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 re

__all__ = ["fix","sci","NotANumber"]

# Compiled regular expression to "decode" a number
decoder = re.compile(r'^([-+]?)0*(\d*)((?:\.\d*)?)(([eE][-+]?\d+)?)$')
# \0 the whole thing
# \1 leading sign or empty
# \2 digits left of decimal point
# \3 fraction (empty or begins with point)
# \4 exponent part (empty or begins with 'e' or 'E')

try:
    class NotANumber(ValueError):
        pass
except TypeError:
    NotANumber = 'fpformat.NotANumber'

def extract(s):
    """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"""
    res = decoder.match(s)
    if res is None: raise NotANumber, s
    sign, intpart, fraction, exppart = res.group(1,2,3,4)
    if sign == '+': sign = ''
    if fraction: fraction = fraction[1:]
    if exppart: expo = int(exppart[1:])
    else: expo = 0
    return sign, intpart, fraction, expo

def unexpo(intpart, fraction, expo):
    """Remove the exponent by changing intpart and fraction."""
    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

def roundfrac(intpart, fraction, digs):
    """Round or extend the fraction to size 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, ''

def fix(x, 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."""
    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

def sci(x, digs):
    """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."""
    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

def test():
    """Interactive test run."""
    try:
        while 1:
            x, digs = input('Enter (x, digs): ')
            print x, fix(x, digs), sci(x, digs)
    except (EOFError, KeyboardInterrupt):
        pass