# 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 . # # [1]: http://www.cs.tut.fi/~jkorpela/c/eng.html SI_PREFIX_UNITS = "yzafpnum kMGTPEZY" CRE_SI_NUMBER = re.compile(r'\s*(?P[\+\-])?' r'(?P\d+)' r'(?P.\d+)?\s*' r'(?P[%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[\+\-]?\d+)?' r'(?P.\d+)?\s*([eE]\s*' r'(?P[\+\-]?\d+))?$') CRE_SI_NUMBER = re.compile(r'^\s*(?P(?P[\+\-]?\d+)?' r'(?P.\d+)?)\s*' r'(?P[%s])?\s*$' % 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))