Octave中find函数用法

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 [e-mail] mailing list.