Irrational number in python

Asked May 08 '17 at 12:05

Active May 08 '17 at 12:16

Viewed 1,703 times

How to represent irrational numbers in Python? e.g. I need sqrt(2) * sqrt(2) to be 2 (or 2.0)

But:

>>> sqrt(2) * sqrt(2)   
2.0000000000000004

Same with floating point:

>>> sqrt(2.0) * sqrt(2.0)    
2.0000000000000004

Decimal too:

>>> Decimal(2).sqrt() * Decimal(2).sqrt()
Decimal('1.999999999999999999999999999')

asked May 08 '17 at 12:05

Ron

7,588
11
38
42

3

Why aren't you rounding? – TLOwater May 08 '17 at 12:06
3

Nothing here is an irrational number – OneCricketeer May 08 '17 at 12:07
Because I need e.g. `sqrt(1.5) * sqrt(1.5)` to be `1.5` too – Ron May 08 '17 at 12:07
It the way it is, in any programming language, you have to round it to use. – Phung Duy Phong May 08 '17 at 12:08
1

@Ron round to the number of places from the original input or round to like the 10th place which will have a knock on effect in your examples to get the right answer. – TLOwater May 08 '17 at 12:10
2

@cricket_007 The square root of 2 is but this fact is **irr**elevant here. – Ma0 May 08 '17 at 12:10
4

Short answer: [use sympy](http://stackoverflow.com/a/42149252/1600898). – user4815162342 May 08 '17 at 12:12
@Ron do you also need `0.1+0.1+0.1` to be `0.3`? Because if you do `0.1+0.1+0.1==0.3` the result is `False` – May 08 '17 at 12:12
1

@PhongPhung nonsense, there are plenty of symbolic languages and libraries which allow manipulation of expressions. – Peter Wood May 08 '17 at 12:13
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@SembeiNorimaki Both `fractions.Fraction` and `decimal.Decimal` can handle 0.1+0.1+0.1 evaluating to exactly 0.3. To do the same with irrationals you need a symbolic package. – user4815162342 May 08 '17 at 12:15
@PeterWood: please en-light me, I'm new – Phung Duy Phong May 08 '17 at 12:26
@PhongPhung e.g. [SymPy](http://www.sympy.org/en/index.html). The tutorial [has an example](http://docs.sympy.org/latest/tutorial/intro.html#what-is-symbolic-computation) using `sqrt(8)`. – Peter Wood May 08 '17 at 12:39
@PeterWood: great, thanks – Phung Duy Phong May 08 '17 at 12:52

Irrational number in python

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