Octave – Gestione matrici – IV – 85

ww2

Sempre nella gestione matici, da qui continuo copiando qui.

Built-in Function: [s, i] = sort (x)
Built-in Function: [s, i] = sort (x, dim)
Built-in Function: [s, i] = sort (x, mode)
Built-in Function: [s, i] = sort (x, dim, mode)

Return a copy of x with the elements arranged in increasing order.
For matrices, sort orders the elements within columns

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If the optional argument dim is given, then the matrix is sorted along the dimension defined by dim. The optional argument mode defines the order in which the values will be sorted. Valid values of mode are "ascend" or "descend".
The sort function may also be used to produce a matrix containing the original row indices of the elements in the sorted matrix.

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For equal elements, the indices are such that equal elements are listed in the order in which they appeared in the original list.
Sorting of complex entries is done first by magnitude (abs (z)) and for any ties by phase angle (angle (z)).

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Nota: usando i mi da errore; non so perché; chiudo la REPL e rilancio, tutto OK

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NaN values are treated as being greater than any other value and are sorted to the end of the list.
The sort function may also be used to sort strings and cell arrays of strings, in which case ASCII dictionary order (uppercase ’A’ precedes lowercase ’a’) of the strings is used.
The algorithm used in sort is optimized for the sorting of partially ordered lists.

Function File: [s, i] = sortrows (A)
Function File: [s, i] = sortrows (A, c)

Sort the rows of the matrix A according to the order of the columns specified in c.
By default (c omitted, or a particular column unspecified in c) an ascending sort order is used. However, if elements of c are negative then the corresponding column is sorted in descending order. If the elements of A are strings then a lexicographical sort is used.
Example: sort by column 2 in descending order, then 3 in ascending order.

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Built-in Function: issorted (a)
Built-in Function: issorted (a, mode)
Built-in Function: issorted (a, "rows", mode)

Return true if the array is sorted according to mode, which may be either "ascending", "descending", or "either".
By default, mode is "ascending". NaNs are treated in the same manner as sort.
If the optional argument "rows" is supplied, check whether the array is sorted by rows as output by the function sortrows (with no options).
This function does not support sparse matrices.

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Built-in Function: nth_element (x, n)
Built-in Function: nth_element (x, n, dim)

Select the n-th smallest element of a vector, using the ordering defined by sort.
The result is equivalent to sort(x)(n).
n can also be a contiguous range, either ascending l:u or descending u:-1:l, in which case a range of elements is returned.
If x is an array, nth_element operates along the dimension defined by dim, or the first non-singleton dimension if dim is not given.
Programming Note: nth_element encapsulates the C++ standard library algorithms nth_element and partial_sort. On average, the complexity of the operation is O(M*log(K)), where M = size (x, dim) and K = length (n). This function is intended for cases where the ratio K/M is small; otherwise, it may be better to use sort.

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Function File: tril (A)
Function File: tril (A, k)
Function File: tril (A, k, pack)
Function File: triu (A)
Function File: triu (A, k)
Function File: triu (A, k, pack)

Nota perso: “tril” non è una parola esistente; Octave (anzi Matlab) l’ha creata da “triangular + lower”

Return a new matrix formed by extracting the lower (tril) or upper (triu) triangular part of the matrix A, and setting all other elements to zero.
The second argument is optional, and specifies how many diagonals above or below the main diagonal should also be set to zero.
The default value of k is zero, so that triu and tril normally include the main diagonal as part of the result.
If the value of k is nonzero integer, the selection of elements starts at an offset of k diagonals above or below the main diagonal; above for positive k and below for negative k.
The absolute value of k must not be greater than the number of subdiagonals or superdiagonals.

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If the option "pack" is given as third argument, the extracted elements are not inserted into a matrix, but rather stacked column-wise one above other.

Built-in Function: v = vec (x)
Built-in Function: v = vec (x, dim)

Return the vector obtained by stacking the columns of the matrix x one above the other.
Without dim this is equivalent to x(:).
If dim is supplied, the dimensions of v are set to dim with all elements along the last dimension. This is equivalent to shiftdim (x(:), 1-dim).

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Function File: vech (x)
Return the vector obtained by eliminating all superdiagonal elements of the square matrix x and stacking the result one column above the other.
This has uses in matrix calculus where the underlying matrix is symmetric and it would be pointless to keep values above the main diagonal.

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Function File: prepad (x, l)
Function File: prepad (x, l, c)
Function File: prepad (x, l, c, dim)

Prepend the scalar value c to the vector x until it is of length l. If c is not given, a value of 0 is used.
If length (x) > l, elements from the beginning of x are removed until a vector of length l is obtained.
If x is a matrix, elements are prepended or removed from each row.
If the optional argument dim is given, operate along this dimension.
If dim is larger than the dimensions of x, the result will have dim dimensions.

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Function File: postpad (x, l)
Function File: postpad (x, l, c)
Function File: postpad (x, l, c, dim)

Append the scalar value c to the vector x until it is of length l. If c is not given, a value of 0 is used.
If length (x) > l, elements from the end of x are removed until a vector of length l is obtained.
If x is a matrix, elements are appended or removed from each row.
If the optional argument dim is given, operate along this dimension.
If dim is larger than the dimensions of x, the result will have dim dimensions.

Built-in Function: M = diag (v)
Built-in Function: M = diag (v, k)
Built-in Function: M = diag (v, m, n)
Built-in Function: v = diag (M)
Built-in Function: v = diag (M, k)

Return a diagonal matrix with vector v on diagonal k.
The second argument is optional. If it is positive, the vector is placed on the k-th superdiagonal. If it is negative, it is placed on the -k-th subdiagonal. The default value of k is 0, and the vector is placed on the main diagonal.

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The 3-input form returns a diagonal matrix with vector v on the main diagonal and the resulting matrix being of size m rows x n columns.
Given a matrix argument, instead of a vector, diag extracts the k-th diagonal of the matrix.

Function File: blkdiag (A, B, C, ...)
Build a block diagonal matrix from A, B, C, ...
All arguments must be numeric and either two-dimensional matrices or scalars. If any argument is of type sparse, the output will also be sparse.

o464

Mi sembra di essere tornato indietro di 30 anni; e se allora avessi avuto Octave che bello sarebbe stato 😀

:mrgreen:

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