diag
diag(x)
diag(x, k)
Create a diagonal matrix or retrieve the diagonal of a matrix. When x is a vector, a matrix with the vector values on the diagonal will be returned. When x is a matrix, a vector with the diagonal values of the matrix is returned. When k is provided, the k-th diagonal will be filled in or retrieved, if k is positive, the values are placed on the super diagonal. When k is negative, the values are placed on the sub diagonal.
Try it yourself:
| $1 | diag(1:3)
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| $2 | diag(1:3, 1)
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| $3 | a = [1, 2, 3; 4, 5, 6; 7, 8, 9]
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| $4 | diag(a)
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