A Weird Computation Result from Python

Question

I wrote a crule1.py as follows.

 def exp_rule1 (mu1, h, alpha): return h**2/(4*mu1**2)

Then I run it in the Interpreter. I got

Python 2.7.6 |Anaconda 1.9.1 (64-bit)| (default, Nov 11 2013, 10:49:15) [MSC v.1500 64 bit (AMD64)] on win32 Type "help", "copyright", "credits" or "license" for more information. Imported NumPy 1.8.0, SciPy 0.13.3, Matplotlib 1.3.1 Type "scientific" for more details. >>> import crul1 as c1 >>> c1.exp_rule1(1, 1, 0) 0

Then I copy the code into interpreter. The result is

>>> def exp_rule1 (mu1, h, alpha): ... return h**2/(4*mu1**2) ... >>> exp_rule1(1, 1, 0) 0.25

It makes me very confused and I cannot fix it. You are very appreciated to point out the problem in this code.

Define "interpreter". Are you running the second in the same shell as the import cru1 as c1, c1.exp_rule1(1, 1, 0) lines? — user395760
– user395760, Commented Mar 4, 2014 at 19:03

chepner · Accepted Answer · 2014-03-04 19:01:54Z

In Python 2.x, / returns an integer value if both its operands are integers. As a special case, n/d is 0 if n < d and both are positive.

For

def exp_rule1 (mu1, h, alpha): return h**2/(4*mu1**2)

you want to ensure that either the numerator or denominator is a floating point value. A simple way to do that is to make the 4 a float instead of an integer.

def exp_rule1 (mu1, h, alpha): return h**2/(4.0*mu1**2)

Another solution is to import the new 3.x behavior for /, which is to always return a float regardless of the operands. If you do this, replace any divisions where you rely on integer division with // instead.

# This allows 1/2 to return 0.5 instead of 0 from __future__ import division

jeffknupp · Accepted Answer · 2014-03-04 18:59:59Z

0

The problem is you're mixing integers and floats. Try making the call exp_rule1(1.0, 1.0, 0.0) and you'll get the right result. Just make sure all the arguments to the function are floats.

answered Mar 4, 2014 at 18:59

jeffknupp

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2 Comments

user395760 Over a year ago

But both cases appear to mix integers and floats in the same way, don't they?

Hooked Over a year ago

Specifically, you are doing integer division instead of float division. 1/2 is zero for ints, .5 for floats.

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