at the end.
There are lots of things you can do with strings.
1. You can assign them to variables. However, the name of the variable must be special to show that its
value is a string & not a number: it must be a single letter followed by $ (shifted U). For instance, type
LET A$="DOUBLE GLOUCESTER"
&
PRINT A$
2. You can add them together. This is often called concatenation, meaning 'chaining together', & that is
exactly what it does. Try
PRINT "GAMMO" + "N RASHERS"
You cannot subtract, multiply or divide strings, or raise them to powers.
3. You can apply some functions to strings to get numbers, & vice versa.
STR$ This is applied to a number, & the result is what would appear on the screen if the number were
displayed by a PRINT statement. For instance STR$ 3.5 = "3.5".
4. Just as for numbers, you can combine these to make string expressions, like
V
AL (STR$ LEN "123456"+"-4")
LEN This is applied to a string, & the result is its length. For instane LEN "CHEESE" = 6.
VAL This applied to a string, & the result is what that string gives when evaluated as an arithmetic
expression. For instance (if A = 9), VAL "1/2+SQRA" = 3.5. If the string to which VAL is applied
contains variables, then two rules must be obeyed.
(i) If the VAL function is part of a larger expression, it must be the first item; e.g. 10 LET X = 7+VAL
"Y" must be changed to 10 LET X = VAL "Y" +7.
(ii) VAL can only appear in the first coordinate of a PRINT AT, PLOT or UNPLOT statement (see
Chapter 17 and 18) e.g. 10 PLOT 5, VAL "X" must be changed to
10 LET Y = VAL "X"
15 PLOT 5, Y

which is evaluated as
Summary
Strings
Operation: + (for strings)
Functions: LEN, VAL, STR$
Exercises
1. Type
LET A$="2+2"
& then
PRINT A$;" = ",VAL A$
Try changing A$ to more complicated things & doing the same, e.g.
LET A$="ATN 1*4"
(The answer here should be .)
2. The string "" with no characters is called the empty or null string. It is only string whose length is 0.
Remember that spaces are significant and an empty string is not the same as one containing spaces.
Do not confuse it with the quote image, "" (a single token, shifted Q). This is a special device to get over
the fact that you cannot write an ordinary string quote in the middle of a string (why not?). When the quote
image appears in a string that has its quotes at the end (for instance in the listing of a program), it shows
up as two quote symbols, to distinguish it from the ordinary quote; but when it is displayed by a PRINT
statement, it is as just one quote symbol.
Try
PRINT "X";"";"X",""""."""";"";""""
3. If you enjoy humiliating computers, type
PRINT "2+2 = ";2+1

Chapter 8 - Computer programming
And now at last you shall write a computer program. Turn the computer off & on, just to make sure that it is
clear. Now type
10 LET BUTTER=75 (& NEWLINE)
& the screen will look like this:
This is different from what happened with EGGS in chapter 6; if you type
PRINT BUTTER
you will see (from the report 2) that the variable BUTTER has not been set up. (Press NEWLINE again &
the screen should go back to looking like the picture.)
Because the LET statement had a number, 10, in front of it, the computer did not execute it straight
away, but saved it for later. 10 is its line number, & is used to refer to it rather in the same way that names
are used to refer to variables. A set of these stored statements is called a
program. Now type
20 PRINT BUTTER
& the screen should look like this:
This is a listing of your program. To have the program carried out (or executed or run), type
RUN (don't forget the NEWLINE)