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How to convert float to sortable integer in Python

P: n/a
Hi all:
I want to convert the float number to sortable integer, like the
function float2rawInt() in java, but I don't know the internal
expression of float, appreciate your help!

Jan 16 '07 #1
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7 Replies


P: n/a
On Tue, 16 Jan 2007 01:21:52 -0800, shellon wrote:
Hi all:
I want to convert the float number to sortable integer, like the
function float2rawInt() in java, but I don't know the internal
expression of float, appreciate your help!
Google says:

Your search - float2rawInt - did not match any documents.

Are you sure about that function? What's a sortable integer?
--
Steven.

Jan 16 '07 #2

P: n/a
shellon wrote:
Hi all:
I want to convert the float number to sortable integer, like the
function float2rawInt() in java, but I don't know the internal
expression of float, appreciate your help!
float comparision works well enough for sorting in Python. What is
the actual requirement?

see also hash() and id() for keying.

Robert
Jan 16 '07 #3

P: n/a
robert <no*****@no-spam-no-spam.invalidwrote:
shellon wrote:
>Hi all:
I want to convert the float number to sortable integer, like the
function float2rawInt() in java, but I don't know the internal
expression of float, appreciate your help!

float comparision works well enough for sorting in Python. What is
the actual requirement?
Maybe this is the problem?
>>sorted([-0.0, 0.0, -0.0, 0.0, -0.0])
[-0.0, 0.0, -0.0, 0.0, -0.0]

Java sorting imposes an artificial total ordering on float or double
values:
The < relation does not provide a total order on all floating-point
values; although they are distinct numbers -0.0 == 0.0 is true and a
NaN value compares neither less than, greater than, nor equal to any
floating-point value, even itself. To allow the sort to proceed,
instead of using the < relation to determine ascending numerical
order, this method uses the total order imposed by
Double.compareTo(java.lang.Double). This ordering differs from the <
relation in that -0.0 is treated as less than 0.0 and NaN is
considered greater than any other floating-point value. For the
purposes of sorting, all NaN values are considered equivalent and
equal.
Jan 16 '07 #4

P: n/a
shellon wrote:
Hi all:
I want to convert the float number to sortable integer, like the
function float2rawInt() in java, but I don't know the internal
expression of float, appreciate your help!
You should know you can sort mixed float/integer values in Python
>>l=[3,2.3,1.45,2,5]
l.sort()
l
[1.45, 2, 2.2999999999999998, 3, 5]

to convert a float to int use the built-in int function:
>>int(2.34)
2

Hope this helps

regards

Wolfgang
Jan 16 '07 #5

P: n/a
I'm sorry I mistake the function name, the function is
floatToRawIntBits(), it convert ieee 754 floating point number to
integer, e.g. if f1>f2 then floatToRawBits(f1) floatToRawBits(f1)
I want convert floating point number to sortable string to index in
Lucene, so I want first to conver the floating point number to integer
first, and than convert the integer to sortable string?

so How to convert a ieee 754 floating point to raw bits like the java
api floatToRawBits do?

Appreciate your help!

Steven D'Aprano wrote:
On Tue, 16 Jan 2007 01:21:52 -0800, shellon wrote:
Hi all:
I want to convert the float number to sortable integer, like the
function float2rawInt() in java, but I don't know the internal
expression of float, appreciate your help!

Google says:

Your search - float2rawInt - did not match any documents.

Are you sure about that function? What's a sortable integer?
--
Steven.
Jan 17 '07 #6

P: n/a
shellon wrote:
I'm sorry I mistake the function name, the function is
floatToRawIntBits(), it convert ieee 754 floating point number to
integer, e.g. if f1>f2 then floatToRawBits(f1) floatToRawBits(f1)
You can get the raw bits of a floating point number by packing into a
string then unpacking it as an integer, but the invariant you give
doesn't hold when you do that. That is, f1>f2 does not guarantee
rawbits(f1)>rawbits(f2). So be warned.

Python floats are double-precision internally, so the integer returned
would have to hold at least 64-bits (i.e., it'd be a long on 32-bit
platforms).

import struct

def double_to_raw_int(d):
return struct.unpack("=q",struct.pack("=d",d))

I want convert floating point number to sortable string to index in
Lucene, so I want first to conver the floating point number to integer
first, and than convert the integer to sortable string?
I presume that, by "sortable string", you mean a numerical
representation (padded or something), but if all you want is a string
containing the bits of the number, struct.pack("=d",d) will get you an
8-byte string representing the bits without having to change it to an
integer first. Check the documentation for the struct module if you
need a certain endianness.

so How to convert a ieee 754 floating point to raw bits like the java
api floatToRawBits do?
See documentation for the struct module.
Carl Banks

Jan 17 '07 #7

P: n/a
At Wednesday 17/1/2007 03:36, shellon wrote:
>I'm sorry I mistake the function name, the function is
floatToRawIntBits(), it convert ieee 754 floating point number to
integer, e.g. if f1>f2 then floatToRawBits(f1) floatToRawBits(f1)
I want convert floating point number to sortable string to index in
Lucene, so I want first to conver the floating point number to integer
first, and than convert the integer to sortable string?
The following two functions may be useful; they do more-or-less the
same thing, but returning a string instead of an integer.
Note that this property: f1>f2 =floatToRawBits(f1) >
floatToRawBits(f2), only holds for nonnegative numbers; this code
has the same restriction.

The code assumes a few things: float(repr(x))==x for 0.5<=x<1 (that
is, repr(x) is exact in that range); the maximum double value has an
exponent (as given by frexp) less than 5000; the minimum positive
value has an exponent greater than -5000; 0 is the only number having
mantissa==0; and surely I'm assuming many more but I'm not aware of
them. I think it's not dependent on a particular FP hardware or representation.
Beasts like denormals, INFs and NaNs are not considered at all. It
appears to work OK for "normal" numbers, but I've not checked all
corner cases. It was only tested on Windows XP.
After such long disclaimer... I hope it's useful :)

--- begin fpsrepr.py ---
import math

def float2srepr(f):
"""Convert a floating point number into a string representation
which can be sorted lexicographically maintaining the original
numerical order (only for numbers >= 0).
That is, given f1,f2 >= 0, f1<f2 <=float2srepr(f1)<float2srepr(f2)
Denormals, INFs, NaNs are not handled at all!

format: Seeeem*
S : sign, "0" for +, else "-"
eeee: exponent + 5000 (never negative)
m* : mantissa, one or more chars as needed,
0.5<=mantissa<1, without the leading "0."
"""
try: m, e = math.frexp(f)
except ValueError,E:
E.args = list(E.args) + [repr(f)]
raise
if m < 0:
sign = '-'
m = -m
else:
sign = '0'
e = e+5000
if m==0.0: e = 0
return "%s%04.4d%s" % (sign, e, repr(m)[2:])

def srepr2float(s):
"""Convert a string in the format used by float2srepr
into a floating point number.
srepr2float(float2srepr(x)) == x
"""
sign, e, m = s[0], s[1:5], s[5:]
neg = sign == '-'
e = int(e)
e = e-5000
m = float('0.'+m)
if m==0: f = 0
else: f = math.ldexp(m, e)
if neg: f = -f
return f

# test

def find_eps(): # smallest x such 1+x!=1
prev, curr = 1.0, 0.5
while 1.0+curr 1.0 and curr < prev:
prev = curr
curr /= 2.0
return prev

def find_minf(): # smallest x such x>0
prev, curr = 1.0, 0.5
while curr 0.0 and curr < prev:
prev = curr
curr /= 2
return prev

eps = find_eps()
print "eps=%r %r" % (eps, 1.0+eps)
minf = find_minf()
print "minf=%r %r" % (minf, -minf)

values = [0,0.1,0.2,0.3,0.4,0.5,0.6,0.7,0.8,0.9,
1.0,1.0+eps,1.0+2*eps,1.0-eps,1.0-2*eps,
2,3,10,10.0000001,10.00000000001,
123.456,1234.567,12345.678,123456.789,1.234e10,
123412341234,123412341234123412341234.0,1.234e20,
1e100,1e200,1e300,math.pi,math.e]
values += [1.0/x for x in values if x>0] + [eps, minf]
values += [-x for x in values]
values = sorted(values)
for x in values:
s = float2srepr(x)
xr = srepr2float(s)
print '%-30r %s' % (x, s)
assert x==xr, '%r!=%r' % (x, xr)

f2svalues = [float2srepr(x) for x in values if x>=0]
s2fvalues = [srepr2float(x) for x in sorted(f2svalues)]
assert s2fvalues == sorted(s2fvalues), s2fvalues
--- end fpsrepr.py ---
--
Gabriel Genellina
Softlab SRL


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Jan 17 '07 #8

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