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   9 <title>section 1.6: Arrays</title>
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  15 <H2>section 1.6: Arrays</H2>
  16
  17 page 22
  18 <p>Note carefully that arrays in C are 0-based,
  19 not 1-based as they are in some languages.
  20
  21 (As we'll see, 0-based arrays turn out to be more convenient
  22 than 1-based arrays more of the time,
  23 but they may take a bit of getting used to at first.)
  24 </p><p>When they say
  25 ``as reflected in the <TT>for</TT> loops
  26 that initialize and print the array,''
  27 they're referring to the fact that the vast majority of
  28 <TT>for</TT> loops in C look like this:
  29 <pre>   for(i = 0; i &lt; 10; ++i)
  30 </pre>and count from 0 to 9.
  31 The loop
  32 <pre>   for(i = 1; i &lt;= 10; ++i)
  33 </pre>would count from 1 to 10, but loops like this are comparatively
  34 rare.
  35 (In fact, whenever you see either ``<TT>= 1</TT>''
  36 or ``<TT>&lt;=</TT>'' in a <TT>for</TT> loop,
  37 it's an indication that something unusual is going on
  38 which you'll want to be aware of,
  39 and it may even be a bug.)
  40 </p><p>page 23
  41 </p><p>They've started going a little fast here,
  42 so read up if they're losing you.
  43 What's this magic expression <TT>c-'0'</TT>
  44 that they're using as an array subscript?
  45 Remember, as we saw first on page 19,
  46 that characters in C are represented by small integers
  47 corresponding to their values in the machine's character set.
  48 In ASCII, which most machines use,
  49 <TT>'A'</TT> is character code 65,
  50 <TT>'0'</TT> (zero) is code 48,
  51 <TT>'9'</TT> is code 57,
  52 and all the other characters have their own values which I won't
  53 bother to list.
  54 If we've just read the character <TT>'9'</TT> from the file,
  55 it has value 57, so <TT>c-'0'</TT> is 57 - 48 which is 9,
  56 and we'll increment cell number 9 in the array,
  57 just like we want to.
  58 Furthermore,
  59 even if we're not using a machine which uses ASCII,
  60 by subtracting <TT>'0'</TT>,
  61 we'll always subtract whatever the right value is to map the
  62 characters from <TT>'0'</TT> to <TT>'9'</TT> down to the array cell
  63 range 0 to 9.
  64 </p>
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  74 This page by <a href="http://www.eskimo.com/~scs/">Steve Summit</a>
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