Issue33372
Created on 2018-04-27 05:33 by an0n.r00t32, last changed 2018-04-27 06:45 by steven.daprano. This issue is now closed.
Messages (5) | |||
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msg315826 - (view) | 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. https://pastebin.com/uKx1FYx3 |
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msg315827 - (view) | 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 "2*1*10/100*10*10+100/10" in the code you pasted returns the integer 10 under Python 2 but the floating point value 30.0 under Python 3. |
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msg315828 - (view) | 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: -590072-2/10*100-200112-2/10*100-18-2/10*100-18-2/10*100-18-2/10*100-18-2/10*100-18-2/10*100-18-2/10*100-18-2/10*100-18-2/10*100+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: -590072-2/10*100-200112-2/10*100-18-2/10*100-18-2/10*100-18-2/10*100-18-2/10*100-18-2/10*100-18-2/10*100-18-2/10*100-18-2/10*100+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. |
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msg315829 - (view) | 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. |
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msg315830 - (view) | 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 | |||
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Date | User | Action | Args |
2018-04-27 06:45:28 | steven.daprano | set | messages: + msg315830 |
2018-04-27 06:42:37 | steven.daprano | set | messages: + msg315829 |
2018-04-27 06:40:31 | steven.daprano | set | nosy:
+ steven.daprano messages: + msg315828 |
2018-04-27 05:44:29 | tim.peters | set | status: open -> closed nosy: + tim.peters messages: + msg315827 resolution: not a bug stage: resolved |
2018-04-27 05:33:07 | an0n.r00t32 | create |