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