lang/C/the.ansi.c.programming.language/notes.accompany.ansi.c/sx5j.html

   1 <!DOCTYPE HTML PUBLIC "-//W3O//DTD W3 HTML 2.0//EN">
   2 <!-- This collection of hypertext pages is Copyright 1995, 1996 by Steve Summit. -->
   3 <!-- This material may be freely redistributed and used -->
   4 <!-- but may not be republished or sold without permission. -->
   5 <html>
   6 <head>
   7 <link rev="owner" href="mailto:scs@eskimo.com">
   8 <link rev="made" href="mailto:scs@eskimo.com">
   9 <title>section 2.10: Assignment Operators and Expressions</title>
  10 <link href="sx5i.html" rev=precedes>
  11 <link href="sx5k.html" rel=precedes>
  12 <link href="sx5.html" rev=subdocument>
  13 </head>
  14 <body>
  15 <H2>section 2.10: Assignment Operators and Expressions</H2>
  16
  17 page 50
  18 <p>You may wonder what it means to say that
  19 ``<I>expr</I><tt>&lt;sub&gt;</tt>1<tt>&lt;/sub&gt;</tt> is computed only once''
  20 since in an assignment like
  21 <pre>   i = i + 2
  22 </pre>we don't ``evaluate''
  23 the <TT>i</TT> on the left hand side of the <TT>=</TT> at all,
  24 we assign to it.
  25 The distinction becomes important,
  26 however, when
  27 the left hand side
  28 (<I>expr</I><tt>&lt;sub&gt;</tt>1<tt>&lt;/sub&gt;</tt>)
  29 is more complicated than a simple variable.
  30 For example,
  31 we could add 2 to
  32 each of
  33 the <TT>n</TT> cells of an array <TT>a</TT>
  34 with code like
  35 <pre>   int i = 0;
  36         while(i &lt; n)
  37                 a[i++] += 2;
  38 </pre>If we tried to use the expanded form,
  39 we'd get
  40 <pre>   int i = 0;
  41         while(i &lt; n)
  42                 a[i++] = a[i++] + 2;
  43 </pre>and by trying to increment <TT>i</TT> twice within the same expression
  44 we'd get (as we'll see) undesired, unpredictable,
  45 and in fact undefined results.
  46 (Of course,
  47 a more natural form of this loop would be
  48 <pre>   for(i = 0; i &lt; n; i++)
  49                 a[i] += 2;
  50 </pre>and with the increment of <TT>i</TT> moved out of the array subscript,
  51 it wouldn't matter so much whether we used <TT>a[i] += 2</TT>
  52 or <TT>a[i] = a[i] + 2</TT>.)
  53 </p><p>page 51
  54 </p><p>To make the point more clear,
  55
  56 the ``complicated expression'' without using <TT>+=</TT> would look like
  57 <pre>   yyval[yypv[p3+p4] + yypv[p1+p2]] = yyval[yypv[p3+p4] + yypv[p1+p2]] + 2
  58 </pre>(What's going on here is that the subexpression
  59 <TT>yypv[p3+p4] + yypv[p1+p2]</TT> is being used as a subscript
  60 to determine which cell of the <TT>yyval</TT> array to increment by 2.)
  61
  62 </p><p>The sentence
  63 on p. 51
  64 that includes the words
  65 ``the assignment statement has a value''
  66 is a bit misleading:
  67 an assignment is really an expression in C.
  68 Like any expression,
  69 it has a value,
  70 and it can therefore participate as a subexpression
  71 in a larger expression.
  72 (If the distinction between the terms ``statement'' and
  73 ``expression''
  74 seems vague,
  75 don't worry;
  76 we'll start talking about statements in the next chapter.)
  77 </p><hr>
  78 <p>
  79 Read sequentially:
  80 <a href="sx5i.html" rev=precedes>prev</a>
  81 <a href="sx5k.html" rel=precedes>next</a>
  82 <a href="sx5.html" rev=subdocument>up</a>
  83 <a href="top.html">top</a>
  84 </p>
  85 <p>
  86 This page by <a href="http://www.eskimo.com/~scs/">Steve Summit</a>
  87 // <a href="copyright.html">Copyright</a> 1995, 1996
  88 // <a href="mailto:scs@eskimo.com">mail feedback</a>
  89 </p>
  90 </body>
  91 </html>