Force Python calculation in double

Question

I want to print a double in Python. But it doesn't work, and I don't know why.

My code:

test = 3000 / (1500 * 2000)
print str(test)

I always get a 0 and not a 0.001 even if I do the following:

test = 3000 / (1500 * 2000)
print '%.10f' % test

I get a 0.000000000 and not 0.001.

How do I tell Python that this should be a double?

If you didn't use and old version of Python, you wouldn't have this problem. — Oleh Prypin, May 04 '13 at 11:02
Duplicate: http://stackoverflow.com/questions/1787249/why-doesnt-this-division-work-in-python, http://stackoverflow.com/questions/2958684/python-division, etc. — Warren Weckesser, May 04 '13 at 11:15

score 5 · Accepted Answer · edited Feb 22 '17 at 02:53

5

In Python 2.x you need to convert at least one of the operands to float, as integer division results in truncated output in Python 2.x:

>>> 3000 / (1500 * 2000)
0
>>> 3000.0 / (1500 * 2000) # Or use float(3000)
0.001
>>> float(3000) / (1500 * 2000)
0.001

Or you can import division of Python 3.x in Python 2.x, where integer division results in true division:

>>> from __future__ import division
>>> 3000.0 / (1500 * 2000)
0.001

Note that this will affect the division in the whole module.

edited Feb 22 '17 at 02:53

Peter Mortensen

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answered May 04 '13 at 10:57

Ashwini Chaudhary

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and how do I convert a len(MyList) to a float? – gurehbgui May 04 '13 at 10:59
1

@gurehbgui use `float()`: `float( len(MyList) )` – Ashwini Chaudhary May 04 '13 at 11:00
Optionally `from future import true_division` if it won't break something else in the module – Jon Clements May 04 '13 at 11:00
2

And another sort of work around is `from operator import truediv; x * truediv (y, z)` – Jon Clements May 04 '13 at 11:02
@JonClements Didn't knew about `operator.truediv`. – Ashwini Chaudhary May 04 '13 at 11:27
Using `Decimal` will also work. +1 for `truediv` that was a new one for me. – Burhan Khalid May 04 '13 at 11:40

score 4 · Answer 2 · answered May 04 '13 at 10:59

4

That's because you only used integer values at the math operation. Thus the result will be a (truncated) integer. If you want a float result, involve at least one float operand.

answered May 04 '13 at 10:59

pram

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score 0 · Answer 3 · edited Feb 22 '17 at 02:54

0

This behavior is fixed in Python 3.x. As pointed out in the other answers, the fix is to cast the divisible number to float.

If you want to make the / work as in Python 3.x, then you could do: from __future__ import division, and then you'll need to use the // operator to achieve the old behavior. You can check the Python documentation for information about operators.

edited Feb 22 '17 at 02:54

Peter Mortensen

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answered May 04 '13 at 11:51

Jordan Jambazov

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Force Python calculation in double

3 Answers3