New submission from Carlos Neves :
Hi,
I am observing unexpected behavior with round built-in function about (1)
significant figures in analytical methods and (2) number of odd and even
numbers obtained by this function.
https://docs.python.org/3/library/functions.html#round
https://en.wikipedia.org/wiki/Significant_figures
1. Significant Figures
==
For example, when I say 1.20 in analytical methods, I am confident about the
last digit, the zero. It has a meaning. But, when I use Python,
>>> round (1.203, 2)
1.2
>>>
the zero not appears. It is not occur when the second digit is not zero.
>>> round (1.213, 2)
1.21
>>>
The zero should be printed like the other numbers to be consistent with the
significant figures. Maybe other functions could present the same behavior.
2. Rounding procedure
=
I wrote the following code to test the number of odd and even numbers during a
round procedure. I should get half-and-a-half of odd and even numbers. But the
result using the round function is different. We observed 5 even more and 5 odd
less. This behavior causes a systematic error.
https://en.wikipedia.org/wiki/Rounding
I hope to be contributing to the improvement of the code.
Thank advanced.
##
# This code count the number of odd and even number with different procedures:
truncate, round simple and round function
# Test condition: Rounding with one digit after the decimal point.
import numpy as np
even_0 = 0
odd_0 = 0
even_1 = 0
odd_1 = 0
even_2 = 0
odd_2 = 0
even_3 = 0
odd_3 = 0
# generate 1000 numbers from 0.000 up to 1 with step of 0.001
x = np.arange(0,1,0.001)
# printing
for i in range(len(x)):
x_truncated = int((x[i]*10)+0.0)/10 # no rounding
x_rounded_simple = int((x[i]*10)+0.5)/10 # rounding up at 5
x_rounded_function = round(x[i],1) # rounding by function with one
digit after the decimal point
# counting odd and even numbers
if int(x[i]*1000) % 2 == 0:
even_0 += 1
else:
odd_0 += 1
if int(x_truncated*10) % 2 == 0:
even_1 += 1
else:
odd_1 += 1
if int(x_rounded_simple*10) % 2 == 0:
even_2 += 1
else:
odd_2 += 1
if int(x_rounded_function*10) % 2 == 0:
even_3 += 1
else:
odd_3 += 1
print ("{0:.3f} {1:.1f} {2:.1f} {3:.1f}".format((x[i]), x_truncated,
x_rounded_simple, x_rounded_function))
print ("Result:")
print ("Raw: Even={0}, Odd={1}".format(even_0,odd_0))
print ("Truncated: Even={0}, Odd={1}".format(even_1,odd_1))
print ("Rounded simple: Even={0}, Odd={1}".format(even_2,odd_2))
print ("Rounded Function: Even={0}, Odd={1}".format(even_3,odd_3))
##
Output
...
0.995 0.9 1.0 1.0
0.996 0.9 1.0 1.0
0.997 0.9 1.0 1.0
0.998 0.9 1.0 1.0
0.999 0.9 1.0 1.0
Result:
Raw: Even=500, Odd=500
Truncated: Even=500, Odd=500
Rounded simple: Even=500, Odd=500
Rounded Function: Even=505, Odd=495
--
components: Library (Lib)
messages: 372878
nosy: Carlos Neves, lemburg, mark.dickinson, rhettinger, stutzbach
priority: normal
severity: normal
status: open
title: Round built-in function not shows zeros acording significant figures and
calculates different numbers of odd and even
type: behavior
versions: Python 3.8
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<https://bugs.python.org/issue41198>
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