Question

PS4.3: Ok I have this odd problem...the post retirement fund...my first three amounts add up, but then amount 4, 5 do not match. I cannot find the problem with my algorithm...any thoughts?

Answer 1

My code is this:

def postretirement(savings, growthrates,expenses):
savingsrecord = []
for i in range(len(growthrates)):
fund = savings + float(savings*growthrates[i]) - expenses
savingsrecord.append(fund)
savings = fund
print "SavingsRecord", savingsrecord

Answer 2

The amounts that were specified are this: [80000.000000000015, 54000.000000000015, 24000.000000000015,
-4799.9999999999854, -34847.999999999985]

My last two records are slightly different.

Answer 3

And are returned as this:
SavingsRecord [80000.0, 54000.0, 24000.0, -4800.0, -39600.0]

Answer 4

Ok I reworked my code a bit, but I am still off...
Results: SavingsRecord [80000.0, 54000.0, 24000.0, -4800.0, -34800.0]

def postretirement(savings, growthrates,expenses):
savingsrecord = []
for i in range(len(growthrates)):
if savings > 0:
fund = savings + float(savings*growthrates[i]) - expenses
savingsrecord.append(fund)
savings = fund
else:
fund = float(savings*growthrates[i]) - expenses
savingsrecord.append(fund)
print "SavingsRecord", savingsrecord

Ideas?

Answer 5

"savings*growthrates[i]"
This should be savings*0.01*growthrates[i] to convert the growthrates to percentages. I too am having issues on this problem. I'll report what I find. My current output is:
[80000.00000000001, 54000.000000000015, 20000.000000000015, -1964.9999999999818, -38563.70919999998]

Answer 6

This is my current code. Still baffled.
#Initialize result with (savings * (1 + .01 * growthRates[0])) in each index, with
#result's length being equal to the number of years of growth rates.
result = [savings * (1 + .01 * growthRates[0]) - expenses] * len(growthRates)

for j in range(1, len(growthRates)+1):
for i in range(j, len(growthRates)):
result[i] = result[i] * (1 + .01 * growthRates[i]) - expenses
return result

Answer 7

i believe you are missing compounding.try
f=[]
f.append(savings)
for i in range(len(growthRates)):
f.append (f[i]*(1+.01*growthRates[i])-expenses)
del f[0]
return f

Answer 8

I'm using the same code I used for the earlier steps in the assignment, so I shouldn't have compounding issues. I would have gotten the wrong answers earlier in the problem set. I think we're having issues with error propagation. I added some prints to my code to track values of variables:
result[i] * (1 + .01 * growthRates[i]) - expenses = 20000.0
result[i] = 50000.0
growthrates[i] = 0
But if I type the values of those variables right into the interpreter:
>>> 50000*(1+.01*1) - 30000
20500.0

Answer 9

Err, nevermind. I dunno how I missed that 0 doesn't equal 1. Derp derp.

Answer 10

Yeah, now I've got it working. My previous method was failing to properly multiply against previous iterations. I like your use of append to iterate, very slick.

Answer 11

Isn't this code:
f=[]
f.append(savings)

the same as:
f = [savings]

Answer 12

I did not add the "1+...." into my algorithm...I will recode that part and see what I get...

Answer 13

Hey - I changed my code and got it to work perfectly - thanks for all your help!

Answer 14

If you want to take a look at the new trial and compare the results:

def postretirement(savings, growthrates,expenses):
"""Growthrates as a list of percentages ex. (0.01 for 10%)"""
savingsrecord = []
for i in range(len(growthrates)):
if savings > 0:
#fund = savings + float(savings*growthrates[i]) - expenses
fund = float(savings*(1+(0.01*growthrates[i]))) - expenses
savingsrecord.append(fund)
savings = fund
print "Expenses", expenses
else:
fund = float(savings*(1+0.01*growthrates[i])) - expenses
savingsrecord.append(fund)
savings = fund
print "SavingsRecord", savingsrecord
return savingsrecord

Answer 15

The commented out code was the original equation I used...don't know why I missed the 1+, but now its there on the line just below. Thanks again.

Answer 16

Of course, this: """Growthrates as a list of percentages ex. (0.01 for 10%)"""
would no longer apply either, as I changed the code to accept just the raw percentage, rather than floating points.

Answer 17

Isn't this code:
f=[]
f.append(savings)

the same as:
f = [savings]
You are making a list of a list. So, if you take as an example, savings = [1,2,3,4,5]
then f =[savings] would return [[1,2,3,4,5]]. I don't know if that would matter, unless you append savings or f with another list, because then you would have to be careful about indexes.
Example: Append savings with 7, then f with 8. Notice the difference?
savings = [1,2,3,4,5,6,7]
f = [[1,2,3,4,5,6,7],8]
So, savings[1] which is not the same as f[1].

Answer 18

Savings[1] = 2
f[1] = 8

Answer 19

> You are making a list of a list. So, if you take as an example,
> savings = [1,2,3,4,5]
Except savings is just an int for that problem, not a list.

Answer 20

If you simply need an int, then the original question: is f.append(savings) the same as f = [savings], is not really relevant, since both involve lists. If you just need an int, then use variables without list functions.