1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
|
<html lang="en">
<head>
<title>The @code{switch} Statement - Untitled</title>
<meta http-equiv="Content-Type" content="text/html">
<meta name="description" content="Untitled">
<meta name="generator" content="makeinfo 4.11">
<link title="Top" rel="start" href="index.html#Top">
<link rel="up" href="Statements.html#Statements" title="Statements">
<link rel="prev" href="The-_0040code_007bif_007d-Statement.html#The-_0040code_007bif_007d-Statement" title="The @code{if} Statement">
<link rel="next" href="The-_0040code_007bwhile_007d-Statement.html#The-_0040code_007bwhile_007d-Statement" title="The @code{while} 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">
<p>
<a name="The-%3ccode%3eswitch%3c%2fcode%3e-Statement"></a>
<a name="The-_003ccode_003eswitch_003c_002fcode_003e-Statement"></a>
Next: <a rel="next" accesskey="n" href="The-_003ccode_003ewhile_003c_002fcode_003e-Statement.html#The-_003ccode_003ewhile_003c_002fcode_003e-Statement">The <code>while</code> Statement</a>,
Previous: <a rel="previous" accesskey="p" href="The-_003ccode_003eif_003c_002fcode_003e-Statement.html#The-_003ccode_003eif_003c_002fcode_003e-Statement">The <code>if</code> Statement</a>,
Up: <a rel="up" accesskey="u" href="Statements.html#Statements">Statements</a>
<hr>
</div>
<h3 class="section">10.2 The <code>switch</code> Statement</h3>
<p><a name="index-g_t_0040code_007bswitch_007d-statement-560"></a><a name="index-g_t_0040code_007bcase_007d-statement-561"></a><a name="index-g_t_0040code_007botherwise_007d-statement-562"></a><a name="index-g_t_0040code_007bendswitch_007d-statement-563"></a>
It is very common to take different actions depending on the value of
one variable. This is possible using the <code>if</code> statement in the
following way
<pre class="example"> if (X == 1)
do_something ();
elseif (X == 2)
do_something_else ();
else
do_something_completely_different ();
endif
</pre>
<p class="noindent">This kind of code can however be very cumbersome to both write and
maintain. To overcome this problem Octave supports the <code>switch</code>
statement. Using this statement, the above example becomes
<pre class="example"> switch (X)
case 1
do_something ();
case 2
do_something_else ();
otherwise
do_something_completely_different ();
endswitch
</pre>
<p class="noindent">This code makes the repetitive structure of the problem more explicit,
making the code easier to read, and hence maintain. Also, if the
variable <code>X</code> should change its name, only one line would need
changing compared to one line per case when <code>if</code> statements are
used.
<p>The general form of the <code>switch</code> statement is
<pre class="example"> switch <var>expression</var>
case <var>label</var>
<var>command_list</var>
case <var>label</var>
<var>command_list</var>
...
otherwise
<var>command_list</var>
endswitch
</pre>
<p class="noindent">where <var>label</var> can be any expression. However, duplicate
<var>label</var> values are not detected, and only the <var>command_list</var>
corresponding to the first match will be executed. For the
<code>switch</code> statement to be meaningful at least one
<code>case </code><var>label</var> <var>command_list</var> clause must be present,
while the <code>otherwise </code><var>command_list</var> clause is optional.
<p>If <var>label</var> is a cell array the corresponding <var>command_list</var>
is executed if <em>any</em> of the elements of the cell array match
<var>expression</var>. As an example, the following program will print
‘<samp><span class="samp">Variable is either 6 or 7</span></samp>’.
<pre class="example"> A = 7;
switch A
case { 6, 7 }
printf ("variable is either 6 or 7\n");
otherwise
printf ("variable is neither 6 nor 7\n");
endswitch
</pre>
<p>As with all other specific <code>end</code> keywords, <code>endswitch</code> may be
replaced by <code>end</code>, but you can get better diagnostics if you use
the specific forms.
<!-- Strings can be matched -->
<p>One advantage of using the <code>switch</code> statement compared to using
<code>if</code> statements is that the <var>label</var>s can be strings. If an
<code>if</code> statement is used it is <em>not</em> possible to write
<pre class="example"> if (X == "a string") # This is NOT valid
</pre>
<p class="noindent">since a character-to-character comparison between <code>X</code> and the
string will be made instead of evaluating if the strings are equal.
This special-case is handled by the <code>switch</code> statement, and it
is possible to write programs that look like this
<pre class="example"> switch (X)
case "a string"
do_something
...
endswitch
</pre>
<ul class="menu">
<li><a accesskey="1" href="Notes-for-the-C-programmer.html#Notes-for-the-C-programmer">Notes for the C programmer</a>
</ul>
</body></html>
|
