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+--- Thread: New guy looking at lists (/thread-15621.html)
New guy looking at lists - sumncguy - Jan-24-2019
Hello folks :
Version/system stats:
Quote:@localhost ~]$ python -V
Python 2.6.6
@localhost ~]$ cat /etc/*release | tail -1
CentOS release 6.10 (Final)
My cheap g code ...
#!/usr/bin/python list = ['abcd', 786, 2.23, 'john', 70.2] print(list) print list list.append(7) print(list)The output :
Quote:@localhost ~]$ ./pyte
['abcd', 786, 2.23, 'john', 70.200000000000003]
['abcd', 786, 2.23, 'john', 70.200000000000003]
['abcd', 786, 2.23, 'john', 70.200000000000003, 7]
The ask:
Why do I see "70.200000000000003" ??
Thanks in adv folks.
RE: New guy looking at lists - ichabod801 - Jan-24-2019
Floating point arithmetic. Decimals are stored as a fraction that approximates the correct value. It is sometimes off by a teeny little bit, as you see in your example.
RE: New guy looking at lists - sumncguy - Jan-24-2019
Ummm .. so how do I work around this in actual execution.
This is just something I found in a tutor.
But if it were real world .. how would I realize that what, lets say, I'm using for a conditional branch isn't quite what I think it is ... Know ?
70.2 isnt the same as 70.20000003. I guess printing out every single list that may contain this issue then script around it. Yeah ?
RE: New guy looking at lists - stullis - Jan-24-2019
You could use round() or import the decimal module to remove the extra digits.
RE: New guy looking at lists - snippsat - Jan-24-2019
It's the way floating point arithmetic work,Basic Answers | Python doc.
Quote:But if it were real world .. how would I realize that what, lets say, I'm using for a conditional branch isn't quite what I think it is ... Know ?For finical calculation or for it to work as you learn at school,use Decimal module.
>>> 0.4 - 0.1
0.30000000000000004
>>> from decimal import Decimal
>>> Decimal('0.4') - Decimal('0.1')
Decimal('0.3')
RE: New guy looking at lists - ichabod801 - Jan-24-2019
Other possible techniques include testing the difference against a really small value or using integers (as in, calculate in pennies instead of fractions of a dollar).
epsilon = 1 / 10 ** 15
...
if abs(x - y) < epsilon:
# basically equal