<html lang="en"> <head> <title>The if Statement - GNU Octave</title> <meta http-equiv="Content-Type" content="text/html"> <meta name="description" content="GNU Octave"> <meta name="generator" content="makeinfo 4.13"> <link title="Top" rel="start" href="index.html#Top"> <link rel="up" href="Statements.html#Statements" title="Statements"> <link rel="next" href="The-switch-Statement.html#The-switch-Statement" title="The switch Statement"> <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="The-if-Statement"></a> <p> Next: <a rel="next" accesskey="n" href="The-switch-Statement.html#The-switch-Statement">The switch Statement</a>, Up: <a rel="up" accesskey="u" href="Statements.html#Statements">Statements</a> <hr> </div> <h3 class="section">10.1 The if Statement</h3> <p><a name="index-g_t_0040code_007bif_007d-statement-690"></a><a name="index-g_t_0040code_007belse_007d-statement-691"></a><a name="index-g_t_0040code_007belseif_007d-statement-692"></a><a name="index-g_t_0040code_007bendif_007d-statement-693"></a> The <code>if</code> statement is Octave's decision-making statement. There are three basic forms of an <code>if</code> statement. In its simplest form, it looks like this: <pre class="example"> if (<var>condition</var>) <var>then-body</var> endif </pre> <p class="noindent"><var>condition</var> is an expression that controls what the rest of the statement will do. The <var>then-body</var> is executed only if <var>condition</var> is true. <p>The condition in an <code>if</code> statement is considered true if its value is non-zero, and false if its value is zero. If the value of the conditional expression in an <code>if</code> statement is a vector or a matrix, it is considered true only if it is non-empty and <em>all</em> of the elements are non-zero. <p>The second form of an if statement looks like this: <pre class="example"> if (<var>condition</var>) <var>then-body</var> else <var>else-body</var> endif </pre> <p class="noindent">If <var>condition</var> is true, <var>then-body</var> is executed; otherwise, <var>else-body</var> is executed. <p>Here is an example: <pre class="example"> if (rem (x, 2) == 0) printf ("x is even\n"); else printf ("x is odd\n"); endif </pre> <p>In this example, if the expression <code>rem (x, 2) == 0</code> is true (that is, the value of <code>x</code> is divisible by 2), then the first <code>printf</code> statement is evaluated, otherwise the second <code>printf</code> statement is evaluated. <p>The third and most general form of the <code>if</code> statement allows multiple decisions to be combined in a single statement. It looks like this: <pre class="example"> if (<var>condition</var>) <var>then-body</var> elseif (<var>condition</var>) <var>elseif-body</var> else <var>else-body</var> endif </pre> <p class="noindent">Any number of <code>elseif</code> clauses may appear. Each condition is tested in turn, and if one is found to be true, its corresponding <var>body</var> is executed. If none of the conditions are true and the <code>else</code> clause is present, its body is executed. Only one <code>else</code> clause may appear, and it must be the last part of the statement. <p>In the following example, if the first condition is true (that is, the value of <code>x</code> is divisible by 2), then the first <code>printf</code> statement is executed. If it is false, then the second condition is tested, and if it is true (that is, the value of <code>x</code> is divisible by 3), then the second <code>printf</code> statement is executed. Otherwise, the third <code>printf</code> statement is performed. <pre class="example"> if (rem (x, 2) == 0) printf ("x is even\n"); elseif (rem (x, 3) == 0) printf ("x is odd and divisible by 3\n"); else printf ("x is odd\n"); endif </pre> <p>Note that the <code>elseif</code> keyword must not be spelled <code>else if</code>, as is allowed in Fortran. If it is, the space between the <code>else</code> and <code>if</code> will tell Octave to treat this as a new <code>if</code> statement within another <code>if</code> statement's <code>else</code> clause. For example, if you write <pre class="example"> if (<var>c1</var>) <var>body-1</var> else if (<var>c2</var>) <var>body-2</var> endif </pre> <p class="noindent">Octave will expect additional input to complete the first <code>if</code> statement. If you are using Octave interactively, it will continue to prompt you for additional input. If Octave is reading this input from a file, it may complain about missing or mismatched <code>end</code> statements, or, if you have not used the more specific <code>end</code> statements (<code>endif</code>, <code>endfor</code>, etc.), it may simply produce incorrect results, without producing any warning messages. <p>It is much easier to see the error if we rewrite the statements above like this, <pre class="example"> if (<var>c1</var>) <var>body-1</var> else if (<var>c2</var>) <var>body-2</var> endif </pre> <p class="noindent">using the indentation to show how Octave groups the statements. See <a href="Functions-and-Scripts.html#Functions-and-Scripts">Functions and Scripts</a>. </body></html>