title: why the first index of the column of a matrix is negative?
body: <p>why in this simple part of code to modify the point in the lower left of the bidimensional array have I to put in the coordinate <code>m[37][-40]</code>?</p>
  
  <pre><code>#include &lt;stdio.h&gt;
  #include &lt;stdlib.h&gt;
  #define MAX 40
  
  int main()
  {
      int i, j;
      char m[MAX][MAX];
  
      for(i=0; i&lt;MAX; i++){
          for(j=0; j&lt;MAX; j++)
              m[i][j]=' ';
      }
  
      m[37][-40]='X';
  
      for(i=0; i&lt;MAX; i++){
          for(j=0; j&lt;MAX; j++)
              printf("%c", m[i][j]);
      }
      return 0;
  }
  </code></pre>
  
  <p>Souldn't it be <code>m[37][0]</code>? Becouse the row is 37 and the column is 0...</p>