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mvp1/si_prefix.py

249 lines
6.8 KiB

# coding: utf-8
from __future__ import division
import math
import re
# Print a floating-point number in engineering notation.
# Ported from [C version][1] written by
# Jukka “Yucca” Korpela <jkorpela@cs.tut.fi>.
#
# [1]: http://www.cs.tut.fi/~jkorpela/c/eng.html
SI_PREFIX_UNITS = "yzafpnum kMGTPEZY"
CRE_SI_NUMBER = re.compile(r'\s*(?P<sign>[\+\-])?'
r'(?P<integer>\d+)'
r'(?P<fraction>.\d+)?\s*'
r'(?P<si_unit>[%s])?\s*' % SI_PREFIX_UNITS)
def split(value, precision=1):
'''
Split `value` into value and "exponent-of-10", where "exponent-of-10" is a
multiple of 3. This corresponds to SI prefixes.
Returns tuple, where the second value is the "exponent-of-10" and the first
value is `value` divided by the "exponent-of-10".
Args
----
value : int, float
Input value.
precision : int
Number of digits after decimal place to include.
Returns
-------
tuple
The second value is the "exponent-of-10" and the first value is `value`
divided by the "exponent-of-10".
Examples
--------
.. code-block:: python
si_prefix.split(0.04781) -> (47.8, -3)
si_prefix.split(4781.123) -> (4.8, 3)
See `si_prefix.format` for more examples.
'''
negative = False
digits = precision + 1
if value < 0.:
value = -value
negative = True
elif value == 0.:
return 0., 0
expof10 = int(math.log10(value))
if expof10 > 0:
expof10 = (expof10 // 3) * 3
else:
expof10 = (-expof10 + 3) // 3 * (-3)
value *= 10 ** (-expof10)
if value >= 1000.:
value /= 1000.0
expof10 += 3
elif value >= 100.0:
digits -= 2
elif value >= 10.0:
digits -= 1
if negative:
value *= -1
return value, int(expof10)
def prefix(expof10):
'''
Args:
expof10 : Exponent of a power of 10 associated with a SI unit
character.
Returns:
str : One of the characters in "yzafpnum kMGTPEZY".
'''
prefix_levels = (len(SI_PREFIX_UNITS) - 1) // 2
si_level = expof10 // 3
if abs(si_level) > prefix_levels:
raise ValueError("Exponent out range of available prefixes.")
return SI_PREFIX_UNITS[si_level + prefix_levels]
def si_format(value, precision=1, format_str='{value} {prefix}',
exp_format_str='{value}e{expof10}'):
'''
Format value to string with SI prefix, using the specified precision.
Args
----
value : int, float
Input value.
precision : int
Number of digits after decimal place to include.
format_str : str
Format string where `{prefix}` and `{value}` represent the SI prefix
and the value (scaled according to the prefix), respectively. The
default format matches the `SI prefix style`_ format.
exp_str : str
Format string where `{expof10}` and `{value}` represent the exponent of
10 and the value (scaled according to the exponent of 10),
respectively. This format is used if the absolute exponent of 10 value
is greater than 24.
Returns
-------
str
`value` formatted according to the `SI prefix style`_.
Examples
--------
For example, with `precision=2`:
.. code-block:: python
1e-27 --> 1.00e-27
1.764e-24 --> 1.76 y
7.4088e-23 --> 74.09 y
3.1117e-21 --> 3.11 z
1.30691e-19 --> 130.69 z
5.48903e-18 --> 5.49 a
2.30539e-16 --> 230.54 a
9.68265e-15 --> 9.68 f
4.06671e-13 --> 406.67 f
1.70802e-11 --> 17.08 p
7.17368e-10 --> 717.37 p
3.01295e-08 --> 30.13 n
1.26544e-06 --> 1.27 u
5.31484e-05 --> 53.15 u
0.00223223 --> 2.23 m
0.0937537 --> 93.75 m
3.93766 --> 3.94
165.382 --> 165.38
6946.03 --> 6.95 k
291733 --> 291.73 k
1.22528e+07 --> 12.25 M
5.14617e+08 --> 514.62 M
2.16139e+10 --> 21.61 G
9.07785e+11 --> 907.78 G
3.8127e+13 --> 38.13 T
1.60133e+15 --> 1.60 P
6.7256e+16 --> 67.26 P
2.82475e+18 --> 2.82 E
1.1864e+20 --> 118.64 E
4.98286e+21 --> 4.98 Z
2.0928e+23 --> 209.28 Z
8.78977e+24 --> 8.79 Y
3.6917e+26 --> 369.17 Y
1.55051e+28 --> 15.51e+27
6.51216e+29 --> 651.22e+27
.. _SI prefix style:
http://physics.nist.gov/cuu/Units/checklist.html
'''
svalue, expof10 = split(value, precision)
value_format = '%%.%df' % precision
value_str = value_format % svalue
try:
return format_str.format(value=value_str,
prefix=prefix(expof10).strip())
except ValueError:
sign = ''
if expof10 > 0:
sign = "+"
return exp_format_str.format(value=value_str,
expof10=''.join([sign, str(expof10)]))
def si_parse(value):
'''
Parse a value expressed using SI prefix units to a floating point number.
Args:
value (str) : Value expressed using SI prefix units (as returned by
`si_format` function).
'''
CRE_10E_NUMBER = re.compile(r'^\s*(?P<integer>[\+\-]?\d+)?'
r'(?P<fraction>.\d+)?\s*([eE]\s*'
r'(?P<expof10>[\+\-]?\d+))?$')
CRE_SI_NUMBER = re.compile(r'^\s*(?P<number>(?P<integer>[\+\-]?\d+)?'
r'(?P<fraction>.\d+)?)\s*'
r'(?P<si_unit>[%s])?\s*$' % SI_PREFIX_UNITS)
match = CRE_10E_NUMBER.match(value)
if match:
# Can be parse using `float`.
assert(match.group('integer') is not None or
match.group('fraction') is not None)
return float(value)
match = CRE_SI_NUMBER.match(value)
assert(match.group('integer') is not None or
match.group('fraction') is not None)
d = match.groupdict()
si_unit = d['si_unit'] if d['si_unit'] else ' '
prefix_levels = (len(SI_PREFIX_UNITS) - 1) // 2
scale = 10 ** (3 * (SI_PREFIX_UNITS.index(si_unit) - prefix_levels))
return float(d['number']) * scale
def si_prefix_scale(si_unit):
'''
Parameters
----------
si_unit : str
SI unit character, i.e., one of "yzafpnum kMGTPEZY".
Returns
-------
int
Multiple associated with `si_unit`, e.g., 1000 for `si_unit=k`.
'''
return 10 ** si_prefix_expof10(si_unit)
def si_prefix_expof10(si_unit):
'''
Parameters
----------
si_unit : str
SI unit character, i.e., one of "yzafpnum kMGTPEZY".
Returns
-------
int
Exponent of the power of ten associated with `si_unit`, e.g., 3 for
`si_unit=k` and -6 for `si_unit=u`.
'''
prefix_levels = (len(SI_PREFIX_UNITS) - 1) // 2
return (3 * (SI_PREFIX_UNITS.index(si_unit) - prefix_levels))