MATLAB Central Newsreader - non-complex elements

non-complex elements

Steve — Sat, 09 Feb 2008 03:36:01 -0500

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.

Re: non-complex elements

Shaun — Sat, 09 Feb 2008 03:58:06 -0500

lookfor real

Re: non-complex elements

Steve — Sat, 09 Feb 2008 04:28:01 -0500

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

Re: non-complex elements

Shaun — Sat, 09 Feb 2008 04:57:01 -0500

>> 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

Re: non-complex elements

Steve — Sat, 09 Feb 2008 05:46:02 -0500

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

Re: non-complex elements

roberson@ibd.nrc-cnrc.gc.ca (Walter Roberson) — Sat, 09 Feb 2008 07:43:16 -0500

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

Re: non-complex elements

Shaun — Sat, 09 Feb 2008 13:58:01 -0500

>> 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

Re: non-complex elements

Steve — Sat, 09 Feb 2008 19:06:02 -0500

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