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## 3.3 Object Sizes

The following functions allow you to determine the size of a variable or expression. These functions are defined for all objects. They return -1 when the operation doesn't make sense. For example, Octave's data structure type doesn't have rows or columns, so the `rows` and `columns` functions return -1 for structure arguments.

Built-in Function: ndims (a)

Returns the number of dimensions of array a. For any array, the result will always be larger than or equal to 2. Trailing singleton dimensions are not counted.

Built-in Function: columns (a)

Return the number of columns of a.

Built-in Function: rows (a)

Return the number of rows of a.

Built-in Function: numel (a)

Returns the number of elements in the object a.

Built-in Function: length (a)

Return the `length' of the object a. For matrix objects, the length is the number of rows or columns, whichever is greater (this odd definition is used for compatibility with MATLAB).

Built-in Function: size (a, n)

Return the number rows and columns of a.

With one input argument and one output argument, the result is returned in a row vector. If there are multiple output arguments, the number of rows is assigned to the first, and the number of columns to the second, etc. For example,

 ```size ([1, 2; 3, 4; 5, 6]) ⇒ [ 3, 2 ] [nr, nc] = size ([1, 2; 3, 4; 5, 6]) ⇒ nr = 3 ⇒ nc = 2 ```

If given a second argument, `size` will return the size of the corresponding dimension. For example

 ```size ([1, 2; 3, 4; 5, 6], 2) ⇒ 2 ```

returns the number of columns in the given matrix.

Built-in Function: isempty (a)

Return 1 if a is an empty matrix (either the number of rows, or the number of columns, or both are zero). Otherwise, return 0.

Built-in Function: isnull (x)

Return 1 if x is a special null matrix, string or single quoted string. Indexed assignment with such a value as right-hand side should delete array elements. This function should be used when overloading indexed assignment for user-defined classes instead of `isempty`, to distinguish the cases:

`A(I) = []`

This should delete elements if `I` is nonempty.

`X = []; A(I) = X`

This should give an error if `I` is nonempty.

Built-in Function: sizeof (val)

Return the size of val in bytes

Built-in Function: size_equal (a, b, …)

Return true if the dimensions of all arguments agree. Trailing singleton dimensions are ignored. Called with a single argument, size_equal returns true.

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