We
have already seen how to tell the computer to print letters and graphics on
the screen using PRINT. We have also seen that ENTER has to be
used to tell the computer to execute the command just typed in. From now on,
we will not use ENTER in the manual each time a command is used, but
assume that you will key it at the end of the line automatically.
Numbers
can be handled by the computer more easily than letters. In the previous chapter
we hinted at this by explaining that the computer expects a number after PRINT
if quotations marks are not used.
So
if we key
PRINT 2 the
number 2 will appear on the screen.
It
is possible to mix letters and numbers:
PRINT 2,"ABC" Notice
that there is a gap on the screen between the 2 and ABC.
Now
key
PRINT 2;"ABC" and
then
PRINT 2 "ABC"
Using a comma between the items after PRINT
spaces them out by 16 columns using a semi-colon leaves no spaces and using
nothing gives an error.
PRINT
can also be used with the mathematical functions on the keyboard. In fact the
ZX Spectrum can be used as an electronic calculator.
For
example:
PRINT 2+2
The answer appears at the top of the screen. Compare this with:
PRINT "2+2"
It is possible
to combine these to give something more useful. Try
PRINT "2+2=";2+2 Try some other
kinds of arithmetic as well:
PRINT 3-2
PRINT 4/5
PRINT 12*2 The *
is used as a multiplication sign instead of × to avoid getting it confused
with the letter x; and / is used for the division sign ÷.
Experiment with
lots of different calculations. If you like you can use negative numbers or
numbers with decimal points in.
If you do enough
to use up the 22 lines of the top part of the screen, then you will notice something
rather interesting happening; it will all move up one line and the top line
will be lost. This is called scrolling.
Calculations
are not always performed in the order you might expect. As an example, try
PRINT 2+3*5 You might expect
this to take 2, add 3, giving 5, and then multiply by 5, giving 25; however
this is not the case. Multiplications - and divisions as well - are performed
before additions and subtractions, so the expression '2+3*5' means 'take 3 and
multiply it by 5, giving 15; and then add that to 2, giving 'IT. 17 should be
the answer displayed on the screen.
Because multiplications
and divisions are done first, we say that they have higher priority than addition
and subtraction. Relative to each other, multiplication and division have the
same priority, which means that the multiplications and divisions are done in
order from left to right. When they are dealt with, we are left with the additions
and subtractions - these again have the same priority as each other, so we do
them in order from left to right.
Let us see how
the computer would work out:
PRINT 20-2*9+4/2*3
i 20-2*9+4/2*3
| i |
20-2*9+4/2*3 |
} |
First
we do the multiplications and
divisions in order from left to right |
| ii |
20-18+4/2*3 |
| iii |
20-18+2*3 |
| iv |
20-18+6 |
} |
and
then the additions and subtractions |
| v |
2+6 |
| vi |
8 |
Although all
you really need to know is whether one operation has a higher or lower priority
than another, the computer does this by having a number between 1 and 16 to
represent the priority of each operation: * and / have priority
8, and + and - have priority 6.
This order of
calculation is absolutely rigid, but you can circumvent it by using brackets;
anything in brackets is evaluated first and then treated as a single number,
so that
PRINT 3*2+2 gives the answer
6+2=8 but
PRINT 3*(2+2)
gives the answer 3*4=12
It is sometimes
useful to give the computer expressions such as this because whenever the computer
is expecting a number from you, you can give it an expression instead and it
will work out the answer. The exceptions to this rule are so few that they will
be stated explicitly in every case.
You can write
numbers with decimal points (use the full stop), and 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 write an exponent part consisting
of the letter e, then maybe -, and, then a number. The exponent
part shifts the decimal point along to the right (or left, for a negative exponent),
thus multiplying (or dividing) the original number by 10 a few times. For instance,
2.34e0=2.34
2.34e3=2340
2.34e-2=0.0234 and so on
(Try printing
these out on the computer.) This is one of the few cases where you can't replace
a number by an expression: for instance, you cannot write
(1.34+1)e(6/2). You can also
have expressions whose values are not numbers, but strings of letters. You have
seen the simplest form of this many times, the string of letters written down
with double quotes around it. This is rather analogous to the simplest form
of numeric expression, which is just a number written down on its own. What
you have not yet seen is the use of + with strings (but not -,
* or /, so there is no problem with priorities here). Adding strings
together just joins them together one after the other: so try
PRINT "jers"+"ey
cow" You can add
together as many strings as you like in a single expression, and if you want,
you can even use brackets.
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