Class: Finance::Amortization
 Inherits:

Object
 Object
 Finance::Amortization
 Defined in:
 lib/finance/amortization.rb
Overview
There are two ways to create an amortization. The first example uses the amortize method for the Numeric class. The second calls Amortization.new directly.
the Amortization class provides an interface for working with loan amortizations.
Instance Attribute Summary collapse

#balance ⇒ DecNum
readonly
The balance of the loan at the end of the amortization period (usually zero).

#payment ⇒ DecNum
readonly
The required monthly payment.

#principal ⇒ DecNum
readonly
The principal amount of the loan.

#rates ⇒ Array
readonly
The interest rates used for calculating the amortization.
Class Method Summary collapse

.payment(principal, rate, periods) ⇒ DecNum
The periodic payment due on a loan.
Instance Method Summary collapse

#==(amortization) ⇒ Numeric
compare two Amortization instances.

#additional_payments ⇒ Array
The amount of any additional payments in each period.

#amortize(rate) ⇒ Object
private
amortize the balance of loan with the given interest rate.

#compute ⇒ Object
private
compute the amortization of the principal.

#duration ⇒ Integer
The time required to pay off the loan, in months.

#initialize(principal, *rates, &block) ⇒ Amortization
constructor
create a new Amortization instance.
 #inspect ⇒ Object

#interest ⇒ Array
The amount of interest charged in each period.

#payments ⇒ Array
The amount of the payment in each period.
Constructor Details
#initialize(principal, *rates, &block) ⇒ Amortization
create a new Amortization instance
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# File 'lib/finance/amortization.rb', line 132 def initialize(principal, *rates, &block) @principal = Flt::DecNum.new(principal.to_s) @rates = rates @block = block # compute the total duration from all of the rates. @periods = (rates.collect { r r.duration }).sum @period = 0 compute end 
Instance Attribute Details
#balance ⇒ DecNum (readonly)
Returns the balance of the loan at the end of the amortization period (usually zero).
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# File 'lib/finance/amortization.rb', line 24 def balance @balance end 
#payment ⇒ DecNum (readonly)
Returns the required monthly payment. For loans with more than one rate, returns nil.
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# File 'lib/finance/amortization.rb', line 27 def payment @payment end 
#principal ⇒ DecNum (readonly)
Returns the principal amount of the loan.
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# File 'lib/finance/amortization.rb', line 30 def principal @principal end 
#rates ⇒ Array (readonly)
Returns the interest rates used for calculating the amortization.
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# File 'lib/finance/amortization.rb', line 33 def rates @rates end 
Class Method Details
.payment(principal, rate, periods) ⇒ DecNum
in most cases, you will probably want to use rate.monthly when calling this function outside of an Amortization instance.
Returns the periodic payment due on a loan.
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# File 'lib/finance/amortization.rb', line 174 def Amortization.payment(principal, rate, periods) if rate.zero? # simplified formula to avoid divisionbyzero when interest rate is zero return (principal / periods).round(2) else return (principal * (rate + (rate / ((1 + rate) ** periods  1)))).round(2) end end 
Instance Method Details
#==(amortization) ⇒ Numeric
compare two Amortization instances
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# File 'lib/finance/amortization.rb', line 39 def ==(amortization) self.principal == amortization.principal and self.rates == amortization.rates and self.payments == amortization.payments end 
#additional_payments ⇒ Array
Returns the amount of any additional payments in each period.
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# File 'lib/finance/amortization.rb', line 49 def additional_payments @transactions.find_all(&:payment?).collect{ p p.difference } end 
#amortize(rate) ⇒ Object
This method is part of a private API. You should avoid using this method if possible, as it may be removed or be changed in the future.
amortize the balance of loan with the given interest rate
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# File 'lib/finance/amortization.rb', line 57 def amortize(rate) # For the purposes of calculating a payment, the relevant time # period is the remaining number of periods in the loan, not # necessarily the duration of the rate itself. periods = @periods  @period amount = Amortization.payment @balance, rate.monthly, periods pmt = Payment.new(amount, :period => @period) if @block then pmt.modify(&@block) end rate.duration.to_i.times do # Do this first in case the balance is zero already. if @balance.zero? then break end # Compute and record interest on the outstanding balance. int = (@balance * rate.monthly).round(2) interest = Interest.new(int, :period => @period) @balance += interest.amount @transactions << interest.dup # Record payment. Don't pay more than the outstanding balance. if pmt.amount.abs > @balance then pmt.amount = @balance end @transactions << pmt.dup @balance += pmt.amount @period += 1 end end 
#compute ⇒ Object
This method is part of a private API. You should avoid using this method if possible, as it may be removed or be changed in the future.
compute the amortization of the principal
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# File 'lib/finance/amortization.rb', line 89 def compute @balance = @principal @transactions = [] @rates.each do rate amortize(rate) end # Add any remaining balance due to rounding error to the last payment. unless @balance.zero? @transactions.find_all(&:payment?)[1].amount = @balance @balance = 0 end if @rates.length == 1 @payment = self.payments[0] else @payment = nil end @transactions.freeze end 
#duration ⇒ Integer
Returns the time required to pay off the loan, in months.
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# File 'lib/finance/amortization.rb', line 122 def duration self.payments.length end 
#inspect ⇒ Object
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# File 'lib/finance/amortization.rb', line 145 def inspect "Amortization.new(#{@principal})" end 
#interest ⇒ Array
Returns the amount of interest charged in each period.
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# File 'lib/finance/amortization.rb', line 159 def interest @transactions.find_all(&:interest?).collect{ p p.amount } end 
#payments ⇒ Array
Returns the amount of the payment in each period.
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# File 'lib/finance/amortization.rb', line 189 def payments @transactions.find_all(&:payment?).collect{ p p.amount } end 