array.fetch(index)                    → obj
array.fetch(index, default )          → obj
array.fetch(index) {|index| block }   → obj

Tries to return the element at position index. If the index lies outside the array, the first form throws an IndexError exception, the second form returns default, and the third form returns the value of invoking the block, passing in the index. Negative values of index count from the end of the array.

a = [ 11, 22, 33, 44 ]
a.fetch(1)               #=> 22
a.fetch(-1)              #=> 44
a.fetch(4, 'cat')        #=> "cat"
a.fetch(4) { |i| i*i }   #=> 16

Source Code

/*
*  call-seq:
*     array.fetch(index)                    -> obj
*     array.fetch(index, default )          -> obj
*     array.fetch(index) {|index| block }   -> obj
*  
*  Tries to return the element at position <i>index</i>. If the index
*  lies outside the array, the first form throws an
*  <code>IndexError</code> exception, the second form returns
*  <i>default</i>, and the third form returns the value of invoking
*  the block, passing in the index. Negative values of <i>index</i>
*  count from the end of the array.
*     
*     a = [ 11, 22, 33, 44 ]
*     a.fetch(1)               #=> 22
*     a.fetch(-1)              #=> 44
*     a.fetch(4, 'cat')        #=> "cat"
*     a.fetch(4) { |i| i*i }   #=> 16
*/

static VALUE
rb_ary_fetch(argc, argv, ary)
   int argc;
   VALUE *argv;
   VALUE ary;
{
   VALUE pos, ifnone;
   long block_given;
   long idx;

   rb_scan_args(argc, argv, "11", &pos, &ifnone);
   block_given = rb_block_given_p();
   if (block_given && argc == 2) {
       rb_warn("block supersedes default value argument");
   }
   idx = NUM2LONG(pos);

   if (idx < 0) {
       idx +=  RARRAY(ary)->len;
   }
   if (idx < 0 || RARRAY(ary)->len <= idx) {
       if (block_given) return rb_yield(pos);
       if (argc == 1) {
           rb_raise(rb_eIndexError, "index %ld out of array", idx);
       }
       return ifnone;
   }
   return RARRAY(ary)->ptr[idx];
}