Issue 22917: Calculating problem - Python tracker (original) (raw)

Hi guys

I am very new to this, (just started my first lines today) so I am using a book to learn Python. BUT there is something wrong:

This is my program (from the book):

This is not quite true outside of USA

and is based on my dim memories of my younger years

print("Firstish Grade") print("1 + 1 =", 1 + 1) print("2 + 4 =", 2 + 4) print("5 - 2 =", 5 - 2) print() print("Thirdish Grade") print("243 - 23 =", 243 - 23) print("12 * 4 =", 12 * 4) print("12 / 3 =", 12 / 3) print("13 / 3 =", 13 // 3, "R", 13 % 3) print() print("Junior High") print("123.56 - 62.12 =", 123.56 - 62.12) print("(4 + 3) * 2 =", (4 + 3) * 2) print("4 + 3 * 2 =", 4 + 3 * 2) print("3 ** 2 =", 3 ** 2)

and this is the programs answer:

Python 3.4.2 (v3.4.2:ab2c023a9432, Oct 6 2014, 22:15:05) [MSC v.1600 32 bit (Intel)] on win32 Type "copyright", "credits" or "license()" for more information.

================================ RESTART ================================

Firstish Grade 1 + 1 = 2 2 + 4 = 6 5 - 2 = 3

Thirdish Grade 243 - 23 = 220 12 * 4 = 48 12 / 3 = 4.0 13 / 3 = 4 R 1

Junior High 123.56 - 62.12 = 61.440000000000005 (4 + 3) * 2 = 14 4 + 3 * 2 = 10 3 ** 2 = 9

As you can see,

print("123.56 - 62.12 =", 123.56 - 62.12) is not = 61.440000000000005

so my guess is that the interpreter has a malfunction.

My pc is a compaq mini 110 running (walking;-)) windows xp srv pack 3

in a std. config.

brg

Henning Mentz - Denmark

Hi Henning,

this is not a bug. This is to do with how floating point numbers represented in computers. I'd suggest https://en.wikipedia.org/wiki/IEEE_floating_point as a starting point. Briefly, due to binary base that the computers operate on, not every number can be stored/represented precisely. In any case there are plenty of other information online that you can find useful, the easiest solution is to output correctly formatted strings corresponding to your answers.

Good luck.

Hi SilentGhost

Thanx for a quick response.

Sorry to hear that it is not a bug, my old GW basic interpretor from 1988 has no problem with this simple calculation (123.56 - 62.12 = 61.44) but my new Python interpreter cannot give me a correct answer.

Yes, I know about precision, I have been using C for several years, but as I am interested in the Raspberry Pi I would like to use Python with it.

And as I would use it as a data collector (from instruments), I need good precision and reliability.

Is there anything I can do? (exept changing to another language)

brg

Henning

SilentGhost skrev den 22/11/2014 21:33:

SilentGhost added the comment:

Hi Henning,

this is not a bug. This is to do with how floating point numbers represented in computers. I'd suggest https://en.wikipedia.org/wiki/IEEE_floating_point [1] as a starting point. Briefly, due to binary base that the computers operate on, not every number can be stored/represented precisely. In any case there are plenty of other information online that you can find useful, the easiest solution is to output correctly formatted strings corresponding to your answers.

Good luck.

nosy: +SilentGhost resolution: -> not a bug status: open -> closed

Python tracker <report@bugs.python.org> <http://bugs.python.org/issue22917 [2]>

[1] https://en.wikipedia.org/wiki/IEEE_floating_point [2] http://bugs.python.org/issue22917