CODEWITHSUNDEEP
python
Hick
Hick

Reputation: 36414

Error while I use math.atan in Python!

When I do 1/2 in Python why does it give me zero? Even if I coerce it with float(1/2) still I get zero. Why? And how can I get around it?

When I give arctan(1/2) I get 0 as answer, but when I give arctan(.5) I get the correct answer!

Upvotes: 0

Views: 2689

Answers (9)

Autoplectic
Autoplectic

Reputation: 7676

Because Python 2.x uses integer division for integers, so:

1/2 == 0

evaluates to True.

You want to do:

1.0/2

or do a

from __future__ import division

Upvotes: 7

Nosredna
Nosredna

Reputation: 86336

Your coercing doesn't stand a chance because the answer is already zero before you hand it to float.

Try 1./2

Upvotes: 1

S.Lott
S.Lott

Reputation: 392050

First, 1/2 is integer division. Until Python 3.0.

>>> 1/2
0
>>> 1.0/2.0
0.5
>>>

Second, use math.atan2 for this kind of thing.

>>> math.atan2(1,2)
0.46364760900080609
>>> math.atan(.5)
0.46364760900080609

Upvotes: 6

Laurence Gonsalves
Laurence Gonsalves

Reputation: 143334

If 1/2 == 0 then float(1/2) will be 0.0. If you coerce it to float after it's been truncated it'll still be truncated.

There are a few options:

  • Add the following import: from __future__ import division. This will make the / operator divide "correctly" in that module. You can use // if you need truncating division.
  • Coerce either of the operands to a float. eg: float(1)/2
  • If you're actually using constants then just add a decimal point instead of using float(), eg: 1.0/2 or 1/2.0 or 1.0/2.0

Upvotes: 1

moshez
moshez

Reputation: 37605

In Python, dividing integers yields an integer -- 0 in this case.

There are two possible solutions. One is to force them into floats: 1/2. (note the trailing dot) or float(1)/2.

Another is to use "from future import division" at the top of your code, and use the behavior you need.

python -c 'from future import division;import math;print math.atan(1/2)' yields the correct 0.463647609001

Upvotes: 1

Jacob
Jacob

Reputation: 78920

As these answers are implying, 1/2 doesn't return what you are expecting. It returns zero, because 1 and 2 are integers (integer division causes numbers to round down). Python 3 changes this behavior, by the way.

Upvotes: 1

S.P
S.P

Reputation: 8766

From the standard:

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.

Upvotes: 2

Matthew Flaschen
Matthew Flaschen

Reputation: 285057

atan(float(1)/2)

If you do: 

atan(float(1/2))

in Python 2.x, but without:

from __future__ import division

the 1/2 is evaluated first as 0, then 0 is converted to a float, then atan(0.0) is called. This changes in Python 3, which uses float division by default even for integers. The short portable solution is what I first gave.

Upvotes: 3

PaV
PaV

Reputation: 753

float(1)/float(2)

If you divide int / int you get an int, so float(0) still gives you 0.0

Upvotes: 2

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