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<title>Object Sizes - GNU Octave</title>
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<p>
Previous: <a rel="previous" accesskey="p" href="User_002ddefined-Data-Types.html#User_002ddefined-Data-Types">User-defined Data Types</a>,
Up: <a rel="up" accesskey="u" href="Data-Types.html#Data-Types">Data Types</a>
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<h3 class="section">3.3 Object Sizes</h3>
<p>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 <code>rows</code> and
<code>columns</code> functions return −1 for structure arguments.
<!-- ndims src/data.cc -->
<p><a name="doc_002dndims"></a>
<div class="defun">
— Built-in Function: <b>ndims</b> (<var>a</var>)<var><a name="index-ndims-228"></a></var><br>
<blockquote><p>Return the number of dimensions of <var>a</var>.
For any array, the result will always be larger than or equal to 2.
Trailing singleton dimensions are not counted.
<pre class="example"> ndims (ones (4, 1, 2, 1))
⇒ 3
</pre>
</blockquote></div>
<!-- columns src/data.cc -->
<p><a name="doc_002dcolumns"></a>
<div class="defun">
— Built-in Function: <b>columns</b> (<var>a</var>)<var><a name="index-columns-229"></a></var><br>
<blockquote><p>Return the number of columns of <var>a</var>.
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<p class="noindent"><strong>See also:</strong> <a href="doc_002drows.html#doc_002drows">rows</a>, <a href="doc_002dsize.html#doc_002dsize">size</a>, <a href="doc_002dlength.html#doc_002dlength">length</a>, <a href="doc_002dnumel.html#doc_002dnumel">numel</a>, <a href="doc_002disscalar.html#doc_002disscalar">isscalar</a>, <a href="doc_002disvector.html#doc_002disvector">isvector</a>, <a href="doc_002dismatrix.html#doc_002dismatrix">ismatrix</a>.
</p></blockquote></div>
<!-- rows src/data.cc -->
<p><a name="doc_002drows"></a>
<div class="defun">
— Built-in Function: <b>rows</b> (<var>a</var>)<var><a name="index-rows-230"></a></var><br>
<blockquote><p>Return the number of rows of <var>a</var>.
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<p class="noindent"><strong>See also:</strong> <a href="doc_002dcolumns.html#doc_002dcolumns">columns</a>, <a href="doc_002dsize.html#doc_002dsize">size</a>, <a href="doc_002dlength.html#doc_002dlength">length</a>, <a href="doc_002dnumel.html#doc_002dnumel">numel</a>, <a href="doc_002disscalar.html#doc_002disscalar">isscalar</a>, <a href="doc_002disvector.html#doc_002disvector">isvector</a>, <a href="doc_002dismatrix.html#doc_002dismatrix">ismatrix</a>.
</p></blockquote></div>
<!-- numel src/data.cc -->
<p><a name="doc_002dnumel"></a>
<div class="defun">
— Built-in Function: <b>numel</b> (<var>a</var>)<var><a name="index-numel-231"></a></var><br>
— Built-in Function: <b>numel</b> (<var>a, idx1, idx2, <small class="dots">...</small></var>)<var><a name="index-numel-232"></a></var><br>
<blockquote><p>Return the number of elements in the object <var>a</var>.
Optionally, if indices <var>idx1</var>, <var>idx2</var>, <small class="dots">...</small> are supplied,
return the number of elements that would result from the indexing
<pre class="example"> <var>a</var>(<var>idx1</var>, <var>idx2</var>, ...)
</pre>
<p>Note that the indices do not have to be numerical. For example,
<pre class="example"> <var>a</var> = 1;
<var>b</var> = ones (2, 3);
numel (<var>a</var>, <var>b</var>);
</pre>
<p class="noindent">will return 6, as this is the number of ways to index with <var>b</var>.
<p>This method is also called when an object appears as lvalue with cs-list
indexing, i.e., <code>object{...}</code> or <code>object(...).field</code>.
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<p class="noindent"><strong>See also:</strong> <a href="doc_002dsize.html#doc_002dsize">size</a>.
</p></blockquote></div>
<!-- length src/data.cc -->
<p><a name="doc_002dlength"></a>
<div class="defun">
— Built-in Function: <b>length</b> (<var>a</var>)<var><a name="index-length-233"></a></var><br>
<blockquote><p>Return the "length" of the object <var>a</var>. For matrix objects, the
length is the number of rows or columns, whichever is greater (this
odd definition is used for compatibility with <span class="sc">matlab</span>).
</p></blockquote></div>
<!-- size src/data.cc -->
<p><a name="doc_002dsize"></a>
<div class="defun">
— Built-in Function: <b>size</b> (<var>a</var>)<var><a name="index-size-234"></a></var><br>
— Built-in Function: <b>size</b> (<var>a, dim</var>)<var><a name="index-size-235"></a></var><br>
<blockquote><p>Return the number of rows and columns of <var>a</var>.
<p>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:
<pre class="example"> size ([1, 2; 3, 4; 5, 6])
⇒ [ 3, 2 ]
[nr, nc] = size ([1, 2; 3, 4; 5, 6])
⇒ nr = 3
⇒ nc = 2
</pre>
<p>If given a second argument, <code>size</code> will return the size of the
corresponding dimension. For example,
<pre class="example"> size ([1, 2; 3, 4; 5, 6], 2)
⇒ 2
</pre>
<p class="noindent">returns the number of columns in the given matrix.
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<p class="noindent"><strong>See also:</strong> <a href="doc_002dnumel.html#doc_002dnumel">numel</a>.
</p></blockquote></div>
<!-- isempty src/data.cc -->
<p><a name="doc_002disempty"></a>
<div class="defun">
— Built-in Function: <b>isempty</b> (<var>a</var>)<var><a name="index-isempty-236"></a></var><br>
<blockquote><p>Return true if <var>a</var> is an empty matrix (any one of its dimensions is
zero). Otherwise, return false.
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<p class="noindent"><strong>See also:</strong> <a href="doc_002disnull.html#doc_002disnull">isnull</a>.
</p></blockquote></div>
<!-- isnull src/ov-null-mat.cc -->
<p><a name="doc_002disnull"></a>
<div class="defun">
— Built-in Function: <b>isnull</b> (<var>x</var>)<var><a name="index-isnull-237"></a></var><br>
<blockquote><p>Return true if <var>x</var> is a special null matrix, string, or single quoted
string. Indexed assignment with such a value on the right-hand side should
delete array elements. This function should be used when overloading
indexed assignment for user-defined classes instead of <code>isempty</code>, to
distinguish the cases:
<dl>
<dt><code>A(I) = []</code><dd>This should delete elements if <code>I</code> is nonempty.
<br><dt><code>X = []; A(I) = X</code><dd>This should give an error if <code>I</code> is nonempty.
</dl>
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<p class="noindent"><strong>See also:</strong> <a href="doc_002disempty.html#doc_002disempty">isempty</a>, <a href="doc_002disindex.html#doc_002disindex">isindex</a>.
</p></blockquote></div>
<!-- sizeof src/ov.cc -->
<p><a name="doc_002dsizeof"></a>
<div class="defun">
— Built-in Function: <b>sizeof</b> (<var>val</var>)<var><a name="index-sizeof-238"></a></var><br>
<blockquote><p>Return the size of <var>val</var> in bytes.
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<p class="noindent"><strong>See also:</strong> <a href="doc_002dwhos.html#doc_002dwhos">whos</a>.
</p></blockquote></div>
<!-- size_equal src/data.cc -->
<p><a name="doc_002dsize_005fequal"></a>
<div class="defun">
— Built-in Function: <b>size_equal</b> (<var>a, b, <small class="dots">...</small></var>)<var><a name="index-size_005fequal-239"></a></var><br>
<blockquote><p>Return true if the dimensions of all arguments agree.
Trailing singleton dimensions are ignored.
Called with a single or no argument, size_equal returns true.
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<p class="noindent"><strong>See also:</strong> <a href="doc_002dsize.html#doc_002dsize">size</a>, <a href="doc_002dnumel.html#doc_002dnumel">numel</a>.
</p></blockquote></div>
<!-- squeeze src/data.cc -->
<p><a name="doc_002dsqueeze"></a>
<div class="defun">
— Built-in Function: <b>squeeze</b> (<var>x</var>)<var><a name="index-squeeze-240"></a></var><br>
<blockquote><p>Remove singleton dimensions from <var>x</var> and return the result.
Note that for compatibility with <span class="sc">matlab</span>, all objects have
a minimum of two dimensions and row vectors are left unchanged.
</p></blockquote></div>
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