Python: using the int function I get -0.91 * 100 = -90 instead of -91. Why?

Asked Jun 09 '16 at 23:02

Active Jun 09 '16 at 23:04

Viewed 38 times

The following program goes from -1 to 0, in steps of 0.01 (-1,-.99,-.98,...,-.02,-.01) , then multiplies each one of these terms by 100, and prints the result.

a=-1
c=100
while a<0:
    print "a*c=",a*c,"int(a*c)=",int(a*c)
    a=a+.01

Now, I should be getting the same output for "a * c" and "int(a * c)", i.e. -100,-99,-98, etc but no, at a=-0.91 , I'm getting -0.91*100 = -90 instead of -91. All the rest of the multiplications are fine. Why ??

By the way I'm using the int function because I need to use the variables a and c in a function that only takes integers.

edited Jun 09 '16 at 23:04

asked Jun 09 '16 at 23:02

kazordoon

2

What language is this? – No Name Jun 09 '16 at 23:04
2

I presume this is python and for `int(-0.91 * 100)` I get `-91`, I would also be wary of `a = a + .01` – Padraic Cunningham Jun 09 '16 at 23:05
sorry, i didn't mention that it is Python – kazordoon Jun 09 '16 at 23:05
Inexact types don't produce exact results. There have been a surprising number of questions on this point lately. – Jun 09 '16 at 23:08
`print(repr(a))` and you may be surprised, I bet you see something like `-0.909999999999` inplace of `-91` – Padraic Cunningham Jun 09 '16 at 23:08
@PadraicCunningham Then try `int(-1+0.01+0.01+0.01+0.01+0.01+0.01+0.01+0.01+0.01)`. It's a floating point problem. You do indeed get `a = -0.9099999999999999 a*c= -90.99999999999999 int(a*c)= -90` – JohannesB Jun 09 '16 at 23:11

Python: using the int function I get -0.91 * 100 = -90 instead of -91. Why?

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