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Unexpected behavior of Floor and IntegerPart

  • To: mathgroup at smc.vnet.net
  • Subject: [mg97872] Unexpected behavior of Floor and IntegerPart
  • From: Curtis Osterhoudt <cfo at lanl.gov>
  • Date: Tue, 24 Mar 2009 05:32:21 -0500 (EST)
  • Organization: LANL
  • Reply-to: cfo at lanl.gov

Dear List, 

   I'm sure this is a result of non-infinite-precision arithmetic, but it would be nice to have a more consistent behavior:

In[26]:= IntegerPart[0.29*100]

Out[26]= 28

In[25]:= Floor[0.29*100]

Out[25]= 28

In[27]:= Floor[(29/100)*100]

Out[27]= 29

In[32]:= (Floor[#1*100] & ) /@ Range[0, 1, 0.01]

Out[32]= {0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, \
17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 28, 30, 31, 32, 33, \
34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, \
51, 52, 53, 54, 55, 56, 57, 57, 59, 60, 61, 62, 63, 64, 65, 66, 67, 
68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, \
84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, 100}

In[33]:= Differences[%]

Out[33]= {1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, \
1, 1, 1, 1, 1, 1, 1, 1, 0, 2, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, \
1, 
   1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 0, 2, 1, 1, 1, 1, 1, 1, 1, \
1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 
   1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1}



An interesting hint as to what may be happening is obtained by:

ListPlot[Differences[(Floor[#1*100] & ) /@ Range[0, 100, 0.01]]]

                     Regards, 
                                    C.O.



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