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# Six steps of software development

### STEP ONE ###
# Problem Analysis
#
#   Problem:
#       Create a program that determines the value of an investment after 10 years.
#   Knowns:
#       Money put in a bank earns interest.
#       Interest accumulates as years pass.
#       Formula for calculating the value of one year is I = P*1+rt
#       I = interest (the calculated amount after a year)
#       P = principal (money put into the bank)
#       rt = rate (yearly)


### STEP TWO ###
# Specification (what do we need?)
#
#   Input:
#       principal - the amount of money being invested.
#       APR - the annual percentage rate expressed as a decimal number (float).
#   Output:
#       Value of investment 10 years into the future.
#   Relationship:
#       Value after one year is principal*(1+apr). Apply formula 10 times.


### STEP THREE ###
# Design (write it using pseudo code)
#
# defining the function:
#   print introduction
#   input amount of the principal (principal)
#   input amount of percentage rate (apr)
#   repeat 10 times:
#       principal = principal * (1 + apr)
#   print the value of principal


### STEP FOUR ###
# Implementation (code it)

def main():
    print('This program calculates the future value',\
          'of an investment.')
    principal = float(input('Enter the initial principal: '))
    apr = float(input('Enter the anual interest rate: ' ))
    investment = float(input('Enter how much you will invest each year: '))
    period = int(input('Enter amount of years you would like to calculate for: '))
    if period == 1:
        years = 'year'
    else:
        years = 'years'

    for i in range(period):
        principal = (principal+investment) * (1 + apr)
    print('The value in', period, years, 'is:', principal)

if __name__ == "__main__":
    main()


### STEP FIVE ###
# Testing and debugging
#
#   Bug testing:
#       What if user inputs a letter?
#       What if the user inputs python code (code injection)?
#       What if user inputs a number with %?
#           try, except and if/else statements to fix that.
#   User interface:
#       Describe input better, so the user doesn't mess it up
#       Expect that the user will mess up however, and work around that.


### STEP SIX ###
# Maintenance
#
# Continue to maintain and develop the software until it dies or becomes irrelevant.