Unfinished machine arithmatic
- To: mathgroup at smc.vnet.net
- Subject: [mg7508] Unfinished machine arithmatic
- From: Ersek_Ted%PAX1A at mr.nawcad.navy.mil
- Date: Sat, 7 Jun 1997 03:48:48 -0400 (EDT)
- Sender: owner-wri-mathgroup at wolfram.com
Consider the following lines involving a mixture of machine numbers
and exact numbers.
In[1]:= 15.0/0.125 +Log[1-Erf[3]]
Out[1]= 109.28
In[2]:= 15.0/0.125 +Log[1-Erf[7]]
In[2]= 120. +Log[1-Erf[7]]
The first line evaluates without any trouble, but
the second line doesn't evaluate to a number even though it
has a numerical value of 68.472 (found using arbitrary precision).
Question: Was the appropriate value given for Out[2].
Lets look at the next line.
In[3]:= 15.0/0.125+Log[1-Erf[7.0]] (* all machine numbers *)
Out[3]= -Infinity
The numerical value of Out[2] is 68.472 and thats a long way from
(-Infinity).
I suppose it WRI decided the following:
- Arbitrary precision should not be used in the case of In[2]
for the sake of efficiency.
- The result of In[2] will not be (-Infinity), because it is far from
correct.
- The only thing left is to give an incomplete calculation for Out[2].
I think the implementation is very good, but I thought users should be
aware that the arithmatic can end up unfinished.
Ted Ersek