Solving an equation, Python

Question

So I am trying to solve this EQUATION in Python 2.7.3:

import math

y = 68
x = -5/4*(-463 + math.sqrt(1216881-16000 * y))
print x

x should print 130, but instead is printing 208.

I cannot see where I have gone wrong.

What do you expect `-5/4` to be? Try it. You may be surprised. — recursive, Dec 17 '14 at 16:23
@JonathanDavies, I recommend you use `from __future__ import division`, though (simpler transition to python 3). — miku, Dec 17 '14 at 16:25

score 3 · Answer 1 · answered Dec 17 '14 at 16:23

Python 2.7.6 (default, Nov 10 2013, 19:24:18) [MSC v.1500 32 bit (Intel)] on win32
Type "help", "copyright", "credits" or "license" for more information.
>>> -5/4
-2
>>> from __future__ import division
>>> -5/4
-1.25

See PEP 238 -- Changing the Division Operator. Python 2 returns the floor on integer division, Python 3 returns a float where needed.

score 3 · Accepted Answer · answered Dec 17 '14 at 16:23

3

the problem is that the result of a division of an integer by another integer returns a integer, change one of your constants to float and you'll get the correct result

try this:

import math

y = 68.0
x = -5/4.0*(-463 + math.sqrt(1216881-16000 * y))
print x

for more information please read:

https://www.python.org/dev/peps/pep-0238/

answered Dec 17 '14 at 16:23

Victor Ribeiro da Silva Eloy

4,051
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25
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Thanks, this works fine, any idea why? It also works with 5.0/4 also. @Victor – Jonathan Davies Dec 17 '14 at 16:25
1

@JonathanDavies, please read my edit – Victor Ribeiro da Silva Eloy Dec 17 '14 at 16:27

Solving an equation, Python

2 Answers2