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MathGroup Archive 2006

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Re: MemberQ

  • To: mathgroup at smc.vnet.net
  • Subject: [mg68558] Re: [mg68503] MemberQ
  • From: Igor Antonio <igora at wolf-ram.com>
  • Date: Wed, 9 Aug 2006 04:20:49 -0400 (EDT)
  • Organization: Wolfram Research, Inc.
  • References: <200608081028.GAA29635@smc.vnet.net>
  • Reply-to: igora at wolf-ram.com
  • Sender: owner-wri-mathgroup at wolfram.com

Bruce Colletti wrote:
> Re Mathematica 5.2.0.0.
> 
> Since 0.7 is in the set {0.0, 0.1, 0.2,..., 0.9, 1.0}, why does MemberQ[Range[0., 1., .1], .7] return False?
>  
> Thankx.
> 
> Bruce

I don't know what function MemberQ uses to identify if something is part 
of the list, but it seems to be related to the precision of the 0.7:


In[8]:=
MemberQ[Range[0., 1.0, 0.1], 0.7]

Out[8]=
False

In[9]:=
MemberQ[Range[0., 1.0, 0.1], 0.1]

Out[9]=
True


In[19]:= Range[0.0, 1.0, 0.1] // InputForm

Out[19]//InputForm=
{0., 0.1, 0.2, 0.30000000000000004, 0.4, 0.5, 0.6000000000000001,
  0.7000000000000001, 0.8, 0.9, 1.}


Two alternatives:

1) use exact numbers

In[14]:=
MemberQ[Range[0, 1, 1/10], 7/10]

Out[14]=
True

2) Use a condition statement, which allows you to use the == function. 
Note that even == will not work in some cases, however.  It will depend 
on the precision of the numbers involved.

In[29]:= MemberQ[Range[0., 1.0, 0.1], x_ /; x == 0.7]

Out[29]= True

Igor

--
Igor C. Antonio
Wolfram Research, Inc.
http://www.wolfram.com

To email me personally, remove the dash.


  • References:
    • MemberQ
      • From: Bruce Colletti <vze269bv@verizon.net>
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