Octave中find函数用法

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本文详细介绍了 Octave 中 find 函数的使用方法及其参数选项，包括如何通过 find 函数定位矩阵中的非零元素索引，并展示了不同参数组合下 find 函数的行为变化。

https://zhuanlan.zhihu.com/p/22484964 链接是知乎上总结的Octave/Matlab教程-Coursera-斯坦福-机器学习

下面是find的用法

'find' is a built-in function from the file libinterp/corefcn/find.cc

-- IDX = find (X)
-- IDX = find (X, N)
-- IDX = find (X, N, DIRECTION)
-- [i, j] = find (...)
-- [i, j, v] = find (...)

Return a vector of indices of nonzero elements of a matrix, as a row if X is a row vector or as a column otherwise.

返回一个矩阵的非零元素的索引向量，如果X是一个行向量则返回的IDX是一个行向量，如果X是列向量,，则返回列向量

比如行向量 X=[0,1,2,3]，find(X)返回[2 3 4];列向量X=[0;1;2;3]，find(X)返回[2;3; 4].。X=[0,1;2,3]两行两列，find(X)返回[2;3; 4]

find (eye (2))
=> [ 1; 4 ]

-- IDX = find (X, N)

If two inputs are given, N indicates the maximum number of elements to find from the beginning of the matrix or vector.
如果有两个输入，N表示从矩阵或者向量的开始位置返回非零元素索引值的最大数量，比如说X=[0;1;2;3]，非零索引是2,3,4，所以find(X,2)表示返回数最多两个，就是[2,3]

如果N=3或者3以上，结果都为[2;3;4];

-- IDX = find (X, N, DIRECTION)
If three inputs are given, DIRECTION should be one of "first" or "last", requesting only the first or last N indices, respectively.
However, the indices are always returned in ascending order.

如果有三个输入，direction为"first"或者"last",first表明返回前面N个索引值，last表明返回最后N个索引值，比如X=[0;1;2;3]，find(X,2,"last")为[3;4]

If two outputs are requested, 'find' returns the row and column indices of nonzero elements of a matrix. For example:

如果有两个输出，find返回一个矩阵的行和列索引值，i为行索引，j为列索引。

[i, j] = find (2 * eye (2))
=> i = [ 1; 2 ]
=> j = [ 1; 2 ]

If three outputs are requested, 'find' also returns a vector containing the nonzero values. For example:

如果有两个输出，find返回一个矩阵的行和列索引值，i为行索引，j为列索引，v为矩阵内的所有非零元素。

[i, j, v] = find (3 * eye (2))
=> i = [ 1; 2 ]
=> j = [ 1; 2 ]
=> v = [ 3; 3 ]

Note that this function is particularly useful for sparse matrices, as it extracts the nonzero elements as vectors, which can then be

used to create the original matrix. For example:

sz = size (a);
[i, j, v] = find (a);

b = sparse (i, j, v, sz(1), sz(2));

See also: nonzeros.

Additional help for built-in functions and operators is available in the online version of the manual. Use the command 'doc <topic>' to search the manual index.

Help and information about Octave is also available on the WWW at http://www.octave.org and via the help@octave.org mailing list.