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<a name="Assignment-Ops"></a>
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<hr>
<a name="Assignment-Expressions"></a>
<h3 class="section">8.6 Assignment Expressions</h3>
<a name="index-assignment-expressions"></a>
<a name="index-assignment-operators"></a>
<a name="index-operators_002c-assignment"></a>
<a name="index-expressions_002c-assignment"></a>
<a name="index-_003d"></a>
<p>An <em>assignment</em> is an expression that stores a new value into a
variable. For example, the following expression assigns the value 1 to
the variable <code>z</code>:
</p>
<div class="example">
<pre class="example">z = 1
</pre></div>
<p>After this expression is executed, the variable <code>z</code> has the value 1.
Whatever old value <code>z</code> had before the assignment is forgotten.
The ‘<samp>=</samp>’ sign is called an <em>assignment operator</em>.
</p>
<p>Assignments can store string values also. For example, the following
expression would store the value <code>"this food is good"</code> in the
variable <code>message</code>:
</p>
<div class="example">
<pre class="example">thing = "food"
predicate = "good"
message = [ "this " , thing , " is " , predicate ]
</pre></div>
<p>(This also illustrates concatenation of strings.)
</p>
<a name="index-side-effect"></a>
<p>Most operators (addition, concatenation, and so on) have no effect
except to compute a value. If you ignore the value, you might as well
not use the operator. An assignment operator is different. It does
produce a value, but even if you ignore the value, the assignment still
makes itself felt through the alteration of the variable. We call this
a <em>side effect</em>.
</p>
<a name="index-lvalue"></a>
<p>The left-hand operand of an assignment need not be a variable
(see <a href="Variables.html#Variables">Variables</a>). It can also be an element of a matrix
(see <a href="Index-Expressions.html#Index-Expressions">Index Expressions</a>) or a list of return values
(see <a href="Calling-Functions.html#Calling-Functions">Calling Functions</a>). These are all called <em>lvalues</em>, which
means they can appear on the left-hand side of an assignment operator.
The right-hand operand may be any expression. It produces the new value
which the assignment stores in the specified variable, matrix element,
or list of return values.
</p>
<p>It is important to note that variables do <em>not</em> have permanent types.
The type of a variable is simply the type of whatever value it happens
to hold at the moment. In the following program fragment, the variable
<code>foo</code> has a numeric value at first, and a string value later on:
</p>
<div class="example">
<pre class="example">octave:13> foo = 1
foo = 1
octave:13> foo = "bar"
foo = bar
</pre></div>
<p>When the second assignment gives <code>foo</code> a string value, the fact that
it previously had a numeric value is forgotten.
</p>
<p>Assignment of a scalar to an indexed matrix sets all of the elements
that are referenced by the indices to the scalar value. For example, if
<code>a</code> is a matrix with at least two columns,
</p>
<div class="example">
<pre class="example">a(:, 2) = 5
</pre></div>
<p>sets all the elements in the second column of <code>a</code> to 5.
</p>
<p>Assigning an empty matrix ‘<samp>[]</samp>’ works in most cases to allow you to
delete rows or columns of matrices and vectors. See <a href="Empty-Matrices.html#Empty-Matrices">Empty Matrices</a>.
For example, given a 4 by 5 matrix <var>A</var>, the assignment
</p>
<div class="example">
<pre class="example">A (3, :) = []
</pre></div>
<p>deletes the third row of <var>A</var>, and the assignment
</p>
<div class="example">
<pre class="example">A (:, 1:2:5) = []
</pre></div>
<p>deletes the first, third, and fifth columns.
</p>
<p>An assignment is an expression, so it has a value. Thus, <code>z = 1</code>
as an expression has the value 1. One consequence of this is that you
can write multiple assignments together:
</p>
<div class="example">
<pre class="example">x = y = z = 0
</pre></div>
<p>stores the value 0 in all three variables. It does this because the
value of <code>z = 0</code>, which is 0, is stored into <code>y</code>, and then
the value of <code>y = z = 0</code>, which is 0, is stored into <code>x</code>.
</p>
<p>This is also true of assignments to lists of values, so the following is
a valid expression
</p>
<div class="example">
<pre class="example">[a, b, c] = [u, s, v] = svd (a)
</pre></div>
<p>that is exactly equivalent to
</p>
<div class="example">
<pre class="example">[u, s, v] = svd (a)
a = u
b = s
c = v
</pre></div>
<p>In expressions like this, the number of values in each part of the
expression need not match. For example, the expression
</p>
<div class="example">
<pre class="example">[a, b] = [u, s, v] = svd (a)
</pre></div>
<p>is equivalent to
</p>
<div class="example">
<pre class="example">[u, s, v] = svd (a)
a = u
b = s
</pre></div>
<p>The number of values on the left side of the expression can, however,
not exceed the number of values on the right side. For example, the
following will produce an error.
</p>
<div class="example">
<pre class="example">[a, b, c, d] = [u, s, v] = svd (a);
-| error: element number 4 undefined in return list
</pre></div>
<p>The symbol <code>~</code> may be used as a placeholder in the list of lvalues,
indicating that the corresponding return value should be ignored and not stored
anywhere:
</p>
<div class="example">
<pre class="example">[~, s, v] = svd (a);
</pre></div>
<p>This is cleaner and more memory efficient than using a dummy variable.
The <code>nargout</code> value for the right-hand side expression is not affected.
If the assignment is used as an expression, the return value is a
comma-separated list with the ignored values dropped.
</p>
<a name="index-_002b_003d"></a>
<p>A very common programming pattern is to increment an existing variable
with a given value, like this
</p>
<div class="example">
<pre class="example">a = a + 2;
</pre></div>
<p>This can be written in a clearer and more condensed form using the
<code>+=</code> operator
</p>
<div class="example">
<pre class="example">a += 2;
</pre></div>
<p><a name="index-_002d_003d"></a>
<a name="index-_002a_003d"></a>
<a name="index-_002f_003d"></a>
Similar operators also exist for subtraction (<code>-=</code>),
multiplication (<code>*=</code>), and division (<code>/=</code>). An expression
of the form
</p>
<div class="example">
<pre class="example"><var>expr1</var> <var>op</var>= <var>expr2</var>
</pre></div>
<p>is evaluated as
</p>
<div class="example">
<pre class="example"><var>expr1</var> = (<var>expr1</var>) <var>op</var> (<var>expr2</var>)
</pre></div>
<p>where <var>op</var> can be either <code>+</code>, <code>-</code>, <code>*</code>, or <code>/</code>,
as long as <var>expr2</var> is a simple expression with no side effects. If
<var>expr2</var> also contains an assignment operator, then this expression
is evaluated as
</p>
<div class="example">
<pre class="example"><var>temp</var> = <var>expr2</var>
<var>expr1</var> = (<var>expr1</var>) <var>op</var> <var>temp</var>
</pre></div>
<p>where <var>temp</var> is a placeholder temporary value storing the computed
result of evaluating <var>expr2</var>. So, the expression
</p>
<div class="example">
<pre class="example">a *= b+1
</pre></div>
<p>is evaluated as
</p>
<div class="example">
<pre class="example">a = a * (b+1)
</pre></div>
<p>and <em>not</em>
</p>
<div class="example">
<pre class="example">a = a * b + 1
</pre></div>
<p>You can use an assignment anywhere an expression is called for. For
example, it is valid to write <code>x != (y = 1)</code> to set <code>y</code> to 1
and then test whether <code>x</code> equals 1. But this style tends to make
programs hard to read. Except in a one-shot program, you should rewrite
it to get rid of such nesting of assignments. This is never very hard.
</p>
<a name="index-increment-operator"></a>
<a name="index-decrement-operator"></a>
<a name="index-operators_002c-increment"></a>
<a name="index-operators_002c-decrement"></a>
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