<html lang="en"> <head> <title>Short-circuit Boolean Operators - Untitled</title> <meta http-equiv="Content-Type" content="text/html"> <meta name="description" content="Untitled"> <meta name="generator" content="makeinfo 4.13"> <link title="Top" rel="start" href="index.html#Top"> <link rel="up" href="Boolean-Expressions.html#Boolean-Expressions" title="Boolean Expressions"> <link rel="prev" href="Element_002dby_002delement-Boolean-Operators.html#Element_002dby_002delement-Boolean-Operators" title="Element-by-element Boolean Operators"> <link href="http://www.gnu.org/software/texinfo/" rel="generator-home" title="Texinfo Homepage"> <meta http-equiv="Content-Style-Type" content="text/css"> <style type="text/css"><!-- pre.display { font-family:inherit } pre.format { font-family:inherit } pre.smalldisplay { font-family:inherit; font-size:smaller } pre.smallformat { font-family:inherit; font-size:smaller } pre.smallexample { font-size:smaller } pre.smalllisp { font-size:smaller } span.sc { font-variant:small-caps } span.roman { font-family:serif; font-weight:normal; } span.sansserif { font-family:sans-serif; font-weight:normal; } --></style> </head> <body> <div class="node"> <a name="Short-circuit-Boolean-Operators"></a> <a name="Short_002dcircuit-Boolean-Operators"></a> <p> Previous: <a rel="previous" accesskey="p" href="Element_002dby_002delement-Boolean-Operators.html#Element_002dby_002delement-Boolean-Operators">Element-by-element Boolean Operators</a>, Up: <a rel="up" accesskey="u" href="Boolean-Expressions.html#Boolean-Expressions">Boolean Expressions</a> <hr> </div> <h4 class="subsection">8.5.2 Short-circuit Boolean Operators</h4> <p><a name="index-short_002dcircuit-evaluation-523"></a> Combined with the implicit conversion to scalar values in <code>if</code> and <code>while</code> conditions, Octave's element-by-element boolean operators are often sufficient for performing most logical operations. However, it is sometimes desirable to stop evaluating a boolean expression as soon as the overall truth value can be determined. Octave's <dfn>short-circuit</dfn> boolean operators work this way. <dl> <dt><var>boolean1</var><code> && </code><var>boolean2</var><dd><a name="index-g_t_0026_0026-524"></a>The expression <var>boolean1</var> is evaluated and converted to a scalar using the equivalent of the operation <code>all (</code><var>boolean1</var><code>(:))</code>. If it is false, the result of the overall expression is 0. If it is true, the expression <var>boolean2</var> is evaluated and converted to a scalar using the equivalent of the operation <code>all (</code><var>boolean1</var><code>(:))</code>. If it is true, the result of the overall expression is 1. Otherwise, the result of the overall expression is 0. <p><strong>Warning:</strong> there is one exception to the rule of evaluating <code>all (</code><var>boolean1</var><code>(:))</code>, which is when <code>boolean1</code> is the empty matrix. The truth value of an empty matrix is always <code>false</code> so <code>[] && true</code> evaluates to <code>false</code> even though <code>all ([])</code> is <code>true</code>. <br><dt><var>boolean1</var><code> || </code><var>boolean2</var><dd><a name="index-g_t_007c_007c-525"></a>The expression <var>boolean1</var> is evaluated and converted to a scalar using the equivalent of the operation <code>all (</code><var>boolean1</var><code>(:))</code>. If it is true, the result of the overall expression is 1. If it is false, the expression <var>boolean2</var> is evaluated and converted to a scalar using the equivalent of the operation <code>all (</code><var>boolean1</var><code>(:))</code>. If it is true, the result of the overall expression is 1. Otherwise, the result of the overall expression is 0. <p><strong>Warning:</strong> the truth value of an empty matrix is always <code>false</code>, see the previous list item for details. </dl> <p>The fact that both operands may not be evaluated before determining the overall truth value of the expression can be important. For example, in the expression <pre class="example"> a && b++ </pre> <p class="noindent">the value of the variable <var>b</var> is only incremented if the variable <var>a</var> is nonzero. <p>This can be used to write somewhat more concise code. For example, it is possible write <pre class="example"> function f (a, b, c) if (nargin > 2 && ischar (c)) ... </pre> <p class="noindent">instead of having to use two <code>if</code> statements to avoid attempting to evaluate an argument that doesn't exist. For example, without the short-circuit feature, it would be necessary to write <pre class="example"> function f (a, b, c) if (nargin > 2) if (ischar (c)) ... </pre> <p class="noindent">Writing <pre class="example"> function f (a, b, c) if (nargin > 2 & ischar (c)) ... </pre> <p class="noindent">would result in an error if <code>f</code> were called with one or two arguments because Octave would be forced to try to evaluate both of the operands for the operator ‘<samp><span class="samp">&</span></samp>’. </body></html>