Lib/fpformat.py

   1 """General floating point formatting functions.
   2
   3 Functions:
   4 fix(x, digits_behind)
   5 sci(x, digits_behind)
   6
   7 Each takes a number or a string and a number of digits as arguments.
   8
   9 Parameters:
  10 x:             number to be formatted; or a string resembling a number
  11 digits_behind: number of digits behind the decimal point
  12 """
  13
  14 import re
  15
  16 __all__ = ["fix","sci","NotANumber"]
  17
  18 # Compiled regular expression to "decode" a number
  19 decoder = re.compile(r'^([-+]?)0*(\d*)((?:\.\d*)?)(([eE][-+]?\d+)?)$')
  20 # \0 the whole thing
  21 # \1 leading sign or empty
  22 # \2 digits left of decimal point
  23 # \3 fraction (empty or begins with point)
  24 # \4 exponent part (empty or begins with 'e' or 'E')
  25
  26 try:
  27     class NotANumber(ValueError):
  28         pass
  29 except TypeError:
  30     NotANumber = 'fpformat.NotANumber'
  31
  32 def extract(s):
  33     """Return (sign, intpart, fraction, expo) or raise an exception:
  34     sign is '+' or '-'
  35     intpart is 0 or more digits beginning with a nonzero
  36     fraction is 0 or more digits
  37     expo is an integer"""
  38     res = decoder.match(s)
  39     if res is None: raise NotANumber, s
  40     sign, intpart, fraction, exppart = res.group(1,2,3,4)
  41     if sign == '+': sign = ''
  42     if fraction: fraction = fraction[1:]
  43     if exppart: expo = int(exppart[1:])
  44     else: expo = 0
  45     return sign, intpart, fraction, expo
  46
  47 def unexpo(intpart, fraction, expo):
  48     """Remove the exponent by changing intpart and fraction."""
  49     if expo > 0: # Move the point left
  50         f = len(fraction)
  51         intpart, fraction = intpart + fraction[:expo], fraction[expo:]
  52         if expo > f:
  53             intpart = intpart + '0'*(expo-f)
  54     elif expo < 0: # Move the point right
  55         i = len(intpart)
  56         intpart, fraction = intpart[:expo], intpart[expo:] + fraction
  57         if expo < -i:
  58             fraction = '0'*(-expo-i) + fraction
  59     return intpart, fraction
  60
  61 def roundfrac(intpart, fraction, digs):
  62     """Round or extend the fraction to size digs."""
  63     f = len(fraction)
  64     if f <= digs:
  65         return intpart, fraction + '0'*(digs-f)
  66     i = len(intpart)
  67     if i+digs < 0:
  68         return '0'*-digs, ''
  69     total = intpart + fraction
  70     nextdigit = total[i+digs]
  71     if nextdigit >= '5': # Hard case: increment last digit, may have carry!
  72         n = i + digs - 1
  73         while n >= 0:
  74             if total[n] != '9': break
  75             n = n-1
  76         else:
  77             total = '0' + total
  78             i = i+1
  79             n = 0
  80         total = total[:n] + chr(ord(total[n]) + 1) + '0'*(len(total)-n-1)
  81         intpart, fraction = total[:i], total[i:]
  82     if digs >= 0:
  83         return intpart, fraction[:digs]
  84     else:
  85         return intpart[:digs] + '0'*-digs, ''
  86
  87 def fix(x, digs):
  88     """Format x as [-]ddd.ddd with 'digs' digits after the point
  89     and at least one digit before.
  90     If digs <= 0, the point is suppressed."""
  91     if type(x) != type(''): x = repr(x)
  92     try:
  93         sign, intpart, fraction, expo = extract(x)
  94     except NotANumber:
  95         return x
  96     intpart, fraction = unexpo(intpart, fraction, expo)
  97     intpart, fraction = roundfrac(intpart, fraction, digs)
  98     while intpart and intpart[0] == '0': intpart = intpart[1:]
  99     if intpart == '': intpart = '0'
 100     if digs > 0: return sign + intpart + '.' + fraction
 101     else: return sign + intpart
 102
 103 def sci(x, digs):
 104     """Format x as [-]d.dddE[+-]ddd with 'digs' digits after the point
 105     and exactly one digit before.
 106     If digs is <= 0, one digit is kept and the point is suppressed."""
 107     if type(x) != type(''): x = repr(x)
 108     sign, intpart, fraction, expo = extract(x)
 109     if not intpart:
 110         while fraction and fraction[0] == '0':
 111             fraction = fraction[1:]
 112             expo = expo - 1
 113         if fraction:
 114             intpart, fraction = fraction[0], fraction[1:]
 115             expo = expo - 1
 116         else:
 117             intpart = '0'
 118     else:
 119         expo = expo + len(intpart) - 1
 120         intpart, fraction = intpart[0], intpart[1:] + fraction
 121     digs = max(0, digs)
 122     intpart, fraction = roundfrac(intpart, fraction, digs)
 123     if len(intpart) > 1:
 124         intpart, fraction, expo = \
 125             intpart[0], intpart[1:] + fraction[:-1], \
 126             expo + len(intpart) - 1
 127     s = sign + intpart
 128     if digs > 0: s = s + '.' + fraction
 129     e = repr(abs(expo))
 130     e = '0'*(3-len(e)) + e
 131     if expo < 0: e = '-' + e
 132     else: e = '+' + e
 133     return s + 'e' + e
 134
 135 def test():
 136     """Interactive test run."""
 137     try:
 138         while 1:
 139             x, digs = input('Enter (x, digs): ')
 140             print x, fix(x, digs), sci(x, digs)
 141     except (EOFError, KeyboardInterrupt):
 142         pass