Re: Newbie question solving lin sys

[William Lash]

Whenever you type in the numbers for you calculation in s you are
losing precision.  Instead of typing in:

>octave:55> 22 + 129.32*s - 330.01*s**2 + 356.74*s**3 - 211.74*s**4 + 
>77.512*s**5 - 18.446*s**6 + 2.9092*s**7 - 0.30191*s**8 + 0.019818*s**9 - 
>0.0007457*s**10 + 0.00001225*s**11

Try the following:

x(1)+x(2)*s+x(3)*s**2+x(4)*s**3+x(5)*s**4+x(6)*s**5+x(7)*s**6+x(8)*s**7+x(9)*s**8+x(10)*s**9+x(11)*s**10+x(12)*s**11

There are better ways to write this in octave, one way is:

s.^[0:11] * x

and you could check all the values with a for loop:

for s = [0:11]
s.^[0:11] * x
end


Bill

---------- Original Message ----------------------------------
From: "John B. Thoo" <address@hidden>
Date:  Fri, 27 Dec 2002 13:40:50 -0800

>Hi.  I'm very new to using Octave.  I have two questions: 1. What am I 
>doing wrong in the following?  2. How can I do it more elegantly?
>
>
>I want to solve the system  Ax = T  where  A  is the  12x12  Vandermonde 
>matrix
>
>       [ 1  0  0   ...  0    ]
>       [ 1  1  1   ...  1    ]
>   A = [ 1  2  2^2 ...  2^11 ]
>       [           ...       ]
>       [ 1 11 11^2 ... 11^11 ]
>
>
>   T = [ 22 28 31 39 46 53 56 55 49 42 33 27 ]'
>
>To do this, I enter in Octave:
>
>octave:51> A = fliplr(vander([0,1,2,3,4,5,6,7,8,9,10,11]));
>octave:52> T = [22;28;31;39;46;53;56;55;49;42;33;27];
>octave:53> A\T
>ans =
>
>     2.2000e+01
>     1.2932e+02
>    -3.3001e+02
>     3.5674e+02
>    -2.1174e+02
>     7.7512e+01
>    -1.8446e+01
>     2.9092e+00
>    -3.0191e-01
>     1.9818e-02
>    -7.4570e-04
>     1.2250e-05
>
>*But* when I go to check my answers, they seem to be off.  To check my 
>answers, I enter in Octave:
>
>octave:54> s = 0;
>octave:55> 22 + 129.32*s - 330.01*s**2 + 356.74*s**3 - 211.74*s**4 + 
>77.512*s**5 - 18.446*s**6 + 2.9092*s**7 - 0.30191*s**8 + 0.019818*s**9 - 
>0.0007457*s**10 + 0.00001225*s**11
>ans = 22
>octave:56> s = 1;
>octave:57> 22 + 129.32*s - 330.01*s**2 + 356.74*s**3 - 211.74*s**4 + 
>77.512*s**5 - 18.446*s**6 + 2.9092*s**7 - 0.30191*s**8 + 0.019818*s**9 - 
>0.0007457*s**10 + 0.00001225*s**11
>ans = 28.002
>octave:58> s = 3;
>
>and so on.  The answers match well for the first few, but then they 
>drift.  To summarize, I get
>
>      s  |  answers from Octave  |  supposed to be (T)
>    ---------------------------------------------------
>      0  |        22             |        22
>      1  |        28.002         |        28
>      2  |        31.017         |        31
>      3  |        38.996         |        39
>      4  |        45.498         |        46
>      5  |        50.205         |        53
>      6  |        46.456         |        56
>      7  |        30.029         |        55
>      8  |       - 5.5781        |        49
>      9  |       -62.227         |        42
>     10  |      -145.80          |        33
>     11  |      -245.29          |        27
>
>
>TIA for your help.
>
>---John.
>
>
>
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Octave is freely available under the terms of the GNU GPL.

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