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# How to convert float to sortable integer in Python

 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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 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
 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
 robert Hi all: I want to convert the float number to sortable integer, like thefunction float2rawInt() in java, but I don't know the internalexpression 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
 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
 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
 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
 At Wednesday 17/1/2007 03:36, shellon wrote: >I'm sorry I mistake the function name, the function isfloatToRawIntBits(), it convert ieee 754 floating point number tointeger, e.g. if f1>f2 then floatToRawBits(f1) floatToRawBits(f1)I want convert floating point number to sortable string to index inLucene, so I want first to conver the floating point number to integerfirst, 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, f10 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 __________________________________________________ Preguntá. Respondé. Descubrí. Todo lo que querías saber, y lo que ni imaginabas, está en Yahoo! Respuestas (Beta). ¡Probalo ya! http://www.yahoo.com.ar/respuestas Jan 17 '07 #8

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