On 3 aug 2008, at 17.16,

Ed**********@VerizonWireless.com wrote:

for nth square root: use math.sqrt n times for example

Ehum. The OP wants to compute the nth root ( not the nth square root)

>
>>>import math

num = 625

how_many_sqrt = 2

for i in range(how_many_sqrt):

.. num = math.sqrt(num)

..
>>>num

5.0

all comparisons work fine for arbitrary floating point numbers...

For readability print them with required precision. for example
>>>a = .2

b = .4

b = b/2

a == b

True
>>>a, b

(0.20000000000000001, 0.20000000000000001)
>>>'%.2f' % a, '%.2f' % b

('0.20', '0.20')
>>>>

thx. Edwin

-----Original Message-----

From: py************************************************ **@python.org

[mailto:py***************************************** *********@python.org

]

On Behalf Of CNiall

Sent: Sunday, August 03, 2008 10:03 AM

To: py*********@python.org

Subject: Decimals not equalling themselves (e.g. 0.2 = 0.2000000001)

I am very new to Python (I started learning it just yesterday), but I

have encountered a problem.

I want to make a simple script that calculates the n-th root of a

given

number (e.g. 4th root of 625--obviously five, but it's just an example

:P), and because there is no nth-root function in Python I will do

this

with something like x**(1/n).

However, with some, but not all, decimals, they do not seem to 'equal

themselves'. This is probably a bad way of expressing what I mean, so

I'll give an example:
>>>0.5

0.5
>>>0.25

0.25
>>>0.125

0.125
>>>0.2

0.20000000000000001
>>>0.33

0.33000000000000002

As you can see, the last two decimals are very slightly inaccurate.

However, it appears that when n in 1/n is a power of two, the decimal

does not get 'thrown off'. How might I make Python recognise 0.2 as

0.2

and not 0.20000000000000001?

This discrepancy is very minor, but it makes the whole n-th root

calculator inaccurate. :\

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