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 regex
  15
  16 # Compiled regular expression to "decode" a number
  17 decoder = regex.compile( \
  18         '^\([-+]?\)0*\([0-9]*\)\(\(\.[0-9]*\)?\)\(\([eE][-+]?[0-9]+\)?\)$')
  19 # \0 the whole thing
  20 # \1 leading sign or empty
  21 # \2 digits left of decimal point
  22 # \3 fraction (empty or begins with point)
  23 # \5 exponent part (empty or begins with 'e' or 'E')
  24
  25 NotANumber = 'fpformat.NotANumber'
  26
  27 # Return (sign, intpart, fraction, expo) or raise an exception:
  28 # sign is '+' or '-'
  29 # intpart is 0 or more digits beginning with a nonzero
  30 # fraction is 0 or more digits
  31 # expo is an integer
  32 def extract(s):
  33         if decoder.match(s) < 0: raise NotANumber
  34         (a1, b1), (a2, b2), (a3, b3), (a4, b4), (a5, b5) = decoder.regs[1:6]
  35         sign, intpart, fraction, exppart = \
  36                 s[a1:b1], s[a2:b2], s[a3:b3], s[a5:b5]
  37         if sign == '+': sign = ''
  38         if fraction: fraction = fraction[1:]
  39         if exppart: expo = eval(exppart[1:])
  40         else: expo = 0
  41         return sign, intpart, fraction, expo
  42
  43 # Remove the exponent by changing intpart and fraction
  44 def unexpo(intpart, fraction, expo):
  45         if expo > 0: # Move the point left
  46                 f = len(fraction)
  47                 intpart, fraction = intpart + fraction[:expo], fraction[expo:]
  48                 if expo > f:
  49                         intpart = intpart + '0'*(expo-f)
  50         elif expo < 0: # Move the point right
  51                 i = len(intpart)
  52                 intpart, fraction = intpart[:expo], intpart[expo:] + fraction
  53                 if expo < -i:
  54                         fraction = '0'*(-expo-i) + fraction
  55         return intpart, fraction
  56
  57 # Round or extend the fraction to size digs
  58 def roundfrac(intpart, fraction, digs):
  59         f = len(fraction)
  60         if f <= digs:
  61                 return intpart, fraction + '0'*(digs-f)
  62         i = len(intpart)
  63         if i+digs < 0:
  64                 return '0'*-digs, ''
  65         total = intpart + fraction
  66         nextdigit = total[i+digs]
  67         if nextdigit >= '5': # Hard case: increment last digit, may have carry!
  68                 n = i + digs - 1
  69                 while n >= 0:
  70                         if total[n] != '9': break
  71                         n = n-1
  72                 else:
  73                         total = '0' + total
  74                         i = i+1
  75                         n = 0
  76                 total = total[:n] + chr(ord(total[n]) + 1) + '0'*(len(total)-n-1)
  77                 intpart, fraction = total[:i], total[i:]
  78         if digs >= 0:
  79                 return intpart, fraction[:digs]
  80         else:
  81                 return intpart[:digs] + '0'*-digs, ''
  82
  83 # Format x as [-]ddd.ddd with 'digs' digits after the point
  84 # and at least one digit before.
  85 # If digs <= 0, the point is suppressed.
  86 def fix(x, digs):
  87         if type(x) != type(''): x = `x`
  88         try:
  89                 sign, intpart, fraction, expo = extract(x)
  90         except NotANumber:
  91                 return x
  92         intpart, fraction = unexpo(intpart, fraction, expo)
  93         intpart, fraction = roundfrac(intpart, fraction, digs)
  94         while intpart and intpart[0] == '0': intpart = intpart[1:]
  95         if intpart == '': intpart = '0'
  96         if digs > 0: return sign + intpart + '.' + fraction
  97         else: return sign + intpart
  98
  99 # Format x as [-]d.dddE[+-]ddd with 'digs' digits after the point
 100 # and exactly one digit before.
 101 # If digs is <= 0, one digit is kept and the point is suppressed.
 102 def sci(x, digs):
 103         if type(x) != type(''): x = `x`
 104         sign, intpart, fraction, expo = extract(x)
 105         if not intpart:
 106                 while fraction and fraction[0] == '0':
 107                         fraction = fraction[1:]
 108                         expo = expo - 1
 109                 if fraction:
 110                         intpart, fraction = fraction[0], fraction[1:]
 111                         expo = expo - 1
 112                 else:
 113                         intpart = '0'
 114         else:
 115                 expo = expo + len(intpart) - 1
 116                 intpart, fraction = intpart[0], intpart[1:] + fraction
 117         digs = max(0, digs)
 118         intpart, fraction = roundfrac(intpart, fraction, digs)
 119         if len(intpart) > 1:
 120                 intpart, fraction, expo = \
 121                         intpart[0], intpart[1:] + fraction[:-1], \
 122                         expo + len(intpart) - 1
 123         s = sign + intpart
 124         if digs > 0: s = s + '.' + fraction
 125         e = `abs(expo)`
 126         e = '0'*(3-len(e)) + e
 127         if expo < 0: e = '-' + e
 128         else: e = '+' + e
 129         return s + 'e' + e
 130
 131 # Interactive test run
 132 def test():
 133         try:
 134                 while 1:
 135                         x, digs = input('Enter (x, digs): ')
 136                         print x, fix(x, digs), sci(x, digs)
 137         except (EOFError, KeyboardInterrupt):
 138                 pass