Caleb Madrigal

Programming, Hacking, Math, and Art

  

Reduce with Python and Clojure

I was just playing around writing an annuity calculator function. Here is my first version:

def calculate_annuity(years, interest=0, addition_per_year=0, starting_amount=0):
    result = []
    running_total = starting_amount
    for year in range(years+1):
        result.append(running_total)
        running_total = (running_total + addition_per_year) * (1 + interest)
    return result

Here is a 2nd version, written using reduce instead of a loop:

def calculate_annuity2(years, interest=0, addition_per_year=0, starting_amount=0):
    return reduce(lambda result,addition: result + [(result[-1] + addition) * (1 + interest)],
                  [addition_per_year] * years, [starting_amount])

And here is a version in Clojure (basically a direct translation of the Python one:

(defn annuity [years interest addition_per_year init]
    (reduce
        #(conj %1 (* (+ (last %1) %2) (+ 1 interest)))
        [starting_amount]
        (for [i (range years)] addition_per_year)))

Here are some example use cases:

; Find investment returns by year for a $1000 investment at 7% interest
; for 10 years.
(println (annuity 10 0.07 0 1000))

; Find annuity value if contributing $1000 each month and starting with $7777
; with 10% interest for 10 years.
(println (annuity 10 0.10 1000 7777))

; Find annuity value if contributing $1000 each month and starting with $777
; with 10% interest for 10 years.
(println (annuity 10 0.07 0 1000))

; Test perpetuity annuity that pays out $6000 per year, given a $100000
; at 6.4% interest.
(println (annuity 30 0.064 -6000 100000))

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