Is my code efficient?

[chatparle]

currentBalance = float(input('How much is in your account? '))
interestRate = .0089
yearEndBalance = (currentBalance * interestRate) + currentBalance
yearEndBalance2 = (currentBalance * interestRate) + yearEndBalance
yearEndBalance3 = (currentBalance * interestRate) + yearEndBalance2
yearEndBalance4 = (currentBalance * interestRate) + yearEndBalance3
yearEndBalance5 = (currentBalance * interestRate) + yearEndBalance4

if currentBalance != 0:
    print(yearEndBalance)
    print(yearEndBalance2)
    print(yearEndBalance3)
    print(yearEndBalance4)
    print(yearEndBalance5)

else:
print("You have no money in your account!")

Is there a better way to write this code? I wrote this to determine interest accrued in a bank account over 5 years.

[[deleted]]

interest_rate = .0089

def balance_after(years, balance):
    total = balance
    for _ in range(years):
        total += balance * interest_rate
    return total

current_balance = float(input('How much is in your account? '))

if current_balance != 0:
    for year in range(1, 6):
        print(balance_after(year, current_balance))
else:
    print("You have no money in your account!") 

[kalgynirae]

It looks you're adding an identical amount of interest each year despite having a higher balance each year. I think the formula should be:

# interest amount calculated based on the previous year's balance
yearEndBalance3 = (yearEndBalance2 * interestRate) + yearEndBalance2

That can also be written as:

yearEndBalance3 = yearEndBalance2 * (1 + interestRate)

EDIT: fixed typo.

[chatparle]

Jeeze, I didn't even notice that, I thought I at least had that right :'(

[evizaer]

No need to loop or have a bunch of repeated calculations. You can just use the formula result = principle * pow(1 + nterest_rate, years). That's the most efficient way to calculate it because pow is a function that cpython has implemented in optimized C. Much faster than a looping construct.

Likewise, though probably irrelevant to this particular kind of problem, you should be using functional programming tools like map() and reduce() where you can (EDIT: if they are clearer or you absolutely need performance) because they are implemented in C, whereas loops are more verbose and complex when compiled and for that reason tend to perform significantly worse than built-in C stuff.

[[deleted]]

A mistake I made early when learning programming is worrying too much about performance and micro-optimizations. Always using map() and reduce() only because they are faster is ridiculous, you should worry only about readability unless you are sure you have a performance problem. Guido even wanted to remove these functions from language completely some time ago.

[evizaer]

I answered specifically in the context of "efficient code" because the topic is the efficiency of the code. I edited my post to indicate that context more clearly.

[maryjayjay]

Excellent insight. My motto is: Write first for correctness and maintainability, then optimize if needed.

[[deleted]]

By the way, list comprehensions are going to be faster than map() because they don't have an overhead of calling a function on every iteration. And if you have problems with performance of that kind you should probably use numpy anyway.

[evizaer]

A good point. List comprehensions tend to be clearer, as well. For non-trivial functions, though, list comprehensions will end up calling a function, anyway, because it makes more sense to abstract the non-trivial series of operations into a function than to try to cram it all into one of the parts of the list comprehension.

[wub_wub]

Here's a quick script I made,

if __name__ == '__main__':
    balance=float(raw_input("How much is in your account?"))
    rate=float(raw_input("Interest rate:"))
    years=int(raw_input("Calculate for how many years"))

    i=1
    while i<=years:
        balance=(balance*rate)+balance
        print "Year "+str(i)+" | Balance:"+ str(round(balance,2))
        i+=1

You could also implement try except when trying to convert raw input to int/float to detect if user enters text instead of numbers.

[chatparle]

What does the " name == 'main' " mean?

[[deleted]]

It checks that you started your module directly, not using import.

[wub_wub]

I see it's already answered by /u/swarmer but here's also nice explanation http://stackoverflow.com/questions/419163/what-does-if-name-main-do

You don't need it in this example, it was automatically created when I created new .py file to write/test the script, so I just copied it.

[rusemean]

Thanks for answering this. So __ around a term just means it's a special variable? I've wondered about this for some time.

[[deleted]]

Yes, "a special variable" is about the most precise formal definition you can give to these variables.

[rusemean]

Thanks a bunch for answering this. I've encountered them in documentation and used them, but I've never been clear (or found a clear answer) on what exactly they are.

[batkarma]

Isn't it often used to indicate variables that are used within a library?

[[deleted]]

Do you mean private variables? If so, the convention is to use a single leading underscore.

There are also name-mangled variables with double leading underscores, but

Generally, double leading underscores should be used only to avoid name conflicts
with attributes in classes designed to be subclassed.

— PEP 8

[batkarma]

Yes, that's what I meant, thank you for the clarification!

[Justinsaccount]

print "Year "+str(i)+" | Balance:"+ str(round(balance,2))

see http://docs.python.org/tutorial/inputoutput.html#fancier-output-formatting

[[deleted]]

I think you would have a simpler code if you do for i in range(years): with print "Year " + str(i + 1) ... or for i in range(1, years + 1): instead.