# how to make symmetric matrix with vector of its non repeated elements?

8 visualizzazioni (ultimi 30 giorni)
Modificato: Jan il 1 Mar 2023
suppose we have non repeated elements of a matrix in a vector . I need to make a symmetric matrix by using this vector in matlab.
for example if we have :
v= [ 1 4 5 6 9 0]
the answer must be the matrix:
1 4 5
4 6 9
5 9 0
##### 1 CommentoMostra NessunoNascondi Nessuno
Luca Ferro il 1 Mar 2023
could you better define the rules of the reshape?

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### Risposta accettata

Jan il 1 Mar 2023
Modificato: Jan il 1 Mar 2023
v = [1 4 5 6 9 0];
d = numel(v);
n = (sqrt(8 * d + 1) - 1) / 2; % Number of rows and colums
A = zeros(n, n);
A(tril(true(n, n))) = v;
A(triu(true(n, n))) = v % [EDITED, faster than transposing the mask]
A = 3×3
1 4 6 4 5 9 5 9 0
Or with a loop (assumably faster for large arrays):
k = 0;
A = zeros(n, n);
for i2 = 1:n
k = k + 1; % Diagonal element
A(i2, i2) = v(k);
for i1 = i2 + 1:n % Below and above diagonal
k = k + 1;
A(i1, i2) = v(k);
A(i2, i1) = v(k);
end
end
A
A = 3×3
1 4 5 4 6 9 5 9 0
##### 1 CommentoMostra NessunoNascondi Nessuno
Jan il 1 Mar 2023
Modificato: Jan il 1 Mar 2023
Some speed comparisons:
tic
for k = 1:1e3
n = (k * (k+1)) / 2;
A = ToSymMatrix_vec(1:n);
end
toc
Elapsed time is 1.327950 seconds.
tic
for k = 1:1e3
n = (k * (k+1)) / 2;
A = ToSymMatrix_loop(1:n);
end
toc
Elapsed time is 0.839996 seconds.
function A = ToSymMatrix_vec(v)
d = numel(v);
n = (sqrt(8 * d + 1) - 1) / 2; % Number of rows and colums
A = zeros(n, n);
A(tril(true(n, n))) = v;
A(triu(true(n, n))) = v;
end
function A = ToSymMatrix_loop(v)
d = numel(v);
n = (sqrt(8 * d + 1) - 1) / 2; % Number of rows and colums
k = 0;
A = zeros(n, n);
for i2 = 1:n
k = k + 1; % Diagonal element
A(i2, i2) = v(k);
for i1 = i2 + 1:n % Below and above diagonal
k = k + 1;
A(i1, i2) = v(k);
A(i2, i1) = v(k);
end
end
end

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### Più risposte (1)

Pratheek il 1 Mar 2023
The first line of the code is to input for the size of the matrix that you want to generate.
% ask the user for the size of the matrix
n = input('Enter the size of the square matrix: ');
% get the input vector
v = [1 4 5 6 9 0]; % or prompt the user to enter the vector as well
% create the symmetric matrix
M = zeros(n);
k = 1;
for i = 1:n
for j = (i+1):n
M(i,j) = v(k);
M(j,i) = v(k);
M(i,i) = v(randi(length(v)));
k = k + 1;
end
end
% display the resulting matrix
disp(M);
##### 1 CommentoMostra NessunoNascondi Nessuno
Jan il 1 Mar 2023
Why do you set the diagonal elements randomly and repeatedly in each iteration of the inner loop?

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