This is an input method which is used in many games. You may wonder what these numbers are. There is a special code for these PEEKs which is different from both the ASCII code and the POKE code. Each key is assigned a certain value which is the same whether it is shifted or not. The value for all keys up is 64. Table 1 shows the code for each separate unshifted character. Using one of these two PEEKs is the most effective way to control game animation from the keyboard. It's better than the GET for these purposes because it tells you what is currently pressed, not what was pressed. This method does not extract characters from the keyboard buffer. This could cause problems if you have a GET or INPUT following one of these PEEKs. The characters pressed for the PEEK were entered into the keyboard buffer, but weren't used. When you do come to a GET or INPUT, the keyboard buffer will empty. This will cause the program to go right through a GET, or to empty unused characters at an INPUT.
Table 1. VIC Character Code for PEEK(197)
<r 8 A 17
1 0 S 41
2 56 D 18
3 1 F 42
4 57 G 19
5 2 H 43
6
58 J 20
7 3 K 44
8 59 L 21
9 4 : 45
0 60 ; 22
CHAPTER THREE
Amortize
AMIHAI GLAZER
This program demonstrates an extraordinary method of getting numbers from the user. You can enter ex-pressions such as 3 + 2 as well as the simple number 5.
You're planning to buy a new house. Or perhaps a new car. But money is short and you must take out a loan. What is the monthly payment on the loan? What is the total interest charge? How much interest can you deduct from your income tax in the first year?
Answers to these and other questions are provided by the program
"Amortize." As an added bonus, the program incorporates some techniques you may want to use in your own programs.
Key in the program. On line 63993, simultaneously press the SHIFT key and the letter "O" key; this is an abbreviation for GOTO.
On line 63992 the PRINT statement consists of a quote mark, a blank space, pressing the CTRL and the "2" keys simultaneously, and finally a quote mark. On line 63996, to enter the PRINT statement type a quote mark, then press the CTRL and the "7" keys simultane ously, then press the space bar seven times, and then close with a quote.
Once the program is in memory, type RUN and you will be prompted for the input. Notice that you can type as input not only numbers, but expressions as well. For example, suppose we let the loan be for ten years, so that for the number of months we enter 10*
12; we let the interest rate be 13 + 3/8, and the amount of the loan (the principal) is $50,000. Your friendly VIC-20 will respond by showing that the monthly payment on the loan (PMT) is $757.65.
You will then learn that after your third payment (MONTH = 3) you still owe $49392.20, that you have paid a total of $1665.15 interest over these three months, and that $552.80 out of your third payment went to pay interest.
Look at the results for the last month, month 120. You will find that the total interest paid on the loan is $40918.87. (Yes, that sure is a lot of interest.) Don't let the small amount of principal remaining, 87 cents, bother you; such inaccuracies are inevitable when you can't make monthly payments including a fraction of a cent. One final caveat: some consumer lenders use the Rule of 78s to determine
CHAPTER THREE
the reduction in principal each month. Therefore, the results the program gives you for any but the last month may be slightly different from what the bank may tell you. But the program will still give you the correct value for the monthly payment, and the correct value for the total interest charge.
The INPUT Technique
That's it for you folks who want to use the program without worrying about how it works. As mentioned, the user's input can be in the form of an expression, not merely a number (which is what the INPUT statement allows). Here's how this is done. Suppose we want to get a value for variable N. In statement 50, the computer printed out the characters "N = ". The user types in any expression, say
] 0* 12. Lines 63990 and 63991 accept the characters for this expression and print it out. The screen will now show N = 10* 12. (The POKEs into locations 204 and 207 allow the cursor to be shown when the GET is invoked.) We then switch (in statement 63992) to printing in white so the user will not be confused by the tricks we are about to play.
In statement 63993 the computer prints G F63996. In statements 63994 and 63995 we POKE (into the keyboard buffer) instructions to go up to the screen line which says N = 10* 12, to execute that line, to go to the screen line which says G F63996, and to execute that line. These instructions are executed when the END in statement 63995 is encountered.
Having executed the instruction on the screen to GOTO 63996, the computer is now executing that statement. The computer switches back to printing in blue, erases from the screen all the garbage which it had printed in its machinations, and returns to the calling program.
The subroutine which starts in statement 63990 can be used in any program you wish. The calling sequence is exactly as shown in statement 50.
There is another useful technique in lines 132-136. These instructions allow the user to stop execution by pressing any key, and to continue execution by pressing that or any other key; the instructions transform the keyboard into a toggle switch. The logic is simple: if no key is pressed when line 132 is encountered, the program does not stop. If any key is pressed, the program waits until all keys are released, and then waits until a key is pressed. Execution then continues.
1 REM AMORTIZE —BY 2 REM AMIHAI GLAZER 3 REM UNIV. OF CALIF.
4 REM IRVINE,CA.92717
5 DEF FNR(X)=INT(100* X+.5)/100
10 PRINT "{CLEAR}{REV}AMORTIZEn 20 PRINT "{03 DOWN}"
30 PRINT "NO. OF PERIODS"
40 PRINT " (IN MONTHS)"
50 PRINT "N= ";:GOSUB 63990 60 PRINT "ANNUAL %INTEREST RATE"
70 PRINT "AR=";:GOSUB 63990 8 0 MR=AR/1200
90 PRINT "PRINCIPAL"
9 5 PRINT lfP="; :
100 PMT=(P*MR)/(1-(1+
105 PMT =FNR(PMT)
110 PRINT "{02 DOWN}PMT=",
111 PRINT "{DOWN}PRESS RETURN KEY"
112 PRINT "TO CONTINUE OR STOP"
113 GETA$:IF A$="" THEN 113 120 PRINT "{02 DOWN}"
130 FOR 1=1 TO N 132 GET A$:IF A$=""
134 GET A$: IF A$<>""
136 GET A$:IF A$=""
140 RDUE =FNR(P*MR) 150 CUMR=FNR(CUMR+RDUE 160 P=P-PMT+RDUE
170 PRINT "{REV}MONTH=";I
180 PRINT " PRINCIPAL =";FNR(P) 190 PRINT " TOTAL INT.="; (CUMR)
INT. DUE =" ; 63992 PRINT " {WHT}"90 63993 PRINT "GO63996"
63994 POKE 631,145:
5:POKE634,145:POKE 63995 POKE 636,145:
:POKE198,8:END
63996 PRINT"{02 UP}":FOR ZZ =1TO3 :PRINT" {BLU}
":NEXT:PRINT"{03 UP}"
CHAPTER THREE
ffi;ffi^
Append G
WAYNE KOZUN ^
An Append routine combines two programs together.
With VIC s relocating loader its easy, and this program
-makes it automatic. ^