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MathGroup Archive 2007

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Re: Next representable number

  • To: mathgroup at smc.vnet.net
  • Subject: [mg73627] Re: [mg73542] Next representable number
  • From: Daniel Lichtblau <danl at wolfram.com>
  • Date: Fri, 23 Feb 2007 04:38:41 -0500 (EST)
  • References: <200702210653.BAA18712@smc.vnet.net>

Andrew Moylan wrote:
> What's a neat way to determine the next (higher) representable number
> (at a given precision)? That is, I want f[x] to be the smallest
> representable number (at precision Precision[x]) that's larger than x.
> 

I'm not sure this is quite correct but here goes.

(1) Convert to binary using RealDigits.

(2) Add one to the last "bit" in the mantissa. No matter if it 
"overflows"; the reverse conversion won't mind. This feature caused 
remarkable controversy in-house a few months ago.

(3) Convert from {mantissa,exponent} to a rational using FromDigits.

(4) Numericize to the original precision.

nextRepresentable[x_Real] := Module[
   {mant, expon, prec=Precision[x]},
   {mant,expon} = RealDigits[x];
   N[FromDigits[{Append[Most[mant],Last[mant]+1],expon}], prec]
   ]

In[40]:= ee = N[7/4,22]
Out[40]= 1.750000000000000000000

In[41]:= ff = nextRepresentable[ee]
Out[41]= 1.750000000000000000001

In[42]:= InputForm[ff-ee]
Out[42]//InputForm= 1.`0.45593195564972433*^-21


Daniel Lichtblau
Wolfram Research


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