Re: ???? How to assign result from Solve to variable ????

*To*: mathgroup at smc.vnet.net*Subject*: [mg3846] Re: ???? How to assign result from Solve to variable ????*From*: ianc (Ian Collier)*Date*: Mon, 29 Apr 1996 00:33:02 -0400*Organization*: Wolfram Research, Inc.*Sender*: owner-wri-mathgroup at wolfram.com

In article <4let71$e4m at dragonfly.wolfram.com>, rpiyaket at ucsd.edu (Ram Piyaket) wrote: > Hi, > I was wondering if anyone could tell me how to deal with this problem. > I want to assign the numerical result that I obtain from the Solve or etc. > command, which typically enclosed in the braces ({...}) to a variable > so that I can reuse it. I just want the numerical number, not the braces. > Can you tell me how? I appreciate all the help from you. Thank you very much. > > Ram The following is taken from the Technical Support FAQ area of Wolfram Research's Web pages. The exact URL is: http://www.wolfram.com/support/Kernel/Basic/UsingRuleSolutions.html. How do I extract a solution from the results of Solve or FindRoot? Solve and FindRoot typically return something like the following: {{x -> 2.5}} At first, it may seem a little confusing, but there is a very good reason for it. The reason Solve or FindRoot doesn't simply return a number, such as 2.5, is because sometimes you want to solve for more than one variable. If Mathematica just returned numbers, you would never know which number belonged to which variable -- especially, if each variable had more than one solution. For example: In[1]:= Solve[{x^2 + y^2 == 1, x + y==.5},{x,y}] Out[1]= {{x -> -0.411438, y -> 0.911438}, {x -> 0.911438, y -> -0.411438}} Without showing the attachment between the solution and the variable names, there is no way that we would know for sure which solutions were for x and which ones were for y, or that -.411438 is a solution for x if and only if .911438 is the solution for y. What Mathematica is returning is a collection of rules. Rules are a well-defined and well-documented concept in Mathematica, and they are easy to extract. If you want to pull all of the rules for x out of the result of the above Solve command, you can do the following: In[2]:= x /. % Out[2]= {-0.411438, 0.911438} The /. is shorthand for ReplaceAll, and causes Mathematica to replace all instances of x on the left-hand side with the rule(s) for x on the right-hand side. In fact, the left-hand side can be any expression of x, y, or any other variables: In[3]:= x^2 - y/2 + x y z /. %% Out[3]= {-0.286438 - 0.375 z, 1.03644 - 0.375 z} This returns the expression evaluated at each of the two solutions; note that z remains unevaluated in the solution because z was not a rule. This and many other frequently asked questions about Mathematica is answered in the Technical Support FAQ area of the Wolfram Research Web pages <http://www.wolfram.com/support/>. I hope this helps. --Ian ----------------------------------------------------------- Ian Collier Wolfram Research, Inc. ----------------------------------------------------------- tel:(217) 398-0700 fax:(217) 398-0747 ianc at wolfram.com Wolfram Research Home Page: http://www.wolfram.com/ ----------------------------------------------------------- ==== [MESSAGE SEPARATOR] ====

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**???? How to assign result from Solve to variable ????**

**Re: ???? How to assign result from Solve to variable ????**