Thread Subject: non-complex elements

When using the 'solve' command, I am getting a solution
vector where I am only interested in the non-complex
element. How can I take only the real values of a vector?
Thanks.

lookfor real

Well, there's isreal, of course, but I'd rather not loop
through each element and apply that command. There must be
a way to use the solve function to only give the real
solutions?

"Shaun " <s@s.com> wrote in message
<foj8ce$r3v$1@fred.mathworks.com>...
> lookfor real

>> x = roots([3 7 0 7 3])
x =
  -2.6180
   0.3333 + 0.9428i
   0.3333 - 0.9428i
  -0.3820
>> x(imag(x)==0)
ans =
   -2.6180
   -0.3820

Ahh, see, I tried this to but no go. It must have something
to do with the 'solve' function?

EDU>> [x,y]=solve('2.8^2*32^2/10e3=x*y','y=x/10e3+(x/213)^4.5')
 
x =
 
 
41.722407862147426448707782826110-73.507292921775458350218631802263*i
 -59.669718037824467652279669253703+53.044012500010886727552493671779*i
 
41.722407862147426448707782826110+73.507292921775458350218631802263*i
                                     
66.677124117691249035503047088223
 -59.669718037824467652279669253703-53.044012500010886727552493671779*i
 
 
 
y =
 
 
.46885547030920421417850750011565e-2+.82603804909503165355563636007046e-2*i
 -.75153331228391432010200444372733e-2-.66808330459501486397430163934960e-2*i
 
.46885547030920421417850750011565e-2-.82603804909503165355563636007046e-2*i
                                        
.12040351329234836422099920311077e-1
 -.75153331228391432010200444372733e-2+.66808330459501486397430163934960e-2*i
 
 
EDU>> x(imag(x)==0)
 
ans =
 
[ empty sym ]
 
 
EDU>> y(imag(y)==0)
 
ans =
 
[ empty sym ]

"Shaun " <s@s.com> wrote in message
<fojbqt$e8m$1@fred.mathworks.com>...
> >> x = roots([3 7 0 7 3])
> x =
> -2.6180
> 0.3333 + 0.9428i
> 0.3333 - 0.9428i
> -0.3820
> >> x(imag(x)==0)
> ans =
> -2.6180
> -0.3820

In article <foj731$a8d$1@fred.mathworks.com>,
Steve <steveDEL.bachmeierDEL@yahoo.com> wrote:
>When using the 'solve' command, I am getting a solution
>vector where I am only interested in the non-complex
>element. How can I take only the real values of a vector?

At the maple level, you would use rsolve. Placing assumptions
on your variables can also help, but even if all of your variables
are real, complex results can still be generated.

Selecting for Im(x) == 0 can be useful; though it would likely
be more useful to solve() for Im(x) = 0, as there might be
important conditions in which the imaginary term vanishes. The
presence of an imaginary term in a solution does not automatically
imply that the solution is non-real: such a solution might be
real (and distinct from the other solutions) under some conditions.
--
   "Beware of bugs in the above code; I have only proved it correct,
   not tried it." -- Donald Knuth

>> z = solve('3*x^4+7*x^3+7*x+3')
z =
   1/2*5^(1/2)-3/2
  -3/2-1/2*5^(1/2)
 1/3+2/3*i*2^(1/2)
 1/3-2/3*i*2^(1/2)
>> z = eval(z)
z =
  -0.3820
  -2.6180
   0.3333 + 0.9428i
   0.3333 - 0.9428i
>> z = z(imag(z)==0)
z =
   -0.3820
   -2.6180

OK, thanks a lot; this seems to work. How come I have to
eval() the vectors first before imag()==0 will work?

"Shaun " <s@s.com> wrote in message
<fokbh9$1of$1@fred.mathworks.com>...
> >> z = solve('3*x^4+7*x^3+7*x+3')
> z =
> 1/2*5^(1/2)-3/2
> -3/2-1/2*5^(1/2)
> 1/3+2/3*i*2^(1/2)
> 1/3-2/3*i*2^(1/2)
> >> z = eval(z)
> z =
> -0.3820
> -2.6180
> 0.3333 + 0.9428i
> 0.3333 - 0.9428i
> >> z = z(imag(z)==0)
> z =
> -0.3820
> -2.6180