Initial revision

diff --git a/Lib/fpformat.py b/Lib/fpformat.py
new file mode 100644 (file)
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--- /dev/null
+++ b/Lib/fpformat.py
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+# General floating point formatting functions.
+
+# Functions:
+# fix(x, digits_behind)
+# sci(x, digits_behind)
+
+# Each takes a number or a string and a number of digits as arguments.
+
+# Parameters:
+# x:             number to be formatted; or a string resembling a number
+# digits_behind: number of digits behind the decimal point
+
+
+import regex
+
+# Compiled regular expression to "decode" a number
+decoder = regex.compile( \
+       '^\([-+]?\)0*\([0-9]*\)\(\(\.[0-9]*\)?\)\(\([eE][-+]?[0-9]+\)?\)$')
+# \0 the whole thing
+# \1 leading sign or empty
+# \2 digits left of decimal point
+# \3 fraction (empty or begins with point)
+# \5 exponent part (empty or begins with 'e' or 'E')
+
+NotANumber = 'fpformat.NotANumber'
+
+# Return (sign, intpart, fraction, expo) or raise an exception:
+# sign is '+' or '-'
+# intpart is 0 or more digits beginning with a nonzero
+# fraction is 0 or more digits
+# expo is an integer
+def extract(s):
+       if decoder.match(s) < 0: raise NotANumber
+       (a1, b1), (a2, b2), (a3, b3), (a4, b4), (a5, b5) = decoder.regs[1:6]
+       sign, intpart, fraction, exppart = \
+               s[a1:b1], s[a2:b2], s[a3:b3], s[a5:b5]
+       if sign == '+': sign = ''
+       if fraction: fraction = fraction[1:]
+       if exppart: expo = eval(exppart[1:])
+       else: expo = 0
+       return sign, intpart, fraction, expo
+
+# Remove the exponent by changing intpart and fraction
+def unexpo(intpart, fraction, expo):
+       if expo > 0: # Move the point left
+               f = len(fraction)
+               intpart, fraction = intpart + fraction[:expo], fraction[expo:]
+               if expo > f:
+                       intpart = intpart + '0'*(expo-f)
+       elif expo < 0: # Move the point right
+               i = len(intpart)
+               intpart, fraction = intpart[:expo], intpart[expo:] + fraction
+               if expo < -i:
+                       fraction = '0'*(-expo-i) + fraction
+       return intpart, fraction
+
+# Round or extend the fraction to size digs
+def roundfrac(intpart, fraction, digs):
+       f = len(fraction)
+       if f <= digs:
+               return intpart, fraction + '0'*(digs-f)
+       i = len(intpart)
+       if i+digs < 0:
+               return '0'*-digs, ''
+       total = intpart + fraction
+       nextdigit = total[i+digs]
+       if nextdigit >= '5': # Hard case: increment last digit, may have carry!
+               n = i + digs - 1
+               while n >= 0:
+                       if total[n] != '9': break
+                       n = n-1
+               else:
+                       total = '0' + total
+                       i = i+1
+                       n = 0
+               total = total[:n] + chr(ord(total[n]) + 1) + '0'*(len(total)-n-1)
+               intpart, fraction = total[:i], total[i:]
+       if digs >= 0:
+               return intpart, fraction[:digs]
+       else:
+               return intpart[:digs] + '0'*-digs, ''
+
+# Format x as [-]ddd.ddd with 'digs' digits after the point
+# and at least one digit before.
+# If digs <= 0, the point is suppressed.
+def fix(x, digs):
+       if type(x) != type(''): x = `x`
+       try:
+               sign, intpart, fraction, expo = extract(x)
+       except NotANumber:
+               return x
+       intpart, fraction = unexpo(intpart, fraction, expo)
+       intpart, fraction = roundfrac(intpart, fraction, digs)
+       while intpart and intpart[0] == '0': intpart = intpart[1:]
+       if intpart == '': intpart = '0'
+       if digs > 0: return sign + intpart + '.' + fraction
+       else: return sign + intpart
+
+# Format x as [-]d.dddE[+-]ddd with 'digs' digits after the point
+# and exactly one digit before.
+# If digs is <= 0, one digit is kept and the point is suppressed.
+def sci(x, digs):
+       if type(x) != type(''): x = `x`
+       sign, intpart, fraction, expo = extract(x)
+       if not intpart:
+               while fraction and fraction[0] == '0':
+                       fraction = fraction[1:]
+                       expo = expo - 1
+               if fraction:
+                       intpart, fraction = fraction[0], fraction[1:]
+                       expo = expo - 1
+               else:
+                       intpart = '0'
+       else:
+               expo = expo + len(intpart) - 1
+               intpart, fraction = intpart[0], intpart[1:] + fraction
+       digs = max(0, digs)
+       intpart, fraction = roundfrac(intpart, fraction, digs)
+       if len(intpart) > 1:
+               intpart, fraction, expo = \
+                       intpart[0], intpart[1:] + fraction[:-1], \
+                       expo + len(intpart) - 1
+       s = sign + intpart
+       if digs > 0: s = s + '.' + fraction
+       e = `abs(expo)`
+       e = '0'*(3-len(e)) + e
+       if expo < 0: e = '-' + e
+       else: e = '+' + e
+       return s + 'e' + e
+
+# Interactive test run
+def test():
+       try:
+               while 1:
+                       x, digs = input('Enter (x, digs): ')
+                       print x, fix(x, digs), sci(x, digs)
+       except (EOFError, KeyboardInterrupt):
+               pass