Re: questions about delayed expression.

*To*: mathgroup at smc.vnet.net*Subject*: [mg19884] Re: [mg19862] questions about delayed expression.*From*: "Andrzej Kozlowski" <andrzej at tuins.ac.jp>*Date*: Sun, 19 Sep 1999 18:47:34 -0400*Sender*: owner-wri-mathgroup at wolfram.com

The answer to the first question is easy. You need to use Inter[c_] = Interval[{sola[[1]], solb[[1]]}] and not :=. If you use := the right hand side is not evaluated until you evaluate Int[.3] but then of course Mathematica treats the c on the right hand side just as a symbol and does not substitute 0.3 for it. I can't think of any significantly simpler way of doing what you are asking about in your second question. The following approach is an alternative but is not really preferable. In[1]:= a[x_] := x + 4 In[2]:= b[x_] := -3 x + 8 In[3]:= sola = Roots[a[x] == c, x][[2]]; In[4]:= solb = Roots[b[x] == c, x][[2]]; In[5]:= Inter[c_] = Interval[{sola, solb}] ; In[6]:= Inter[0.3] Out[6]= Interval[{-3.7, 2.56667}] In[7]:= Inter[0.4] Out[7]= Interval[{-3.6, 2.53333}] -- Andrzej Kozlowski Toyama International University JAPAN http://sigma.tuins.ac.jp http://eri2.tuins.ac.jp ---------- >From: d8442803 at student.nsysu.edu.tw (Wen-Feng Hsiao) >To: mathgroup at smc.vnet.net >Subject: [mg19884] [mg19862] questions about delayed expression. >Date: Sun, Sep 19, 1999, 2:20 PM > > Hi, > > The following is the process that I run in my notebook. > > In[1]:= > a[x_] := x + 4 > b[x_] := -3 x + 8 > > In[2]:= > sola = x /. Solve[{a[x] == c}, x]; > solb = x /. Solve[{b[x] == c}, x]; > > In[3]:= > Inter[c_] := Interval[{sola[[1]], solb[[1]]}] > > In[4]:= > Inter[.3] > Inter[.4] > > Out[4]= > \!\(Interval[{\(-4\) + c, \(8 - c\)\/3}]\) > > Out[5]= > \!\(Interval[{\(-4\) + c, \(8 - c\)\/3}]\) > > My questions are: > 1. Why Inter[.3] and Inter[.4] cannot be evaluated? Their results should > not be the same. This is not my intention. > > 2. I don't know if there is any better way to extract the 'root(s)' from > the output of 'Solve' command. The output form is {{x->root1}, {x- >>root2}, ...{}}. If I use 'ReplaceAll'(/.) command, it will remain a list > of solutions of x. It seems I can only use element operation to extract > the root(s) from the solution list? In my case, I use sola[[1]] and > solb[[1]]. > > Please give me suggestions. Thanks for your help. >