why do I get this behavior from a while loop?

[S. Chris Colbert]

This seems strange to me, but perhaps I am just missing something:

In [12]: t = 0.
In [13]: time = 10.

In [14]: while t < time:
....: print t
....: t += 1.
....:
....:
0.0
1.0
2.0
3.0
4.0
5.0
6.0
7.0
8.0
9.0

In [15]: t = 0.

In [16]: time = 10.

In [17]: while t < time:
....: print t
....: t += 0.1
....:
....:
0.0
0.1
0.2
0.3
<--snip-->
9.4
9.5
9.6
9.7
9.8
9.9
10.0


I would think that second loop should terminate at 9.9, no?

I am missing something fundamental?

Cheers!

Chris

 

[Diez B. Roggisch]

This seems strange to me, but perhaps I am just missing something:

In [12]: t = 0.
In [13]: time = 10.

In [14]: while t < time:
....: print t
....: t += 1.
....:
....:
0.0
1.0
2.0
3.0
4.0
5.0
6.0
7.0
8.0
9.0

In [15]: t = 0.

In [16]: time = 10.

In [17]: while t < time:
....: print t
....: t += 0.1
....:
....:
0.0
0.1
0.2
0.3
<--snip-->
9.4
9.5
9.6
9.7
9.8
9.9
10.0


I would think that second loop should terminate at 9.9, no?

I am missing something fundamental?

Yes. The lack of precision that floating points suffer from (nothing
python-specific), and the rounding-behavior of printing them.
.... print repr(t)
.... t += .1
....
0.0
0.10000000000000001
<snip/>
9.8999999999999808
9.9999999999999805

Diez

 

[Paul Rudin]

....: print t

Try replacing with: print "%0.20f" % t


The thing you're missing is that floating point arithmetic isn't (in
general) exact - but when it's printed it's rounded.

 

[Steven D'Aprano]

I would think that second loop should terminate at 9.9, no?

I am missing something fundamental?

"What Every Computer Scientist Should Know About Floating Point"
http://docs.sun.com/source/806-3568/ncg_goldberg.html

 

[S. Chris Colbert]

What a newbie mistake for me to make.

I appreciate the replies everyone!

Cheers,

Chris