REM ANNUITY.BAS, Ver 1.0, December 11, 1982\
	\
	Phil Cary\
	2025 Tyre Circle\
	Las Cruces, NM 88001\
	\
	Mesilla Valley RCP/M (505) 522-8856\
	\
	This program must be compiled with CB80.  It calculates the missing\
	variable of an ordinary annuity problem.  Repeated calculations may\
	be made without reentering the unchanged variables - only a ".".\
	A clear screen code can be substituted for the nulls at clear.screen.\
	\
	The interest rate calculation is iterative and make take a while\
	depending upon how far off the true interest rate is from the built-in\
	first guess rate of 50%.  This high guess value is needed to reach\
	dymamic range interest rate limits of approximately .0002% to\
	approximately 99.9991%.  There may be lesser limits for some untested\
	input values.\
	\
	A compiled version of this program is available on Mesilla Valley\
	RCP/M, (505) 522-8856, as ANNUITY.OBJ.

REM following notice required in composite program by Digital Research, Inc.

	license.notice$="Portions of this program (c)1982 Digital Research Inc"

REM constants

	true% = 1
	false% =0
	null$ = ""
	escape$ = CHR$(27)
	clear.screen$ = null$ + null$ REM Put clear screen code for your\
		terminal here, i.e. escape$ + "E" for H-19

	ON ERROR GOTO process.error

REM start of program

	PRINT clear.screen$
	PRINT " ORDINARY ANNUITY"
	
restart:
	PRINT:PRINT "Enter known values, <CR> for unknown value, " +\
		"period <.> for same value on rerun."

	PRINT
	INPUT "   Present Value(Q to Quit): $";LINE reply$
	GOSUB strip.commas:

		IF (answer$ = "Q" OR answer$ = "q") THEN STOP

		IF answer$ = "."\
		THEN present.value = present.value\
		ELSE present.value = VAL(answer$)

		IF present.value = 0 THEN function% = 1


	INPUT "   Monthly Payment:          $";LINE reply$
	GOSUB strip.commas

		IF answer$ = "."\
		THEN monthly.payment = monthly.payment\
		ELSE monthly.payment = VAL(answer$)

		IF monthly.payment = 0 THEN function% = 2

	INPUT "   Annual Interest Rate:     %";LINE reply$
	GOSUB strip.commas
	
		IF answer$ = "."\
		THEN annual.interest.rate = annual.interest.rate\
		ELSE annual.interest.rate = VAL(answer$)

		IF annual.interest.rate = 0 THEN function% = 3:\
			annual.interest.rate = 50   REM a guess to start

		monthly.interest.rate = annual.interest.rate / 1200

	INPUT "   Number Of Payments:        ";LINE reply$
	GOSUB strip.commas
	
		IF answer$ = "."\
		THEN number.of.payments% = number.of.payments%\
		ELSE number.of.payments% = VAL(answer$)

		IF number.of.payments% = 0 THEN function% = 4


REM define function used in calculations

	DEF partial.solution (monthly.interest.rate, number.of.payments%) =\
		1-1/(1+monthly.interest.rate)^number.of.payments%

REM go perform the selected function

	ON function% GOSUB\
		compute.present.value,\
		compute.monthly.payment,\
		compute.interest.rate,\
		compute.number.of.payments

	GOTO restart

compute.present.value:

	present.value = monthly.payment *\
			(partial.solution\
			(monthly.interest.rate,number.of.payments%)/\
			monthly.interest.rate)

	PRINT
	PRINT USING " The Present Value Is: $$#,###,###,###.##";present.value
	RETURN

compute.monthly.payment:

	monthly.payment = present.value *\
			  (monthly.interest.rate/partial.solution\
			  (monthly.interest.rate,number.of.payments%))

	IF interest.rate.flag% THEN RETURN

	PRINT
	PRINT USING " The Monthly Payment Is: $$###,###,###.##";monthly.payment
	RETURN
 
compute.interest.rate:

	interest.rate.flag% = true%

	target.payment = monthly.payment
	entered.monthly.interest.rate = monthly.interest.rate

	PRINT:PRINT " Iterating calculation, please wait.";

	maximum.iterations% = 50

recompute.interest.rate:

	FOR iteration% = 1 TO maximum.iterations%

	GOSUB compute.monthly.payment

	IF ABS(target.payment - monthly.payment) < .001\
	THEN annual.interest.rate = monthly.interest.rate * 1200:\
		PRINT "  ";iteration%;"iterations.":\
		PRINT:PRINT USING " The Annual Interest Rate Is: ##.####";\
			annual.interest.rate;: PRINT " %":\
		monthly.payment = target.payment:\
		interest.rate.flag% = false%: RETURN

	correction = ABS(monthly.interest.rate - guess.new.rate)/2

	guess.new.rate = monthly.interest.rate

	IF monthly.payment < target.payment\
	THEN monthly.interest.rate = monthly.interest.rate + correction\
	ELSE monthly.interest.rate = monthly.interest.rate - correction

	NEXT iteration%

	PRINT:PRINT:PRINT "No solution after";maximum.iterations%;"iterations."
	PRINT "There is";
	PRINT USING "$$##,###,###.##";target.payment - monthly.payment;
	PRINT " difference between actual payment and the computed "
	PRINT "payment, and the computed annual interest rate is now";
	PRINT USING "###.####";monthly.interest.rate * 1200;:PRINT "%."
	PRINT
	PRINT "You may have entered values that result in a negative" +\
		" amortization, "
	PRINT "or the dynamic range of the interest rate calculations has been"
	PRINT "exceeded with the values entered.  Try again with different" +\
		" values."
	monthly.payment = target.payment:\
	monthly.interest.rate = entered.monthly.interest.rate:\
	interest.rate.flag% = false%:\
	GOTO restart\

compute.number.of.payments:

	number.of.payments% =\
		-LOG(1-(present.value *\
		 monthly.interest.rate/monthly.payment))/\
		 LOG(1 + monthly.interest.rate)

	PRINT
	PRINT USING " The Number Of Payments Is: ###";number.of.payments%
	RETURN

strip.commas:

	answer$ = null$

	FOR character.position% = 1 TO LEN(reply$)

		character$ = MID$(reply$,character.position%,1)
		IF character$ = "," THEN character$ = null$
		answer$ = answer$ + character$

	NEXT character.position%
	RETURN

process.error:

	IF ERR = "DZ" THEN PRINT:PRINT " A division by zero was attempted." +\
		" Please recheck your entries."
	GOTO restart