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