Nest Egg

This file is part of Timex Sinclair Public Domain Library Tape 1003 . Download the collection to get this file.

Developer(s): David Draker

Date: 198x

Type: Program

Platform(s): TS 1000

Nest Egg is a retirement savings calculator that models the growth and depletion of a capital sum over multiple years, accounting for variable interest rates and variable monthly withdrawal schedules. The program allows up to five user-specified years in which new capital injections can be added, and supports an optional accumulation period before withdrawals begin. Interest rates can be set as a constant or varied across four time bands (year 1, years 2–3, years 4–6, and all remaining years), while withdrawals similarly step through four bands (2, 3, 5, and remaining years). The core calculation at line 1240 applies annual compound interest and deducts twelve months of withdrawals in a single integer expression: INT(A*Q/100+A-12*E). Originally published in Sync magazine (January/February 1984, pages 42–50).

Program Analysis

Program Structure

The program is organized into a main flow with several subroutines and detour sections reached by GOTO. Initialization occupies lines 2–27, data entry runs from line 30 to around line 940, a summary display covers lines 950–1170, and the main calculation loop runs from line 1180 to 1270. Helper subroutines are grouped at lines 2500, 3000, and 3500. Two short input detours live at lines 1500 and 2000.

Line range	Purpose
2–27	Variable initialization
30–530	Input: capital, new-capital years, interest rates, accumulation period, withdrawals
540–930	Branching input for varying vs. constant interest and withdrawal rates
950–1170	Summary printout of chosen parameters
1180–1270	Main year-by-year calculation loop
1280–1380	End-of-run prompt and restart option
1500–1520	Constant interest rate input detour
2000–2020	Constant withdrawal input detour
2500–2560	Subroutine: apply new capital injection for current year
3000–3040	Subroutine: select interest rate band for current year
3500–3540	Subroutine: select withdrawal band for current year
4000–4010	SAVE and auto-restart

Variable Usage

Variable	Role
`A`	Current capital balance
`B$`	General yes/no input string
`C`	Constant interest rate
`D`	Constant monthly withdrawal amount
`E`	Active monthly withdrawal for current year
`F,G,H,I`	Varying interest rates (4 time bands)
`J`	Accumulation years (no withdrawals)
`K,L,M,N`	Varying monthly withdrawals (4 time bands)
`Q`	Active interest rate for current year
`S`	Counter for new-capital years entered
`W`	Number of new-capital injection years (max 5)
`WA–WE`	Year numbers for capital injections
`XA–XE`	Amounts of capital injections
`X`	Current year’s capital injection amount
`Y`	Current year counter (starts at 1)

Main Calculation Loop

The loop at lines 1180–1270 calls subroutines conditionally to set the active rate Q and withdrawal E, then applies the core formula at line 1240:

LET A=INT(A*Q/100+A-12*E)

This computes one year’s end balance by adding interest (A*Q/100) to the principal and subtracting twelve months of withdrawals (12*E). INT truncates to whole dollars. The loop terminates when A drops to zero or below.

Capital Injection Subroutine (lines 2500–2560)

The subroutine at 2500 checks whether the current year Y matches any of the five stored injection years (WA–WE). If so, it sets X to the corresponding amount. The compound OR condition at line 2550 then adds X to A. A subtle issue exists: X is only updated when a match is found, but it is never reset to zero between years. If no match occurs on a given year, X retains its last value; however, the OR guard at line 2550 prevents a spurious addition because it only fires on a matching year.

Stepped Rate and Withdrawal Bands

Both the interest rate subroutine (3000–3040) and the withdrawal subroutine (3500–3540) use explicit year comparisons to implement stepped time bands. The withdrawal bands are offset by J (accumulation years), so, for example, the first withdrawal band applies during years J+1 and J+2. The accumulation guard at line 1200 sets E=0 during the accumulation period regardless of what the withdrawal subroutine returned.

New-Capital Entry Loop

Rather than a true loop, the program uses a linear sequence of input pairs (lines 160–530) with a counter S and early exits via IF S=W THEN GOTO 550 to skip unused entries. This allows from one to five injection years to be collected without array variables, at the cost of duplicated code for each of the five possible entries.

Notable Techniques and Idioms

Re-use of B$ for all yes/no questions, pre-initialized to "YES" at line 70 as a default.
Conditional PRINT statements (lines 970–1130) use IF variable>0 to display only parameters that were actually set, producing a clean summary without branching to separate print blocks.
The \' /\ . sequences in lines 1000–1040 render as a percent sign composed of block graphics — a common workaround since % is not directly available in the character set.
All variables are explicitly initialized to zero at lines 2–27, ensuring a clean state on restart via GOTO 2 at line 1340.

Bugs and Anomalies

Line 80 contains a typo: “CPITAL” instead of “CAPITAL”.
Line 410 is missing from the listing (the sequence jumps from 400 to 420), though this does not affect execution.
When the user chooses a constant interest rate (branch to line 1500), C is set but Q is only assigned at line 1230 inside the loop. If the varying-rate subroutine is also invoked (it isn’t, since F remains 0), Q could be overwritten; the conditional guards prevent this correctly.
The program terminates the loop when A<=0 but prints the negative balance without a special message, which could be confusing to the user.
The maximum of five new-capital years is enforced only by the program structure (five hard-coded input pairs), not by validating the user’s entry for W; entering W>5 would cause the counter never to reach W, and the fifth pair would still be the last collected.

Content

Appears On

Timex Sinclair Public Domain Library Tape 1003

Assembled by Tim Ward from many sources. Contains programs 10122 – 10175.

Related Products

Hatch Your Nest Egg

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Source Code
   1 REM NEST EGG(SYNC JAN/FEB 84 PP 42-50)
   2 LET C=0
   3 LET D=0
   4 LET E=0
   5 LET F=0
   6 LET G=0
   7 LET H=0
   8 LET I=0
   9 LET J=0
  10 LET K=0
  11 LET L=0
  12 LET M=0
  13 LET N=0
  14 LET S=0
  15 LET W=0
  16 LET X=0
  17 LET Y=1
  18 LET WA=0
  19 LET WB=0
  20 LET WC=0
  21 LET WD=0
  22 LET WE=0
  23 LET XA=0
  24 LET XB=0
  25 LET XC=0
  26 LET XD=0
  27 LET XE=0
  30 CLS 
  40 PRINT "ENTER INITIAL AMOUNT OF CAPITAL"
  50 INPUT A
  60 CLS 
  70 LET B$="YES"
  80 PRINT "WILL THERE BE ANY YEARS IN WHICH YOU INTRODUCE NEW CPITAL?"
  90 INPUT B$
 100 IF B$<>"YES" THEN GOTO 540
 110 CLS 
 120 PRINT "HOW MANY YEARS WILL SEE NEW ADDITIONS OF CAPITAL?(MAXIMUM OF 5 YEARS)"
 130 INPUT W
 140 CLS 
 150 PRINT "FIRST ENTER THE YEAR NUMBER AT THE BEGINNING OF WHICH YOU WILL ADD NEW CAPITAL, AND THEN ENTER THE AMOUNT OF NEW CAPITAL"
 160 INPUT WA
 170 INPUT XA
 180 CLS 
 190 LET S=S+1
 200 IF S=W THEN GOTO 550
 210 PRINT "YEAR ";WA;" NEW CAPITAL $";XA
 220 PRINT 
 230 PRINT 
 240 PRINT "ENTER THE NEXT YEAR NUMBER AT THE BEGINNING OF WHICH YOU WILL ADD NEW CAPITAL, AND THEN ENTER THE AMOUNT OF NEW CAPITAL"
 250 INPUT WB
 260 INPUT XB
 270 CLS 
 280 LET S=S+1
 290 IF S=W THEN GOTO 550
 300 PRINT "YEAR ";WB;" NEW CAPITAL $";XB
 310 PRINT 
 320 PRINT 
 330 PRINT "ENTER THE NEXT YEAR AND THEN ENTER THE CAPITAL."
 340 INPUT WC
 350 INPUT XC
 360 CLS 
 370 LET S=S+1
 380 IF S=W THEN GOTO 550
 390 PRINT "YEAR ";WC;" NEW CAPITAL $";XC
 400 PRINT 
 420 PRINT "ENTER THE NEXT YEAR AND THEN ENTER THE NEW CAPITAL."
 430 INPUT WD
 440 INPUT XD
 450 CLS 
 460 LET S=S+1
 470 IF S=W THEN GOTO 550
 480 PRINT "YEAR ";WD;" NEW CAPITAL $";XD
 490 PRINT 
 500 PRINT 
 510 PRINT "ENTER THE NEXT YEAR AND THEN ENTER THE NEW CAPITAL."
 520 INPUT WE
 530 INPUT XE
 540 CLS 
 550 PRINT "DO YOU WISH TO VARY THE INTEREST RATE OVER THE YEARS?"
 560 INPUT B$
 570 CLS 
 580 IF B$<>"YES" THEN GOTO 1500
 590 PRINT "WHAT INTEREST RATE DO YOU WANT FOR THE FIRST YEAR?"
 600 INPUT F
 610 PRINT 
 620 PRINT "WHAT INTEREST RATE FOR THE NEXT 2 YEARS?"
 630 INPUT G
 640 PRINT 
 650 PRINT "AND FOR THE SUBSEQUENT 3 YEARS?"
 660 INPUT H
 670 PRINT 
 680 PRINT "FINALLY, WHAT SINGLE INTEREST RATE FOR ALL REMAINING YEARS?"
 690 INPUT I
 700 CLS 
 710 PRINT "DO YOU WISH TO ACCUMULATE INTEREST FOR A NUMBER OF YEARS BEFORE YOU BEGIN TO WITHDRAW MONEY?"
 720 INPUT B$
 730 CLS 
 740 IF B$<>"YES" THEN GOTO 780
 750 PRINT "HOW MANY YEARS DO YOU WISH TO ACCUMULATE INTEREST WITHOUT WITHDRAWALS?"
 760 INPUT J
 770 CLS 
 780 PRINT "DO YOU WISH TO VARY THE RATE OF WITHDRAWALS OVER THE YEARS?"
 790 INPUT B$
 800 CLS 
 810 IF B$<>"YES" THEN GOTO 2000
 820 CLS 
 830 PRINT "HOW MUCH WILL YOU WITHDRAW MONTHLY IN THE FIRST 2 YEARS?"
 840 PRINT 
 850 INPUT K
 860 PRINT "HOW MUCH MONTHLY IN THE NEXT 3 YEARS?"
 870 INPUT L
 880 PRINT 
 890 PRINT "AND MONTHLY IN THE NEXT 5 YEARS?"
 900 INPUT M
 910 PRINT 
 920 PRINT "AND FINALLY, MONTHLY  FOR ALL THE REMAINING YEARS?"
 930 INPUT N
 940 CLS 
 950 PRINT "  BASED ON AN INITIAL CAPITAL OF $ ";A
 960 PRINT 
 970 IF W>0 THEN PRINT "  SOME NEW CAPITAL IN"
 980 IF W>0 THEN PRINT W;" YEARS "
 990 PRINT 
\n1000 IF C>0 THEN PRINT "  A CONSTANT INTEREST RATE OF   ";C;" \' /\ ."
\n1010 IF F>0 THEN PRINT "  A VARYING INTEREST RATE OF   ";F;" \' /\ .  IN THE FIRST YEAR"
\n1020 IF F>0 THEN PRINT G;" \' /\ . IN THE NEXT TWO YEARS"
\n1030 IF F>0 THEN PRINT H;" \' /\ . IN THE NEXT THREE YEARS"
\n1040 IF F>0 THEN PRINT I;" \' /\ . FOR REMAINING YEARS"
\n1050 PRINT 
\n1060 IF J>0 THEN PRINT "  INTEREST GROWTH FOR THE FIRST ";J;" YEARS"
\n1070 PRINT 
\n1080 IF K>0 THEN PRINT "  AND VARYING WITHDRAWALS OF  $ ";K;" MONTHLY FOR TWO YEARS"
\n1090 IF K>0 THEN PRINT "$";L;" MONTHLY FOR THREE YEARS"
\n1100 IF K>0 THEN PRINT "$";M;" MONTHLY FOR FIVE YEARS"
\n1110 IF K>0 THEN PRINT "$";N;" MONTHLY FOR OTHER YEARS"
\n1120 IF K=0 THEN PRINT "  AND CONSTANT MONTHLY WITHDRAWALS OF"
\n1130 IF K=0 THEN PRINT "$";D
\n1140 PRINT 
\n1150 PRINT 
\n1160 PRINT 
\n1170 PRINT 
\n1180 IF K>0 THEN GOSUB 3500
\n1190 IF D>0 THEN LET E=D
\n1200 IF J>=Y THEN LET E=0
\n1210 IF W>0 THEN GOSUB 2500
\n1220 IF F>0 THEN GOSUB 3000
\n1230 IF C>0 THEN LET Q=C
\n1240 LET A=INT (A*Q/100+A-12*E)
\n1250 PRINT "YEAR ";Y;" CAPITAL LEFT $ ";A
\n1260 LET Y=Y+1
\n1270 IF A>0 THEN GOTO 1180
\n1280 PRINT 
\n1290 PRINT 
\n1300 PRINT 
\n1310 PRINT 
\n1320 PRINT "THAT ENDS THE CALCULATIONS. WOULD YOU LIKE TO CHANGE ANY OF YOUR ESTIMATES?"
\n1330 INPUT B$
\n1340 IF B$="YES" THEN GOTO 2
\n1350 PRINT 
\n1360 PRINT 
\n1370 PRINT "OK. THAT ENDS THE RUNNING OF THE PROGRAM. TRY IT AGAIN IF YOU HAVE MORE QUESTIONS."
\n1380 STOP 
\n1500 PRINT "WHAT CONSTANT INTEREST RATE WOULD YOU LIKE TO USE?"
\n1510 INPUT C
\n1520 GOTO 700
\n2000 PRINT "WHAT CONSTANT AMOUNT DO YOU WISH TO WITHDRAW EVERY MONTH?"
\n2010 INPUT D
\n2020 GOTO 940
\n2500 IF Y=WA THEN LET X=XA
\n2510 IF Y=WB THEN LET X=XB
\n2520 IF Y=WC THEN LET X=XC
\n2530 IF Y=WD THEN LET X=XD
\n2540 IF Y=WE THEN LET X=XE
\n2550 IF Y=WA OR Y=WB OR Y=WC OR Y=WD OR Y=WE THEN LET A=A+X
\n2560 RETURN 
\n3000 IF Y=1 THEN LET Q=F
\n3010 IF Y=2 OR Y=3 THEN LET Q=G
\n3020 IF Y=4 OR Y=5 OR Y=6 THEN LET Q=H
\n3030 IF Y>6 THEN LET Q=I
\n3040 RETURN 
\n3500 IF Y=1+J OR Y=2+J THEN LET E=K
\n3510 IF Y=3+J OR Y=4+J OR Y=5+J THEN LET E=L
\n3520 IF Y=6+J OR Y=7+J OR Y=8+J OR Y=9+J OR Y=10+J THEN LET E=M
\n3530 IF Y>10+J THEN LET E=N
\n3540 RETURN 
\n4000 SAVE "1014%3"
\n4010 RUN 

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