c64-users_guide-03-beginning_basic_programming.pdf

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Up to now we've performed

some simple

single line of instructions

into the computer.

operations

by entering

Once .:j:aIlI~U. was de-

immediately.

pressed,

the operation

that we specified was performed

mode.

This is called the

IMMEDIATE

or CALCULATOR

But to accomplish

anything significant,

we must be able to have the

computer

operate with more than a single line statement.

A number of

statements

combined

together

is called a PROGRAM and

use the full power of the Commodore

64.

To see

this:

how easy

it is to write

your first Commodore

allows

you to

try

Clear the screen by holding the

ItJ.~fj:[.],',I:I

key.

64 program,

key, and then depressing

the

Type NEW and press .~j:alll=!/.. (This just clears out any numbers that

might have been left in the computer from your experimenting.)

Now type the following exactly as shown (Remember to hit .:j:alll~j/.

after each line)

1121

2121

?"CONNODORE

64"

GOTO

1121

Now, type RUN and hit .~j:alll:j/.-watch

what happens. Your screen

will come alive with COMMODORE 64. After you've finished watching

the display, hit .~m/'I.'t(ll:l to stop the program.

A number of important concepts were introduced in this short pro-

gram that are the basis for all programming.

Notice that here we preceded each statement with a number. This

LINEnumber tells the computer in what order to work with each state-

ment. These numbers are also a reference point, in case the program

needs to get back to a particular line. line numbers can be any whole

number (integer) value between 0-63,999.

1~ PRINT "COMMODORE

64"

STATEMENT

LINE

NUMBER

It is good programming practice to number lines in increments of

10-in

case you need to insert some statements later on.

Besides PRINT, our program also used another BASIC command,

GOTO. This instructs the computer to go directly to a particular line and

perform it, then continue from that point.

[

HI' PRINT "COMMODORE

64"

2" GOTO 1"

In our example, the program prints the message in line 10, goes to

the next line (20), which instructs it to go back to line 10 and print the

message over again. Then the cycle repeats. Since we didn't give the

computer a way out of this loop, the program will cycle endlessly, until

we physically stop it with the .:ml' u.UI key.

Once you've stopped the program, type: LIST. Your program will be

displayed, intact, because it's still in the computer's memory. Notice,

too, that the computer converted the? into PRINTfor you. The program

can now be changed, saved, or run again.

Another important difference between typing something in the im-

mediate mode and writing a program is that once you execute and

clear the screen of an immediate statement, it's lost. However, you can

always get a program back by just typing LIST.

By the way, when it comes to abbreviations don't forget that the

computer may run out of space on a line if you use too many.

EDITING TIPS

If you make a mistake on a line, yoju have a number

options.

of editing

1. You can retype a line anytime, and the computer will automatically

substitute the new line for the old one.

2. An unwanted line can be erased by simply typing the line number

and

.II.IIIII~.

3. You can also easily edit an existing line, using the cursor keys and

editing keys.

Suppose you made a typing mistake in a line of the example. To

correct it without retyping the entire line, try this:

Type LIST, then using the

and 11..m."I'.~1keys together

move the

cursor up until it is positioned on the line that needs to be changed.

Now, use the cursor-right key to move the cursor to the character yo'u

want to change, typing the change over the old character. Now hit

.~I:lIII:I~. and the corrected line will replace the old one.

If you need more space on the line, position the cursor where the

space is needed and hit IDIIiI and .1~1o"t1J.4I'I~. same time and a

at the

space will open up. Now just type in the additional information and hit

.:I:lIIIII~.

Likewise, you can delete unwanted characters by placing the

cursor to the right of the unwanted character and hitting the .1~Io"tIJ.4I'I~.

key.

To verify that changes were entered, type LIST again, and the cor-

rected program will be displayed! And lines don't have to be entered in

numerical order. The computer will automatically place them in the

proper sequence.

Try editing our sample program on page 33 by changing line 10 and

adding a comma to the end of the line. Then RUN the program again.

HII

"COMMODORE",

DON'T FORGET MOVETHE

CURSOR PAST LINE 20 BEFORE

YOU RUN THE PROGRAM.

VARIABLES

Variables are some of the most used features of any programming

language, because variables can represent much more information in

the computer. Understanding how variables operate will make comput-

ing easier and allow us to accomplish feats that would not be possible

otherwise.

COMMODORE

COMMODOR

COMMODORE

BREAK IN 10

READY

COMMODORE

~M~O~E

COMMODORE

COMM06oRE

COMMODORE

COM~J~ORE

COMMODORE

COMMO~ORE

COMMODORE

COMMOBORE

COMMOIDORE

Imagine a number of boxes within the computer that can each hold a

number or a string of text characters. Each of these boxes is to be

labeled with a name that we choose. That name is called a variable

and represents the information in the respective box.

For example, if we say:

113

X% = 15

20 X = 23.5

313X$ = "THESUMOF X%+X = "

The computer might represent

23.5

THE SUM OF X% +X

the variables

like this:

A variable name represents the box, or memory location, where the

current value of the variable is stored. As you can see, we can assign

either an integer number, floating point number, or a text string to a

variable.

The % symbol following a variable name indicates the variable will

represent an integer number. The following are valid integer variable

names:

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