Mortgage Calculator

Added a simple mortgage calculator that calculates payments and shows the balance over time.

Author: aefarrell

math/mortgage/mortgage.bas

@@ -0,0 +1,250 @@
+/*
+Mortgage Calculator
+Calculates mortgage payments, amortization schedule, and plots balance over time.
+Assumes the mortgage compounds monthly.
+*/
+
+Option explicit
+
+Dim float p 'principle
+Dim float r 'interest rate
+Dim float m 'monthly payment
+Dim integer payoff 'time till paid off
+Dim integer yrs 'years
+Dim integer n 'number of periods
+
+start:
+CLS RGB(black)
+Print "Mortgage Calculator"
+Print "-------------------"
+Input "Loan Amount    : $", p
+Input "Int. Rate (APR): %", r
+Input "Number of Years:  ", yrs
+
+If r<0 Or r>100 Then
+ Print "Interest rate must be 0%<r<100%"
+ GoTo start
+End If
+
+n = 12*yrs
+r = r/100/12
+m = payment(p,r,n)
+payoff = n
+
+Dim float bal(n) 'remaining balance
+Dim float itr(n) 'interest'
+Dim float pmts(n) 'payments
+Array Set m, pmts()
+amort
+balplot
+
+Dim integer exit_flag=0
+On key opts
+Do While exit_flag=0
+ Pause 100
+Loop
+
+End
+
+' functions
+Function payment(p0,r,n) As float
+ payment = (p0*r)/(1-(1+r)^-n)
+End Function
+
+Function balance(p0,r,i,n) As float
+ Local float a, c
+ a = (1+r)^i
+ c = payment(p0,r,n)
+ balance = p0*a - c*(a-1)/r
+End Function
+
+' subroutines
+Sub addpmt
+ Local float apmt
+ Local integer mnth
+ Input "Add Payment:$",apmt
+ Input "Pay Period : ",mnth
+ If mnth>=0 And mnth<=n Then
+  pmts(mnth) = pmts(mnth) + apmt
+ Else
+  Print "invalid pay period"
+ End If
+End Sub
+
+Sub modpmt
+ Local float cpmt
+ Local integer start,stop,i
+ Input "Modified Payment:$",cpmt
+ Input "Pay Period Start: ",start
+ Input "Pay Period Stop : ",stop
+ If start>=0 And stop<=n Then
+  For i=start To stop
+   pmts(i) = cpmt
+  Next i
+ Else
+  Print "invalid pay periods"
+ End If
+End Sub
+
+Sub amort
+ Local integer i
+ Local float it
+ Local float newbal
+ bal(0) = p
+ itr(0) = 0
+ pmts(0) = 0
+ For i=1 To n
+  it = bal(i-1)*r
+  newbal = bal(i-1) + it - pmts(i)
+  itr(i) = it
+  If newbal > 0 Then
+   bal(i) = newbal
+  Else
+   ' loan paid off
+   payoff = i
+   bal(i) = 0
+   pmts(i) = bal(i-1) + it
+   Local integer j
+   For j=i+1 To n
+    bal(j)  = 0
+    itr(j)  = 0
+    pmts(j) = 0
+   Next j
+   Exit For
+  End If
+ Next i
+End Sub
+
+Sub printsum nbr
+ Local string l1$,l2$,l3$,l4$,l5$,l6$, fmt$
+ l1$ = "Initial Loan   : "
+ l2$ = "Monthly Payment: "
+ l3$ = "Total Paid     : "
+ l4$ = "Interest Paid  : "
+ l5$ = "Loan Repayment : "
+ l6$ = "Interest Saved : "
+ fmt$ = "$%10.2f"
+
+ Cat l1$,Format$(p,fmt$)
+ Cat l2$,Format$(m,fmt$)
+ Cat l3$,Format$(Math(sum pmts()),fmt$)
+
+ Local float ipmt
+ ipmt = Math(sum itr())
+ Cat l4$,Format$(ipmt, fmt$)
+
+ Local integer years, months
+ years = payoff\12
+ months = payoff - 12*years
+ Cat l5$,Format$(years," %g years")
+ Cat l5$,Format$(months," %g months")
+ Cat l6$,Format$(n*m-p-ipmt,fmt$)
+
+ Print #nbr, l1$
+ Print #nbr, l2$
+ Print #nbr, l3$
+ Print #nbr, l4$
+ Print #nbr, l5$
+ Print #nbr, "----------------------"
+ Print #nbr, l6$
+
+End Sub
+
+Sub balplot
+ Local float x(n),y(n),yo(n)
+ Local integer x1,x2,y1,y2
+ x1 = 5 : x2 = MM.HRES-5
+ y1 = 0 : y2 = MM.VRES\2
+
+ Local integer i
+ For i=0 To n
+  x(i) = i
+  yo(i) = balance(p,r,i,n)
+ Next i
+ Math window x(),x1,x2,x()
+ Math window yo(),y2,y1,yo()
+ Math window bal(),y2,y1,y()
+
+ CLS RGB(black)
+ Line graph x(),yo(),RGB(red)
+ Line graph x(),y(),RGB(green)
+ Line x1,y1,x1,y2,1,RGB(white)
+ Line x1,y2,x2,y2,1,RGB(white)
+
+ Local integer xtks,xscl,ytks,yscl
+ xtks = yrs
+ xscl = (x2-x1)\xtks
+ ytks = Fix(p/10000)
+ yscl = (y2-y1)\ytks
+ For i=0 To ytks
+  Pixel x1-1,y1+i*yscl,RGB(green)
+ Next i
+
+ For i=0 To xtks
+  Pixel x1+i*xscl,y2+1,RGB(green)
+ Next i
+
+ Print @(0,y2) ""
+ printsum 0
+
+End Sub
+
+Sub printfile
+ Local string fname$
+ Local string fmt$ = "   $%8.2f "
+ Line Input "Filename:",fname$
+ Open fname$ For output As #1
+ printsum 1
+ Local string hd$
+ hd$ = " Period "
+ Cat hd$,"    Interest "
+ Cat hd$,"     Payment "
+ Cat hd$,"   Remaining "
+ Print #1,""
+ Print #1,hd$
+ Print #1,"----------------------------------------------"
+ Local integer i
+ Local string l$
+ For i=0 To n
+  l$ = Format$(i,"  %3.0f  ")
+  Cat l$,Format$(itr(i),fmt$)
+  Cat l$,Format$(pmts(i),fmt$)
+  Cat l$,Format$(bal(i),fmt$)
+  Print #1,l$
+ Next i
+ Close #1
+ Print #0,"Done."
+End Sub
+
+Sub opts
+ Local string k$
+ k$ = Inkey$
+ Select Case Asc(k$)
+ Case 97  'a
+  addpmt
+  amort
+  CLS RGB(black)
+  balplot
+ Case 109 'm
+  modpmt
+  amort
+  CLS RGB(black)
+  balplot
+ Case 112 'p
+  printfile
+ Case 114 'r
+  Array Set m, pmts()
+  amort
+  CLS RGB(black)
+  balplot
+ Case 113 'q
+  exit_flag = 1
+ Case 104 'h
+  Print "a) Additional payment"
+  Print "m) Modify payment schedule"
+  Print "p) Print result to text file"
+  Print "r) Reset"
+  Print "q) Quit"
+  Print "h) Help"
+ End Select
+End Sub