lang/C/the.ansi.c.programming.language/c.programming.notes/sx12e.html

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   9 <title>12.5 Example: Reading a Data File</title>
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  15 <H2>12.5 Example: Reading a Data File</H2>
  16
  17 <p>Suppose you had a data file consisting of rows and columns of numbers:
  18 <pre>
  19         1       2       34
  20         5       6       78
  21         9       10      112
  22 </pre>
  23 Suppose you wanted to read these numbers into an array.
  24 (Actually, the array will be an array of arrays,
  25 or a ``multidimensional'' array;
  26 see section
  27
  28 4.1.2.)
  29 We can write code to do this by putting together several pieces:
  30 the <TT>fgetline</TT> function we just showed,
  31 and the <TT>getwords</TT> function from
  32
  33 chapter 10.
  34 Assuming that the data file is named <TT>input.dat</TT>,
  35 the code would look like this:
  36 <pre>
  37 #define MAXLINE 100
  38 #define MAXROWS 10
  39 #define MAXCOLS 10
  40
  41 int array[MAXROWS][MAXCOLS];
  42 char *filename = "input.dat";
  43 FILE *ifp;
  44 char line[MAXLINE];
  45 char *words[MAXCOLS];
  46 int nrows = 0;
  47 int n;
  48 int i;
  49
  50 ifp = fopen(filename, "r");
  51 if(ifp == NULL)
  52         {
  53         fprintf(stderr, "can't open %s\n", filename);
  54         exit(EXIT_FAILURE);
  55         }
  56
  57 while(fgetline(ifp, line, MAXLINE) != EOF)
  58         {
  59         if(nrows &gt;= MAXROWS)
  60                 {
  61                 fprintf(stderr, "too many rows\n");
  62                 exit(EXIT_FAILURE);
  63                 }
  64
  65         n = getwords(line, words, MAXCOLS);
  66
  67         for(i = 0; i &lt; n; i++)
  68                 array[nrows][i] = atoi(words[i]);
  69         nrows++;
  70         }
  71 </pre>
  72 Each trip through the loop reads one line from the file,
  73 using <TT>fgetline</TT>.
  74 Each line is broken up into ``words'' using <TT>getwords</TT>;
  75 each ``word'' is actually one number.
  76 The numbers are however still represented as strings,
  77 so each one is converted to an <TT>int</TT> by calling <TT>atoi</TT>
  78 before being stored in the array.
  79 The code checks for two different error conditions
  80 (failure to open the input file,
  81 and too many lines in the input file)
  82 and if one of these conditions occurs,
  83 it prints an error message, and exits.
  84 The <TT>exit</TT> function is a Standard library function
  85 which terminates your program.
  86 It is declared in <TT>&lt;stdlib.h&gt;</TT>,
  87 and accepts one argument,
  88 which will be the <dfn>exit status</dfn> of the program.
  89 <TT>EXIT_FAILURE</TT> is a code,
  90 also defined by
  91 <TT>&lt;stdlib.h&gt;</TT>,
  92 which indicates that the program failed.
  93 Success is indicated by a code of <TT>EXIT_SUCCESS</TT>, or simply 0.
  94 (These values can also be returned from <TT>main()</TT>;
  95 calling <TT>exit</TT> with a particular status value
  96 is
  97 essentially equivalent
  98 to returning that same status value from <TT>main</TT>.)
  99 </p><hr>
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