compounding.py

# -*- coding: utf-8 -*-
"""
Simple compunding interest calculator.
Created on Sat May  7 12:48:37 2016

@author: Chris Darringer
"""

def main():
    # read inputs
    principal = readPrincipal()
    rate = readInterestRate()
    years = readYears()
    compounding = readCompounding()
    
    # map compounding type to a denominator
    periodsPerYear = getPeriodsPerYear(compounding)
        
    # print the schedule
    balance = principal    
    for year in range(years):
        for period in range(periodsPerYear): 
            interest = balance * ((rate / 100) / periodsPerYear)
            newBalance = balance + interest
            print "Year: ", year, "Period: ", period, "Beginning balance: ", \
                balance, "Interest: ", interest, "New balance: ", newBalance
            balance = newBalance
    
    # print the final balance using the comppounding formula
    balance = principal * ((1 + ((rate / 100) / periodsPerYear)) ** (years * periodsPerYear))
    print "Balance calculated from compounding formula: ", round(balance, 2)

def getPeriodsPerYear(compounding) :
    compoundingTypeMap = {
        'a':1,
        's':2,
        'q':4,
        'm':12,
        'd':360
    }
    return compoundingTypeMap[compounding]    

def readPrincipal():
    principal = 0
    while principal <= 0.0:
        try:
            answer = raw_input("What is the principal amount in dollars (i.e. 100.00)?  ")
            principal = float(answer)
        except ValueError:
            print "That's not a valid principal, please try again."
    return principal

def readInterestRate():
    rate = 0.0
    while rate == 0.0:
        try:
            answer = raw_input("What is the interest rate percent (i.e. 5.2)?  ")
            rate = float(answer)
        except ValueError:
            print "That's not a valid percentage, please try again."
    return rate

def readYears():
    years = 0
    while years <= 0:
        try:
            answer = raw_input("How many years is the principal invested?  ")
            years = int(answer)
        except ValueError:
            print "That's not a valid number of years, please try again."
    return years

def readCompounding():
    compounding = 'X'
    while compounding not in ['a', 's', 'q', 'm', 'd']:
        try:
            question = "What type of Compounding?\n";
            question = question + "(A=Annual, S=Semiannual, Q=Quarterly, M=Monthly, D=Daily)  "
            answer = raw_input(question)
            compounding = answer.lower()
        except ValueError:
            print "That's not a valid compounding type, please try again."
    return compounding


main()