*To*: mathgroup@smc.vnet.net*Subject*: [mg12133] RE: [mg12048] Very odd behaviour in Mod/*From*: Ersek_Ted%PAX1A@mr.nawcad.navy.mil*Date*: Mon, 27 Apr 1998 01:46:16 -0400

I would like to be able to do the following: In[17]:= t = Mod[Q, 2 Pi] Out[17]= 1000000000000000000 - 318309886183790670 Pi In[18]:= N[t,3] Out[18]= 4.83677 Where the correct value is: 4.831039164951128133....... If you want to be sure even a few digits are right, you can currently use N[t,17]. Then Mathematica will go to all the trouble to get 17 digits correct, when you only care about a few of them. Ted Ersek ---------- |From: "megalith"@jimc.demon.co.uk@PMDF@PAXMB1 To: mathgroup@smc.vnet.net |To: ersek ted; "mathgroup"@smc.vnet.net@PMDF@PAXMB1 |Subject: [mg12133] [mg12048] Very odd behaviour in Mod/N |Date: Friday, April 24, 1998 8:57AM | |<<File Attachment: 00000000.TXT>> |Hi there, |I've observed some very odd behaviour when using Mod and N. It seems |that, when handling very large numbers, Mathematica (3.0.1, Win32) does |not display machine-precision results correctly. Here's an example |which behaves as it should: | |In[12]:= Q = 10^8 |Out[12]= 100000000 | |In[13]:= t = Mod[Q, 2 Pi] |Out[13]= 100000000 - 31830988 Pi | |In[14]:= N[t] |Out[14]= 1.9427 | |In[15]:= N[t, 20] |Out[15]= 1.9426951345040144600 | |Now for a very similar example which obviously behaves wrongly: | |In[16]:= Q = 10^18 |Out[16]= 1000000000000000000 | |In[17]:= t = Mod[Q, 2 Pi] |Out[17]= 1000000000000000000 - 318309886183790670 Pi | |In[18]:= N[t] |Out[18]= 128. | |In[19]:= N[t, 20] |Out[19]= 4.831039164951128133 | |What exactly is going on here? | |Thanks, cheers, |Adrian Cable. |