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