C/the.ansi.c.programming.language/c.programming.notes/sx10e.html

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   9 <title>10.5 ``Equivalence'' between Pointers and Arrays</title>
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  15 <H2>10.5 ``Equivalence'' between Pointers and Arrays</H2>
  16
  17 <p>There are a number of similarities between arrays and pointers in C.
  18 If you have an array
  19 <pre>
  20         int a[10];
  21 </pre>
  22 you can refer to <TT>a[0]</TT>, <TT>a[1]</TT>, <TT>a[2]</TT>, etc.,
  23 or to <TT>a[i]</TT> where <TT>i</TT> is an <TT>int</TT>.
  24 If you declare a pointer variable <TT>ip</TT>
  25 and set it to point to the beginning of an array:
  26 <pre>
  27         int *ip = &amp;a[0];
  28 </pre>
  29 you can refer to <TT>*ip</TT>, <TT>*(ip+1)</TT>, <TT>*(ip+2)</TT>, etc.,
  30 or to <TT>*(ip+i)</TT> where <TT>i</TT> is an <TT>int</TT>.
  31 </p><p>There are also differences, of course.
  32 You cannot assign two arrays; the code
  33 <pre>
  34         int a[10], b[10];
  35         a = b;                          /* WRONG */
  36 </pre>
  37 is illegal.
  38 As we've seen, though,
  39 you <em>can</em> assign two pointer variables:
  40 <pre>
  41         int *ip1, *ip2;
  42         ip1 = &amp;a[0];
  43         ip2 = ip1;
  44 </pre>
  45
  46 Pointer assignment is straightforward;
  47 the pointer on the left is simply made to point wherever the
  48 pointer on the right does.
  49 We haven't copied the data pointed to
  50 (there's still just one copy, in the same place);
  51 we've just made two pointers point to that one place.
  52 </p><p>The similarities between arrays and pointers
  53 end up being quite useful,
  54 and in fact C builds on the similarities,
  55 leading to what is called
  56 ``the equivalence of arrays and pointers in C.''
  57 When we speak of this ``equivalence''
  58 we do not mean that
  59 arrays and pointers are
  60 the same thing
  61 (they are in fact quite different),
  62 but
  63 rather
  64 that they can be used in related ways,
  65 and that certain operations may be used between
  66
  67 them.
  68 </p><p>The first such operation is that it is possible
  69 to (apparently) assign an array to a pointer:
  70 <pre>
  71         int a[10];
  72         int *ip;
  73         ip = a;
  74 </pre>
  75 What can this mean?
  76 In that last assignment
  77 <TT>ip = a</TT>,
  78 aren't we mixing apples and oranges again?
  79 It turns out that we are not;
  80 C defines the result of this assignment
  81 to be
  82 that
  83 <TT>ip</TT> receives a pointer to the first element of <TT>a</TT>.
  84 In other words,
  85 it is as if you had written
  86 <pre>
  87         ip = &amp;a[0];
  88 </pre>
  89 </p><p>The second facet of the equivalence
  90 is that you can use the ``array subscripting'' notation <TT>[i]</TT>
  91 on pointers, too.
  92 If you write
  93 <pre>
  94         ip[3]
  95 </pre>
  96 it is just as if you had written
  97 <pre>
  98         *(ip + 3)
  99 </pre>
 100 So when you have a pointer that points to a block of memory,
 101 such as an array or a part of an array,
 102 you can treat that pointer
 103 ``as if'' it <em>were</em>
 104
 105 an array,
 106 using the convenient <TT>[i]</TT> notation.
 107 In other words,
 108 at the beginning of this section when we talked about
 109 <TT>*ip</TT>, <TT>*(ip+1)</TT>, <TT>*(ip+2)</TT>,
 110 and <TT>*(ip+i)</TT>,
 111 we could have written
 112 <TT>ip[0]</TT>, <TT>ip[1]</TT>, <TT>ip[2]</TT>,
 113 and <TT>ip[i]</TT>.
 114 As we'll see, this can be quite useful
 115 (or at least convenient).
 116 </p><p>The third facet of the equivalence
 117 (which is actually a more general version of the first one we mentioned)
 118 is that <em>whenever</em> you mention the name of an array
 119 in a context where the ``value'' of the array would be needed,
 120 C automatically generates a pointer to the first element of the array,
 121 as if you had written <TT>&amp;array[0]</TT>.
 122 When you write something like
 123 <pre>
 124         int a[10];
 125         int *ip;
 126         ip = a + 3;
 127 </pre>
 128 it is as if you had written
 129 <pre>
 130         ip = &amp;a[0] + 3;
 131 </pre>
 132 which
 133 (and you might like to convince yourself of this)
 134 gives the same result as if you had written
 135 <pre>
 136         ip = &amp;a[3];
 137 </pre>
 138 </p><p>For example,
 139 if the character array
 140 <pre>
 141         char string[100];
 142 </pre>
 143 contains some string,
 144 here is another way to find its length:
 145 <pre>
 146         int len;
 147         char *p;
 148
 149         for(p = string; *p != '\0'; p++)
 150                 ;
 151
 152         len = p - string;
 153 </pre>
 154 After the loop, <TT>p</TT> points to the <TT>'\0'</TT>
 155 terminating the string.
 156 The expression
 157 <TT>p - string</TT>
 158 is equivalent to
 159 <TT>p - &amp;string[0]</TT>,
 160 and gives the length of the string.
 161 (Of course, we could also call <TT>strlen</TT>;
 162 in fact here we've essentially written
 163 another implementation of <TT>strlen</TT>.)
 164 </p><hr>
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 173 This page by <a href="http://www.eskimo.com/~scs/">Steve Summit</a>
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