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

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

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Jan 2023-3-1

编辑：Jan 2023-3-1

在 MATLAB Online 中打开

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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Answer 2

Pratheek 2023-3-1

0
链接

此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/1921070-how-to-make-symmetric-matrix-with-vector-of-its-non-repeated-elements#answer_1182655

在 MATLAB Online 中打开

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);