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MathGroup Archive 1995

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Re: How do you input a solution from FindRoot into another equation?

  • To: mathgroup at christensen.cybernetics.net
  • Subject: [mg760] Re: How do you input a solution from FindRoot into another equation?
  • From: wagner at bullwinkle.cs.Colorado.EDU (Dave Wagner)
  • Date: Tue, 18 Apr 1995 00:51:00 -0400
  • Organization: University of Colorado, Boulder

In article <3mfauj$l4o at news0.cybernetics.net>,
Robert Dack <rdack at acs.ucalgary.ca> wrote:
>
>Can someone tell me how to take the output from the FindRoot
>cammand and input it into the next command line?  The output
>comes out in the form { x -> 2.435 }, so the usual [%] command on
>the next line won't work.
>

You can extract the right-hand side of a rule by subscripting part 2:

(x -> 2.435)[[2]] => 2.435

In the example, since the rule is inside a list, you would need to use the
subscript [[1,2]], as shown below:

In[18]:=
    FindRoot[BesselJ[0,x], {x,2.}]
Out[18]=
    {x -> 2.40483}

In[19]:=
    %[[1,2]]
Out[19]=
    2.40483

Note that the Solve command returns doubly-nested lists. Here are
some examples of how to extract the answers.

In[20]:=
    Solve[x^2==2, x]
Out[20]=
    {{x -> -Sqrt[2]}, {x -> Sqrt[2]}}

In[21]:=
    { %[[1,1,2]], %[[2,1,2]] }
Out[21]=
    {-Sqrt[2], Sqrt[2]}

In[22]:=
    Solve[{x^2-2 == y, x+y==0}]
Out[22]=
    {{y -> -1, x -> 1}, {y -> 2, x -> -2}}

In[23]:=
    { %[[1,1,2]], %[[1,2,2]], %[[2,1,2]] }
Out[23]=
    {-1, 1, 2}

Extracting the x-value of the second solution in the last example
is left as an exercise for the reader.


		Dave Wagner
		Principia Consulting
		(303) 786-8371
		princon at csn.net
		http://www.csn.net/princon


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