Calculating time it takes savings to double in python

Question

I need to write a script for calculating the length of time it takes an input to double on 5% APR

here is what I have, I am just getting the incorrect answer. Is it a math or code issue? Any help appreciated.

i = float(input("What is your principal amount"))
r = float(.05)
year = 0
while i < (i * 2):
  ie = (i * r)
  i = i + ie
  year += 1
  continue
else:
  print("The number of years it takes for your sum to double is", year)

Asked By: PieCharmer

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Source

Answer 1

The problem is that you tell the loop to stop when i is greater than i*2, however, every time the loop runs i*2 is recalculated. To fix this, you should make a variable to store the end number, then compare it to that. Like this:

i = float(input("What is your principal amount"))
r = float(.05)
year = 0
doubled = i * 2
while i < doubled:
  ie = (i * r)
  i = i + ie
  year += 1
  continue
else:
  print("The number of years it takes for your sum to double is", year)

If you use i < i*2, then when i is 1 the statement will be read as 1 < 2. When i is 2, the statement will be read as 2 < 4. When i is 3, the statement will be read as 2 < 6. And so on… The loop will only end when the numbers get so large that Python just crashes.

On the other hand, if you use a variable, like doubled = i*2, then i*2 will only be evaluated once.

However, in the real world you should always prefer math to loops. I understand that this is a homework assignment, but future visitors to this question should unconditionally use the formula instead. This is the code that uses the formula:

from math import log

apr = 0.05  # as a decimal
doublingTime = log(2) / log(1 + apr)
print(doublingTime)

Answered By: Michael M.

Calculating time it takes savings to double in python

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