Up to now
we have been telling the computer what to do, directly from the keyboard. Although
it is possible to combine commands together, only limited applications are feasible
using this method.
The
great thing about computers is that they are programmable. This means that we
can give them a series of instructions to make them do things in a sequence.
Every
computer has its own language which allows us to communicate with it. Some languages
are very simple - so that the computer can understand them easily. Unfortunately
languages that are simple for the computer to understand are difficult for humans.
In some ways the reverse is also true - languages simple enough for us to understand
are relatively difficult for the computer - and even have to be translated or
interpreted.
The
ZX Spectrum uses a high level language called BASIC.
BASIC
stands for Beginners All-purpose Symbolic Instruction Code and the language
was designed at Dartmouth college in New Hampshire, USA, in 1964. It is very
widely used on personal computers, but although it is broadly similar on all
of them, there are subtle differences. That is why this manual is written specifically
for the ZX Spectrum. But ZX Spectrum BASIC is not too far from a (non-existent)
consensus BASIC and so you should not have too much trouble adapting any BASIC
program to work on the ZX Spectrum. Unlike other BASICs ZX Spectrum BASIC does
not allow the command LET to be omitted when values are being assigned
to variables.
There
is a limit to how many instructions can be stored in the computer. The ZX Spectrum
indicates this limit by emitting a buzz.
When
programming in BASIC it is necessary to let the computer know the order in which
the instructions are to be executed. Hence each line of the sequence of instructions
has a number at its beginning. It is normal to start at 10 and to increase this
by 10 for each new line. This allows other lines to be inserted if they have
been omitted, or the program to be modified.
Let's
look at a simple program. Consider the series of commands at the end of the
last chapter. If we wanted to repeat the series of commands it would be necessary
to enter them each time. A program overcomes that necessity.
Type
in the following with ENTER after each line.
10 LET b$="What
is your age? "
20 LET a$="Your age is "
30 INPUT (b$);age
40 PRINT a$;age Note
that it is not necessary to enter any spaces, except inside quotes.
Nothing
will actually happen until we tell the computer to start working on the program.
That is done by using RUN (the keyword on R).
Enter
this command and see what happens.
You
may also have noticed a right facing arrow when each line has been entered.
This indicates the last line entered. If you want to see the program again key
ENTER again, (or LIST). You can use RUN to execute the
program as many times as you like. When you no longer need this program, you
can remove it by using the NEW command. This wipes out the program stored
in memory, and gives you a 'clean slate' ready to put in a new one.
Key
NEW, then LIST and see what happens.
To
recap:
When you type in a command preceded by a number, then this tells the computer
that it is not simply a command, but a program line. The computer does not execute
it, but stores it away for later.
The
ZX Spectrum helpfully writes on the screen (or lists) all the program lines
that you have entered with a 
against the last line that you entered.
The
computer will not execute any of these fines immediately, but just stores them
away inside itself.
To
get the computer to execute these lines, you must use the command RUN.
If
you press ENTER on its own you will get the listing back.
Let's
consider another simple program. This one will be a bit more mathematical and
print out the squares of all the numbers between 1 and 10 (the square of a number
is just that number multiplied by itself).
To
generate numbers from 1 to 10 introduces another concept in programming in BASIC.
This is the method by which we get the computer to count. Earlier we have seen
that numbers can be stored in the computer's memory by attaching a 'name' to
them - or technically, assigning a value to a variable. Let the variable x
start with the value 1 and increase in steps of 1 to 10. This is done by using
the command FOR ... TO ... STEP.
So
to enter this program key NEW to get rid of the previous one and type
the following:
10 FOR x=1 TO
19 STEP 1 (Normally
the STEP 1 part can be omitted if counting is going up in steps of one).
The
next line must now tell the computer what to do with x at whatever value
it is, so key:
20 PRINT x,
x*x Finally
we need a line to tell the computer to go to the next value of x, therefore
key
30 NEXT x On
reaching this instruction the computer goes back to line 10 and repeats the
sequence. When x exceeds 10 the computer goes to the next line in the
program i.e. line 40.
The
program should now appear on the screen as follows:
10 FOR x=1 TO
19 STEP 1
20 PRINT x, x*x
30 NEXT x For
completeness we should really have another line telling the computer that the
program has ended when x=10 so key
40 STOP
If
the program is now RUN two columns should appear, the first with values
of x, the second with values of x*x, or x squared. It is
possible to label these columns by adding another line, like this
5 PRINT "x","x*x" Notice
that although this has been entered after all the other lines, because
it has a lower line number the computer automatically puts it in the correct
place.
Try
writing programs using other mathematical functions. if you have any doubt about
how to use them refer to the appropriate pages in the BASIC programming manual.
|
|
