Issue 33372: Wrong calculation - Python tracker (original) (raw)
Created on 2018-04-27 05:33 by an0n.r00t32, last changed 2022-04-11 14:58 by admin. This issue is now closed.
Messages (5)
Author: (an0n.r00t32)
Date: 2018-04-27 05:33
I found that python calculate this wrong. Other calculators gave me 9997809307 not 9997809507 (python), even python3 calculated it correctly. Here is the computation expression which Python 2.7 calculated wrong.
Author: Tim Peters (tim.peters) * 
Date: 2018-04-27 05:44
Please find a minimal example that illustrates the problem you think you've found, and paste the plain text into the bug report.
In the meantime, I'm closing this as "not a bug". The division operator applied to integers in Python 2 defaults doing truncating integer division, and in Python 3 defaults to doing floating point division instead. So this example all by itself is enough to show a difference:
Under Python 2.7.11:
1/8 0
Under Python 3.6.5:
1/8 0.125
Both are expected.
In exactly the same way, the subexpression "2110/1001010+100/10" in the code you pasted returns the integer 10 under Python 2 but the floating point value 30.0 under Python 3.
Author: Steven D'Aprano (steven.daprano) *
Date: 2018-04-27 06:40
I agree with Tim that this is likely to be the difference between Python 2 truncating division and Python 3 division.
For the record, I get the following results:
9997809507L Python 2.7 9997809307.0 Python 3.5 9997809307 R 9997809307 Javascript (Rhino) 9997809307 OpenXion
I tried it in Ruby 1.8 as well, and sometimes got a syntax error and sometimes 9999599931. I have no idea why the odd results.
I'm not mad enough to type the whole thing into my calculator, but I am mad enough to simplify it by hand, and I get:
-590072-2/10100-200112-2/10100-18-2/10100-18-2/10100-18-2/10100-18-2/10100-18-2/10100-18-2/10100-18-2/10100-18-2/10100+9998599835
Even Ruby gets the right answer (9997809507) for that :-)
In Python 2, 2/10 returns 0 unless you have run from __future__ import division, so the -2/10*100 terms all go to zero, and we get:
-590072-2/10100-200112-2/10100-18-2/10100-18-2/10100-18-2/10100-18-2/10100-18-2/10100-18-2/10100-18-2/10100-18-2/10100+9998599835
which evaluates to 9997809307.0 just as Python 2.7 says.
So Tim was right, this is entirely due to the difference between / as truncating division in Python 2 and true division in Python 3.
Oh, and one last thing... in Python 2, you can get the expected result by either using the future import above, or by adding a decimal point to the numbers being divided, turning them into floats and forcing Python to use floating point true division.
Author: Steven D'Aprano (steven.daprano) *
Date: 2018-04-27 06:42
Even Ruby gets the right answer (9997809507) for that :-)
Oops, I forgot to say... Ruby 1.8 also does truncating division like Python 2.
Author: Steven D'Aprano (steven.daprano) *
Date: 2018-04-27 06:45
Ah sheesh I copied and pasted the wrong bits. After the division terms go to zero, you get
-590072-200112-18-18-18-18-18-18-18-18+9998599835
which goes to 9997809507 as calculated by Ruby and Python 2, and I promise that's the end of me replying to myself :-)
History
Date
User
Action
Args
2022-04-11 14:58:59
admin
set
github: 77553
2018-04-27 06:45:28
steven.daprano
set
messages: +
2018-04-27 06:42:37
steven.daprano
set
messages: +
2018-04-27 06:40:31
steven.daprano
set
nosy: + steven.daprano
messages: +
2018-04-27 05:44:29
tim.peters
set
status: open -> closed
nosy: + tim.peters
messages: +
resolution: not a bug
stage: resolved
2018-04-27 05:33:07
an0n.r00t32
create