Commodore 64 Users Guide C64 03 Beginning Basic Programming

c64-users_guide-03-beginning_basic_programming

User Manual:

Open the PDF directly: View PDF .
Page Count: 10

Up to now we've performed some simple operations by entering a

single line of instructions into the computer. Once .:j:aIlI~U. was de-

pressed, the operation that we specified was performed immediately.

This is called the IMMEDIATE or CALCULATORmode.

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 allows you to

use the full power of the Commodore 64.

To see how easy it is to write your first Commodore 64 program, try

this:

Clear the screen by holding the _key, and then depressing the

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

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 ?"CONNODORE 64" I

2121 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"

r L 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 of editing

options.

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~.at the same time and a

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 LISTagain, 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.

~DON'T FORGETTO MOVETHE

CURSOR PAST LINE 20 BEFORE

YOU RUN THE PROGRAM.

HII PRINT "COMMODORE",

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:

113X% = 15

20 X = 23.5

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

The computer might represent the variables like this:

X% 15

X 23.5

X$ THE SUM OF X% +X

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:

Al%

NM%

The '$' following the variable name indicates the variable will repre-

sent a text string. The following are examples of string variables:

MI$

Floating point variables follow the same format, with the type indi-

cator:

In assigning a name to a variable there are a few things to keep in

mind. First, a variable name can have one or two characters. The first

character must be an alphabetic character from A to Z; the second

character can be either alphabetic or numeric (in the range 0 to 9). A

third character can be included to indicate the type of variable (integer

or text string), % or $.

You can use variable names having more than two alphabetic

characters, but only the first two are recognized by the computer. So

PA and PARTNO are the same and would refer to the same variable

box.

The last rule for variable names is simple: they can't contain any

BASIC keywords (reserved words) such as GOTO, RUN, etc. Refer back

to Appendix D for a complete list of BASIC reserved words.

To see how variables can be put to work, type in the complete pro-

gram that we introduced earlier and RUN it. Remember to hit .:j:lIlI:U.

after each line in the program.

If you did everything as shown, you should get the following result

printed on the screen.

We've put together all the tricks learned so far to format the display

as you see it and print the sum of the two variables.

In lines 10 and 20 we assigned an integer value to X% and assigned a

floating point value to X. This puts the number associated with the vari-

able in its box. In line 30, we assigned a text string to X$. Line 40

combines the two types of PRINTstatements to print a message and the

actual value of X% and X. Line 50 prints the text string assigned to X$

and the sum of X% and X.

Note that even though X is used as part of each variable name, the

identifiers % and $ make X%, X, and X$ unique, thus representing

three distinct variables.

But variables are much more powerful. If you change their value, the

new value replaces the original value in the same box. This allows you

to write a statement like:

X=X+l

This would never be accepted in normal algebra, but is one of the

most used concepts in programming. It means: take the current value of

X, add one to it and place the new sum into the box representing X.

IF . . . THEN

Armed with the ability to easily update the value of variables, we can

now try a program such as:

What we've done is introduce two new BASIC commands, and pro-

vided some control over our runaway little print program introduced at

the start of this chapter.

IF . . . THEN adds some logic to the program. It says IF a condition

holds true THEN do something. IF the condition no longer holds true,

THEN do the next line in the program.

A number of conditions can be set up in using an IF ...THEN state-

ment:

SYMBOL

< =

The use of anyone of these

powerful.

1" CT="

[2" ?"COMMODORE64"

30' CT = CT + 1

4" IF CT < 5 THEN2"

5" END

MEANING

Less Than

Greater Than

Equal To

Not Equal To

Greater Than or Equal To

Less Than or Equal To

conditions is simple, yet surprisingly

In the sample program, we've set up a "loop" that has some con-

straints placed on it by saying: IF a value is less than some number

THEN do something.

Line 10 sets CT (CounT) equal to O. Line 20 prints our message. Line 30

adds one to the variable CT. This line counts how many times we do the

loop. Each time the loop is executed, CT goes up by one.

Line 40 is our control line. If CT is less than 5, meaning we've exe-

cuted the loop less than 5 times, the program goes back to line 20 and

prints again. When CT becomes equal to 5-indicating 5 COMMODORE

64's were printed-the program goes to line 50, which signals to END

the program.

Try the program and see what we mean. By changing the CT limit in

line 40 you can have any number of lines printed.

IF . . . THEN has a multitude of other uses, which we'll see in future

examples.

FOR . . . NEXT LOOPS

There is a simpler, and preferred way to accomplish what we did in

the previous example by using a FOR ...NEXT loop. Consider the

following:

As you can see, the program has become much smaller and more

direct.

CT starts at 1 in line 10. Then, line 20 does some printing. In Line 30

CT is incremented by 1. The NEXT statement in line 30 automatically

sends the program back to line 10 where the FOR part of the FOR. . .

NEXTstatement is located. This process will continue until CT reaches the

limit you entered.

The variable used in a FOR . . . NEXT loop can be incremented by

smaller amounts than 1, if needed.

Try this:

If you enter and run this program, you'll see the numbers from 1 to

10, by .5, printed across the display.

All we're doing here is printing the values that NB assumes as it goes

through the loop.

You can even specify whether the variable is increasing or decreas-

ing. Substitute the following for line 10:

10 FOR NB =10 to 1 STEP-.5

and watch the opposite occur, as NB goes from 10 to 1 in descending

order.

Commodore 64 Users Guide C64 03 Beginning Basic Programming

c64-users_guide-03-beginning_basic_programming

c64-users_guide-03-beginning_basic_programming

c64-users_guide-03-beginning_basic_programming

Navigation menu

Versions of this User Manual:

Views

Navigation