Can I assign a variable to a number in a matrix from another matrix?

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Anastasia Zistatsis 2021-3-2

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评论： Walter Roberson 2021-3-11

Using the matrix that comes from eig(), I would like to assign individual values in it to 3 separate variables, and then plug those into a separate matrix. However, I get an error. How do I do this?

E = eig(A)
E(1,1) = w1;
E(2,1) = w2;
E(3,1) = w3;
ans = [m1*w1^2 0 0;
       0    m2*w2^2 0;
       0 0 m3*w3^2 0]
Unrecognized function or variable 'w1'.
Error in eigen (line 23)
E(1,1) = w1;

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Answer 1

Walter Roberson 2021-3-2

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A = magic(7)
A = 7×7
    30    39    48     1    10    19    28
    38    47     7     9    18    27    29
    46     6     8    17    26    35    37
     5    14    16    25    34    36    45
    13    15    24    33    42    44     4
    21    23    32    41    43     3    12
    22    31    40    49     2    11    20
E = eig(A)
E = 7×1
  175.0000
  -56.4848
  -31.0882
  -25.3967
   56.4848
   25.3967
   31.0882
w1 = E(1,1)
w1 = 175
w2 = E(2,1)
w2 = -56.4848
w3 = E(3,1)
w3 = -31.0882
m1 = 3; m2 = 11; m3 = -7;
output = [
            m1*w1^2     0       0;
            0           m2*w2^2 0;
            0           0       m3*w3^2]
output = 3×3
    9.1875         0         0
         0    3.5096         0
         0         0   -0.6765

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Anastasia Zistatsis 2021-3-3

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I apologize for the weight, here you go @Walter Roberson

m1 = 1;
m2 = 1;
m3 = 1;
k = 2;
A = [2*k -k -k;
    -k 2*k -k;
    -k -k 2*k];
E = eig(A)
w1 = E(1,1)
w2 = E(2,1)
w3 = E(3,1)
output = [m1*(w1)^2    0          0;
            0       m2*(w2)^2     0;
            0          0      m3*(w3)^2]

Walter Roberson 2021-3-11

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syms k

A = [2*k -k -k;

-k 2*k -k;

-k -k 2*k];

E = eig(A)

E =

If you are expecting 2.449 then your k would have to be roughly 0.816

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Answer 2

Steven Lord 2021-3-2

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There's a way to do this without creating 2*n individual variables.

A = magic(7);
E = eig(A);
w = E(1:3);
m = [3; 11; -7];
output1 = diag(m.*(w.^2))
output1 = 3×3
    9.1875         0         0
         0    3.5096         0
         0         0   -0.6765
w1 = E(1,1);
w2 = E(2,1);
w3 = E(3,1);
m1 = 3; m2 = 11; m3 = -7;
output2 = [ m1*w1^2     0       0;
            0           m2*w2^2 0;
            0           0       m3*w3^2]
output2 = 3×3
    9.1875         0         0
         0    3.5096         0
         0         0   -0.6765