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Byte 04-08-2006 03:21 PM

Working with decimal points
 
How come:

sum = 1/4
print sum

returns 0? 1/4=0.25, not 0. How do I fix this?

-- /usr/bin/byte


mtallen@gmail.com 04-08-2006 03:30 PM

Re: Working with decimal points
 
Byte wrote:
> How come:
>
> sum = 1/4
> print sum
>
> returns 0? 1/4=0.25, not 0. How do I fix this?


Make sure there is at least one float in your equation. In your example
Python is doing interger math for you and returing the floor. You need
to give it a hint that you would like to do floating point math.

>>> sum = 1.0/4
>>> print sum

0.25
>>> sum = 1/4.0
>>> print sum

0.25
>>>



Byte 04-08-2006 03:34 PM

Re: Working with decimal points
 
That dosnt work either:

sum = 0.1+1/4
print sum

Just returns 0.1


Fredrik Lundh 04-08-2006 03:35 PM

Re: Working with decimal points
 
"Byte" wrote:

> How come:
>
> sum = 1/4
> print sum
>
> returns 0?


because 1 and 4 are integer objects, so 1/4 is an integer division, which
rounds down to the nearest integer.

> 1/4=0.25, not 0. How do I fix this?


use floating point numbers:

1.0/4.0 = 0.25

or convert one of the numbers to a float:

float(1)/4 = 0.25

</F>




Byte 04-08-2006 03:45 PM

Re: Working with decimal points
 
Fredrik Lundh's way works: thank a million!


Peter Hansen 04-08-2006 03:47 PM

Re: Working with decimal points
 
Byte wrote:
> That dosnt work either:
>
> sum = 0.1+1/4
> print sum
>
> Just returns 0.1


That's because the 1/4 is executed first, and the problem mentioned
still applies (i.e. you get a 0, then add it to 0.1).

The best fix for you might be simply to place this line at the start
(before all other code) of your module:

from __future__ import division

That will change the way simple division works to give you the results
you expected. See the online docs for more background on this.

-Peter


Steven D'Aprano 04-08-2006 04:05 PM

Re: Working with decimal points
 
On Sat, 08 Apr 2006 08:21:06 -0700, Byte wrote:

> How come:
>
> sum = 1/4
> print sum
>
> returns 0? 1/4=0.25, not 0. How do I fix this?


By default, / does integer division, not floating point. In integer
division, 1/4 is 0, exactly as calculated.

(How many fours go into one? Zero fours go into one, with one remainder.)

There are two ways to change this:

(1) Convert at least one of the numbers to a float first:

>>> 1.0/4

0.25
>>> 1/float(4)

0.25

but be careful, this one doesn't do what you want:

>>> float(1/4)

0.0


(2) Run the following command at the start of your program or in your
interactive session:

>>> from __future__ import division


Now division will behave as you expect:

>>> 1/4

0.25

and you can use // for integer division.

Now that you can do floating point division, I peer into my crystal
ball and predict your next question: why is Python so inaccurate?

>>> 1/10

0.10000000000000001

Answer: it isn't. You've discovered a mathematical limitation that 1/10
cannot be written exactly in any fixed number of binary places, just like
1/3 cannot be written exactly in any fixed number of decimal places.

See also:

http://www.python.org/doc/faq/genera...-so-inaccurate

http://docs.python.org/tut/node16.html


Hope this helps.


--
Steven.


mtallen@gmail.com 04-08-2006 04:55 PM

Re: Working with decimal points
 

Byte wrote:
> That dosnt work either:
>
> sum = 0.1+1/4
> print sum
>
> Just returns 0.1


You get precedence right? Your equation does not evaluate from left to
right. 1/4 happens first, and since there are no floats you get 0.

in that equation you basically are doing this:

sum = 1/4
print sum
0

sum = 0.1 + sum
print sum
0.1


Fredrik Lundh 04-08-2006 05:16 PM

Re: Working with decimal points
 
"Byte" wrote:

> That dosnt work either:
>
> sum = 0.1+1/4
> print sum
>
> Just returns 0.1


division has higher precedence than addition, so 1/4 is calculated first,
and the result is then added to 0.1.

and as I've already explained, 1/4 is an integer division, so the result
is rounded down to the the nearest integer (0). in other words, your
expression boils down to

0.1 + 0

which is 0.1 [1].

the precedence order is explained here:

http://docs.python.org/ref/summary.html

</F>

1) or at least a close approximation of it; see
http://docs.python.org/tut/node16.html





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