Determine the number of bits in a negative number
in [VbScript]
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From: Csaba Gabor on 2 Dec 2009 04:18 I'd like to determine the number of bits (the position of the highest on bit) in a negative number (say -1) without using knowledge of its type. Valid operations would be AND, OR, XOR, negation, and one's compliment can be done as: 'Get a -1 of the same type as myvar Dim minus1: minus1 = myvar - (myvar + 1) Dim onesComp: onesComp = myvar XOR minus1 You could also use multiplication or division by 2 or other mathematical operators/functions, but then sign extension comes into play. The standard solution is to presume the answer given the type of the variable, but that is not very generic. Csaba Gabor from Vienna By the way, this is equivalent to the problem of determining the number of bits (both on and off bits) in a given variable.
From: mr_unreliable on 2 Dec 2009 12:46 Csaba Gabor wrote: > I'd like to determine the number of bits (the position > of the highest on bit) in a negative number (say -1) > without using knowledge of its type. > The "brute force" way would be to "shift left" one bit at a time, until you hit your 1-bit. Problem is, you can't shift left in script. There are a number of shifting functions written in VB, but they require calling the system api. Most scripters abhor calling the system api, and so your best bet is to wait around for a more knowledgable person to come along with a "pure script" solution. cheers, jw
From: Tom Lavedas on 2 Dec 2009 13:27 On Dec 2, 12:46 pm, mr_unreliable <kindlyReplyToNewsgr...(a)notmail.com> wrote: > Csaba Gabor wrote: > > I'd like to determine the number of bits (the position > > of the highest on bit) in a negative number (say -1) > > without using knowledge of its type. > > The "brute force" way would be to "shift left" one bit > at a time, until you hit your 1-bit. > > Problem is, you can't shift left in script. There are > a number of shifting functions written in VB, but they > require calling the system api. > > Most scripters abhor calling the system api, and so > your best bet is to wait around for a more knowledgable > person to come along with a "pure script" solution. > > cheers, jw JScript has SHIFT operators, so the old trick of using JScript in VBS will work ... wsh.echo ShiftRight(clng(-1) and &h7fffffff&, 30) Function ShiftRight(arg, n) with createobject("htmlfile") .write "<script>var a=" & arg & ">>" & n & ";</script>" ShiftRight = .parentWindow.a end with end function Function ShiftLeft(arg, n) with createobject("htmlfile") .write "<script>var a=" & arg & "<<" & n & ";</script>" ShiftLeft = .parentWindow.a end with end function I also propose ANDing the number (converted to a Long - 32 bit - number) with a mask that zeros the minus sign bit. Then the other bits can be tested. The shift that first returns a value of zero is the most significant bit (I didn't bother to show the test loop). BTW, your shift nomenclature seems backward to me. I guess you were thinking of shifting the mask and not the number. However, starting from the MSB end would still seem most efficient and would still shift the mask to the right, not the left wouldn't it? _____________________ Tom Lavedas
From: mr_unreliable on 2 Dec 2009 15:46 Tom Lavedas wrote: > BTW, your shift nomenclature seems backward to me. I guess you were > thinking of shifting the mask and not the number. However, starting > from the MSB end would still seem most efficient and would still shift > the mask to the right, not the left wouldn't it? > I wasn't clear in expressing what I was thinking -- which was to put a MASK in the left-most bit, then shifting the DATA left one bit at a time, masking to see what the left-most bit was, and then counting the number of zero's, until a 1-bit was reached. But then, my verbal skills have always been lacking, even worse than GWB! Regardless, as jscript has a shift function, that sounds like the way to go. cheers, jw
From: Paul Randall on 2 Dec 2009 17:17
"mr_unreliable" <kindlyReplyToNewsgroup(a)notmail.com> wrote in message news:u1ybAb3cKHA.5156(a)TK2MSFTNGP04.phx.gbl... > Csaba Gabor wrote: >> I'd like to determine the number of bits (the position >> of the highest on bit) in a negative number (say -1) >> without using knowledge of its type. >> > > The "brute force" way would be to "shift left" one bit > at a time, until you hit your 1-bit. > > Problem is, you can't shift left in script. There are > a number of shifting functions written in VB, but they > require calling the system api. > > Most scripters abhor calling the system api, and so > your best bet is to wait around for a more knowledgable > person to come along with a "pure script" solution. If the task must be done without using "knowledge of its type", then double precision floats might be involved, which could be a lot of bits that don't shift nicely. |