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   9 <title>10.3 Pointer Subtraction and Comparison</title>
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  15 <H2>10.3 Pointer Subtraction and Comparison</H2>
  16
  17 <p>As we've seen,
  18 you can add an integer to a pointer to get a new pointer,
  19 pointing somewhere beyond the original
  20 (as long as it's in the same array).
  21 For example, you might write
  22 <pre>
  23         ip2 = ip1 + 3;
  24 </pre>
  25 Applying a little algebra, you might wonder whether
  26 <pre>
  27         ip2 - ip1 = 3
  28 </pre>
  29 and the answer is, yes.
  30 When you subtract two pointers,
  31 as long as they point into the same array,
  32 the result is the number of elements separating them.
  33 You can also ask
  34 (again, as long as they point into the same array)
  35 whether one pointer is greater or less than another:
  36 one pointer is ``greater than'' another
  37 if it points beyond where the other one points.
  38 You can also compare pointers for equality and inequality:
  39 two pointers are equal
  40 if they point to the same variable or to the same cell in an array,
  41 and are (obviously) unequal if they don't.
  42 (When testing for equality or inequality,
  43 the two pointers do not have to point into the same array.)
  44 </p><p>One common use of pointer comparisons is when copying arrays
  45 using pointers.
  46 Here is a code fragment which copies 10 elements
  47 from <TT>array1</TT> to <TT>array2</TT>, using pointers.
  48 It uses an end pointer, <TT>ep</TT>,
  49 to keep track of when it should stop copying.
  50 <pre>
  51         int array1[10], array2[10];
  52         int *ip1, *ip2 = &amp;array2[0];
  53         int *ep = &amp;array1[10];
  54         for(ip1 = &amp;array1[0]; ip1 &lt; ep; ip1++)
  55                 *ip2++ = *ip1;
  56 </pre>
  57 As we mentioned,
  58 there is no element <TT>array1[10]</TT>,
  59 but
  60 it is legal to compute a pointer to this (nonexistent) element,
  61 as long as we only use it in pointer comparisons like this
  62 (that is,
  63 as long as we never try to fetch or store the value that it points
  64 to.)
  65 </p><hr>
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