CODEWITHSUNDEEP
pythonmathpython-2.7
user2215964
user2215964

Reputation: 21

Wrong answer with equation in python

try it with Wolphram Alpha - this is correct result.

try it with python 2.7.x:

u = 4/3 * 6.67*1e-11*3.14*6378000*5515
print (u)

Answer would be 7.36691253546

What is here incorrect?

Upvotes: 1

Views: 872

Answers (2)

Martijn Pieters
Martijn Pieters

Reputation: 1121216

Integer division. 4/3 evaluates to 1 as it is rounded down.

Use 4.0 instead to force floating point arithmetic:

>>> 4.0/3 * 6.67*1e-11*3.14*6378000*5515
9.822550047279998

or use Python 3, where floating point division is the default, or use from __future__ import division to achieve the same in Python 2:

Python 2.7.5 (default, May 22 2013, 12:00:45) 
[GCC 4.2.1 (Based on Apple Inc. build 5658) (LLVM build 2336.11.00)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> from __future__ import division
>>> 4/3 * 6.67*1e-11*3.14*6378000*5515
9.822550047279998

This behaviour is documented under the Binary arithmetic operators section:

The / (division) and // (floor division) operators yield the quotient of their arguments. The numeric arguments are first converted to a common type. Plain or long integer division yields an integer of the same type; the result is that of mathematical division with the ‘floor’ function applied to the result. Division by zero raises the ZeroDivisionError exception.

See PEP 238 about why this behaviour was changed in Python 3, and for a reference to the from __future__ import division statement.

Upvotes: 6

jamylak
jamylak

Reputation: 133504

The problem is the integer division 4/3 in Python 2.7

>>> print (4.0/3) * 6.67*1e-11*3.14*6378000*5515
9.82255004728

In Python 3 (where / is float division and // is integer division) This would have worked without changing it to 4.0/3, alternatively you may use 

from __future__ import division

Upvotes: 3

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