RE: Simple Concept I can't figure out! Making rules?
- To: mathgroup at smc.vnet.net
- Subject: [mg38084] RE: [mg38046] Simple Concept I can't figure out! Making rules?
- From: "David Park" <djmp at earthlink.net>
- Date: Tue, 3 Dec 2002 04:29:00 -0500 (EST)
- Sender: owner-wri-mathgroup at wolfram.com
Stan, I think it is a shame that any engineering college would frown upon using Mathematica. If used correctly, it can only speed learning and give you a lot more practice. It is also a shame that students can't come to college already knowing the basics of Mathematica so they could concentrate on the material to be learned. The argument that the student is supposed to be learning methods and not use Mathematica's advanced automatic routines: one can always skip the advanced routines and program Mathematica to do the steps of the method to be learned. Anyway, on to your question. You could do something like this... xsol = Solve[2x - 1 == 0, x][[1, 1]] answer = x /. xsol // N which returns x -> 1/2 0.5 answer 0.5 Look up ReplaceAll and Rule in Help. What I like to do is save xsol as a rule and not store the x value in some variable answer, or in x itself. I would also tend to stay with exact solutions as long as possible. Then you can use xsol and N something like this... x^2 + Sin[x] % /. xsol % // N which gives x^2 + Sin[x] 1/4 + Sin[1/2] 0.729426 Remember that % means the previous output (in time) and when you use it for statements within one cell it is completely unambiguous. /. is the shortcut notation for ReplaceAll. So % /. xsol means to take the previous output, and replace all occurrences of x using the rule xsol (which is x -> 1/2). David Park djmp at earthlink.net http://home.earthlink.net/~djmp/ From: Stan [mailto:snarten at runbox.com] To: mathgroup at smc.vnet.net Even though it's frowned upon by the engineering college, i use mathematica a lot of the stuff that would take pages of code in another system. I have most basic functions and operations down, but I am lacking one basic thing I need to do to be able to program more problems successfully: here is my problem: Lets say you solve an equation using "Solve" Solve[2x - 1 == 0, x] // N {{x -> 0.5}} How would I then use the result of "Solve" (0.5) and assign it to a new variable, like "answer" for further calculations? I think it has something to do with /. and -> rules of some sort, but I can't get this simple concept to work! Thanks for the help. -Stan